Slides: fix remarks after test presentation
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@ -11,6 +11,7 @@
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\usepackage{booktabs}
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\usepackage{makecell}
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\usepackage{ifthen}
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\usepackage{colortbl}
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\usepackage{../shared/my_listings}
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%\usepackage{../shared/my_hyperref}
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@ -18,17 +19,35 @@
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\usepackage{../shared/common}
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\usepackage{../shared/todo}
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\usepackage{inconsolata}
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\lstset{basicstyle=\footnotesize\ttfamily}
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\renewcommand\theadalign{c}
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\renewcommand\theadfont{\scriptsize\bfseries}
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\setbeamertemplate{navigation symbols}{}
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\setbeamertemplate{headline}{}
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\newcommand{\thenalert}[1]{\only<1>{#1}\only<2>{\alert{#1}}}
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\newcommand{\slidecountline}{
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\ifthenelse{\theframenumber = 0}
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{}
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{\insertframenumber/\inserttotalframenumber}}
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\newcommand{\sectionline}{
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\ifthenelse{\thesection = 0}
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{}
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{\Roman{section}~-- \insertsection}}
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\AtBeginSection[]{
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\begin{frame}
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\vfill
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\centering
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\begin{beamercolorbox}[sep=8pt,center,shadow=true,rounded=true]{title}
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\usebeamerfont{title}\insertsectionhead\par%
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\end{beamercolorbox}
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\vfill
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\end{frame}
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}
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\lstdefinelanguage{gdb}{
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morekeywords={gdb},
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@ -38,8 +57,8 @@
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\author[\slidecountline]{Théophile \textsc{Bastian} \\
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\small{Under supervision of Francesco Zappa Nardelli}}
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\title[DWARF unwinding data compilation]
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{Speeding up stack unwinding by compiling DWARF debugging data}
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\title[\sectionline]
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{Speeding up stack unwinding by compiling DWARF debug data}
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\date{March\ --\ August 2018}
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%\subject{}
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%\logo{}
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@ -51,16 +70,18 @@
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\addtocounter{framenumber}{-1}
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\titlepage{}
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\vspace{-1em}
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\vspace{-2em}
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\begin{center}
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Slides: \url{https://tobast.fr/m2/slides.pdf} \\
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Report: \url{https://tobast.fr/m2/report.pdf}
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\begin{align*}
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\text{Slides: } &\text{\url{https://tobast.fr/m2/slides.pdf}} \\
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\text{Report: } &\text{\url{https://tobast.fr/m2/report.pdf}}
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\end{align*}
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\end{center}
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\end{frame}
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\begin{frame}
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\begin{frame}{~}
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\addtocounter{framenumber}{-1}
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\tableofcontents
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\tableofcontents[hideallsubsections]
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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@ -70,26 +91,36 @@
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\subsection{Introduction}
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\begin{frame}[fragile]{We often use stack unwinding!}
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\begin{lstlisting}[language=gdb, numbers=none, escapechar=|]
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Program received signal SIGSEGV, Segmentation fault.
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0x0000555555554625 in fct_b (m=0x5c) at segfault.c:5
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5 printf("%l\n", *m);
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|\pause| (gdb) backtrace
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#0 0x0000555555554625 in fct_b (m=0x5c) at segfault.c:5
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#1 0x0000555555554663 in fct_a (n=42) at segfault.c:10
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#2 0x0000555555554674 in main () at segfault.c:14
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|\pause| (gdb) frame 1
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#1 0x0000555555554663 in fct_a (n=42) at segfault.c:10
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10 fct_b((int*)(some_fct_a_var + 8));
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|\pause| (gdb) print some_fct_a_var
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$1 = 84
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\end{lstlisting}
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\begin{columns}[c]
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\begin{column}{0.70\textwidth}
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\begin{lstlisting}[language=gdb, numbers=none, escapechar=|]
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Program received signal SIGSEGV.
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0x54625 in fct_b at segfault.c:5
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5 printf("%l\n", *b);
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\pause{}
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\begin{center}
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\textbf{\Large How does it work?!}
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\end{center}
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\vspace{1em}
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|\pause| (gdb) backtrace
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#0 0x54625 in fct_b at segfault.c:5
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#1 0x54663 in fct_a at segfault.c:10
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#2 0x54674 in main at segfault.c:14
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|\pause| (gdb) frame 1
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#1 0x54663 in fct_a at segfault.c:10
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10 fct_b((int*) a);
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|\pause| (gdb) print a
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$1 = 84
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\end{lstlisting}
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\vspace{-1em}
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\pause{}
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\begin{center}
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\textbf{\Large How does it work?!}
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\end{center}
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\end{column}
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\begin{column}{0.35\textwidth}
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\pause{}
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\includegraphics[width=0.95\linewidth]{img/stack/call_stack}
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\end{column}
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\end{columns}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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@ -97,85 +128,64 @@ $1 = 84
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\begin{frame}{Call stack and registers}
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\begin{columns}[c]
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\begin{column}{0.65\textwidth}
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\begin{itemize}
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\item Programs use a \alert{call stack}
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\item Organized in \alert{stack frames}
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\begin{itemize}
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\item Local variables
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\item Function parameters
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\item Keep track of nesting, registers and ``return
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point''
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\end{itemize}
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\end{itemize}
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\begin{column}{0.55\textwidth}
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\begin{center}
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\large\bf
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How do we get the grandparent RA\@?
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Common registers:
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\begin{itemize}
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\item \reg{rip}: program counter (PC)
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\medskip
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\item \reg{rsp}: stack pointer
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Isn't it as trivial as \texttt{pop()}?
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\item \reg{rbp}: base pointer
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\begin{itemize}
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\item Saves \reg{rsp}
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\item Easy access
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\item Wastes a register
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\item Not often used (x86\_64)
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\end{itemize}
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\end{itemize}
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\vspace{2em}
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\only<2>{We only have \reg{rsp} and \reg{rip}.}
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\end{center}
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\end{column}
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\begin{column}{0.35\textwidth}
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\includegraphics[width=0.95\linewidth]{../shared/imgs/call_stack}
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\begin{column}{0.45\textwidth}
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\includegraphics[width=0.95\linewidth]{img/stack/call_stack}
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\end{column}
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\end{columns}
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\end{frame}
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\begin{frame}{Isn't it as trivial as \texttt{pop()}?}
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\begin{itemize}
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\item This is only a \alert{blob of binary data} without mandatory
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structure
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\item We ignore \alert{which registers were saved}
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\item We ignore \alert{whether \reg{rbp} was used}
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\item We ignore \alert{where the return address is stored}
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\item We ignore \alert{where the previous frame begins}
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\end{itemize}
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\medskip
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But\ldots{} if we know how to \alert{unwind one}, we can \alert{recurse}!
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{DWARF tables}
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\begin{frame}[fragile, shrink]{DWARF unwinding data}
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\begin{lstlisting}[numbers=none, language=]
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00009b30 48 009b34 FDE cie=0000 pc=0084950..0084b37
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LOC CFA rbx rbp r12 r13 r14 r15 ra
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0084950 rsp+8 u u u u u u c-8
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0084952 rsp+16 u u u u u c-16 c-8
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0084954 rsp+24 u u u u c-24 c-16 c-8
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0084956 rsp+32 u u u c-32 c-24 c-16 c-8
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0084958 rsp+40 u u c-40 c-32 c-24 c-16 c-8
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0084959 rsp+48 u c-48 c-40 c-32 c-24 c-16 c-8
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008495a rsp+56 c-56 c-48 c-40 c-32 c-24 c-16 c-8
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0084962 rsp+64 c-56 c-48 c-40 c-32 c-24 c-16 c-8
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0084a19 rsp+56 c-56 c-48 c-40 c-32 c-24 c-16 c-8
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0084a1d rsp+48 c-56 c-48 c-40 c-32 c-24 c-16 c-8
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0084a1e rsp+40 c-56 c-48 c-40 c-32 c-24 c-16 c-8
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0084a20 rsp+32 c-56 c-48 c-40 c-32 c-24 c-16 c-8
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0084a22 rsp+24 c-56 c-48 c-40 c-32 c-24 c-16 c-8
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0084a24 rsp+16 c-56 c-48 c-40 c-32 c-24 c-16 c-8
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0084a26 rsp+8 c-56 c-48 c-40 c-32 c-24 c-16 c-8
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0084a30 rsp+64 c-56 c-48 c-40 c-32 c-24 c-16 c-8
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\end{lstlisting}
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\newcolumntype{a}{>{\columncolor{RedOrange}}l}
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\begin{frame}{DWARF unwinding data}
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\vspace{2em}
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\tt \footnotesize
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\begin{tabular}{
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>{\columncolor{YellowGreen}}l
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>{\columncolor{Thistle}}l
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l l l l l l
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>{\columncolor{Apricot}}l}
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~LOC & CFA & rbx & rbp & r12 & r13 & r14 & r15 & ra \\
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0084950 & rsp+8 & u & u & u & u & u & u & c-8 \\
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0084952 & rsp+16 & u & u & u & u & u & c-16 & c-8 \\
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0084954 & rsp+24 & u & u & u & u & c-24 & c-16 & c-8 \\
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0084956 & rsp+32 & u & u & u & c-32 & c-24 & c-16 & c-8 \\
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0084958 & rsp+40 & u & u & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084959 & rsp+48 & u & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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\rowcolor{Aquamarine} 008495a & rsp+56 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084962 & rsp+64 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a19 & rsp+56 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a1d & rsp+48 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a1e & rsp+40 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a20 & rsp+32 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a22 & rsp+24 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a24 & rsp+16 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a26 & rsp+8 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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0084a30 & rsp+64 & c-56 & c-48 & c-40 & c-32 & c-24 & c-16 & c-8 \\
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\end{tabular}
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\pause{}
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\vspace{-4cm}
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\vspace{-3cm}
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\hfill\includegraphics[height=3cm, angle=45, origin=c]{img/dwarf_logo}
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\hspace{-1cm}
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\end{frame}
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\begin{frame}[fragile]{The real DWARF}
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@ -194,22 +204,31 @@ $1 = 84
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DW_CFA_def_cfa_offset: 40
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DW_CFA_offset: r12 (r12) at cfa-40
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DW_CFA_advance_loc: 1 to 0000000000084959
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DW_CFA_def_cfa_offset: 48
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DW_CFA_offset: r6 (rbp) at cfa-48
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DW_CFA_advance_loc: 1 to 000000000008495a
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[...]
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\end{lstlisting}
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\begin{itemize}
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\item[\textbf{$\longrightarrow$}] \textbf{\alert{constructed} on-demand
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by a \alert{Turing-complete, slow bytecode}!}
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\end{itemize}
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\end{frame}
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\begin{frame}{Why does slow matter?}
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\textbf{Do we really care about speed for unwinding?}
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\begin{itemize}
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\item{} After all, we're talking about \alert{debugging procedures} ran
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by a \alert{human being} (slower than the machine).
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\ldots{}or are we?
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\end{itemize}
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\pause{}
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\begin{center}
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\textbf{\Large{}No!}
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\end{center}
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\begin{itemize}
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\pause{}\item{} Pretty much any \alert{program analysis tool}
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\pause{}\item{} \alert{Profiling} with polling profilers
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\pause{}\item{} \alert{Exception handling} in C++
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@ -224,7 +243,25 @@ $1 = 84
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Compiling DWARF}
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\section{Compiling stack unwinding data ahead-of-time}
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\subsection*{}
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\begin{frame}{Compilation overview}
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\begin{itemize}
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\item Compiled to \alert{C code}
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\item C code then \alert{compiled to native binary} (gcc)
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\begin{itemize}
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\item[$\leadsto$] gcc optimisations for free
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\end{itemize}
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\item Compiled as \alert{separate \texttt{.so} files}, called \ehelfs{}
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\bigskip{}
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\item Morally a \alert{monolithic switch} on IPs
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\item Each case contains assembly that computes a \alert{row of the
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table}
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\end{itemize}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Example}
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@ -241,40 +278,24 @@ $1 = 84
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Compilation Strategy}
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\begin{frame}{Compilation overview}
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\begin{itemize}
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\item Compiled to \alert{C code}
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\item C code then \alert{compiled to native binary} (gcc)
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\begin{itemize}
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\item[$\leadsto$] gcc optimisations for free
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\end{itemize}
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\item Compiled as \alert{separate \texttt{.so} files}, called \ehelfs{}
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\bigskip{}
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\item Morally a \alert{monolithic switch} on IPs
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\item Each case fills the context structure
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\end{itemize}
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\end{frame}
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\begin{frame}{Compilation choices}
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\textbf{In order to keep the compiler \alert{simple} and \alert{easily
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testable}, the whole DWARF5 instruction set is not supported.}
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\begin{itemize}
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\item Tailored for \alert{x86\_64} (while DWARF is
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architecture-agnostic)
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\item Only supports \alert{unwinding registers}: \reg{rip}, \reg{rsp},
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\reg{rbp}, \reg{rbx}
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\item Focus on \alert{x86\_64}
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\item Focus on unwinding return address \\
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\vspace{0.3ex}
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$\leadsto$ \textit{Allows building a backtrace}
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\begin{itemize}
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\item[$\leadsto$] suitable for perf, not for gdb
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\end{itemize}
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\item Supports the \alert{wide majority} ($> 99.9\%$) of instructions
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used (see later)
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\begin{itemize}
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\item Only supports few common expressions: already $~ 90\,\%$
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of expressions used
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\end{itemize}
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\item Among \alert{4000} randomly sampled filed, only \alert{24}
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containing unsupported instructions
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\item \alert{suitable for perf, not for gdb}
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\item Only supports \alert{unwinding registers}: \reg{rip}, \reg{rsp},
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\reg{rbp}, \reg{rbx}
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\item Supports the \alert{wide majority} ($> 99.9\%$) of instructions
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used
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\item Among \alert{4000} randomly sampled filed, only \alert{24}
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containing unsupported instructions
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\end{itemize}
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\end{itemize}
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\end{frame}
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@ -282,13 +303,15 @@ $1 = 84
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\begin{itemize}
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\item \alert{libunwind}: \textit{de facto} standard library for
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unwinding
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\item Uses DWARF in background
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\item Relies on DWARF
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\bigskip{}
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\item \texttt{libunwind-eh\_elf}: alternative implementation using
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\ehelfs{}
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\item{} Result: \alert{alternative implementation} of libunwind, nearly
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plug-and-play for existing projects!
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\item[$\leadsto$] \alert{alternative implementation} of libunwind,
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almost plug-and-play for existing projects!
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\begin{itemize}
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\item[$\leadsto$] It is \alert{easy} to use \ehelfs{}: just
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link against the right library!
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|
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@ -310,24 +333,20 @@ $1 = 84
|
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\item Remark: a lot of lines appear often.
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\begin{itemize}
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\item[$\leadsto$] \emph{outline} them!
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\item[$\leadsto$] \textbf{\emph{outline} them!}
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\end{itemize}
|
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\end{itemize}
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\pause{}
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\textbf{Outlining:}
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\begin{itemize}
|
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\item On libc, $20\,827$ rows $\rightarrow$ $302$ outlined ($1.5\,\%$)
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\item Turn the big switch into a binary search \alert{if/else tree}
|
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\item \alert{Extract} the conditional bodies, put them afterwards
|
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\item Jump to them using a \alert{label/goto}
|
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\end{itemize}
|
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||||
\pause{}
|
||||
|
||||
\bigskip{}
|
||||
\begin{center}
|
||||
$\leadsto$ only \textbf{2.5 times heavier than DWARF}
|
||||
$\leadsto$ only \textbf{2.5 times bigger than DWARF}
|
||||
\end{center}
|
||||
\end{frame}
|
||||
|
||||
|
|
@ -337,7 +356,7 @@ $1 = 84
|
|||
|
||||
\subsection{A word on formalization}
|
||||
|
||||
\begin{frame}{A word on formalization}
|
||||
\begin{frame}[t]{A word on formalization}
|
||||
\begin{itemize}
|
||||
\item First task: \alert{writing semantics} for DWARF, written as
|
||||
mapping to C code.
|
||||
|
|
@ -346,6 +365,12 @@ $1 = 84
|
|||
\item What remains to prove is mostly \alert{simple or classic
|
||||
optimisations}
|
||||
\end{itemize}
|
||||
|
||||
\pause{}
|
||||
\vspace{-3cm}
|
||||
\begin{center}
|
||||
\includegraphics[width=0.8\linewidth, angle=10]{img/dw_spec.png}
|
||||
\end{center}
|
||||
\end{frame}
|
||||
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
|
@ -360,19 +385,11 @@ $1 = 84
|
|||
\item Yet be fair: don't always unwind from totally different places
|
||||
\item Distribute evenly: if possible, also from within libraries
|
||||
\end{enumerate}
|
||||
|
||||
\pause{}\vspace{1em}
|
||||
|
||||
\begin{itemize}
|
||||
\item 2 $\implies$ exit hand-crafted program. CSmith did not work
|
||||
either.
|
||||
\item 5 $\implies$ cannot call the unwinding procedure by hand
|
||||
\end{itemize}
|
||||
\end{frame}
|
||||
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\begin{frame}{perf instrumentation}
|
||||
\textbf{\alert{perf} is a polling profiler.}
|
||||
\textbf{\alert{perf} is the state-of-the-art polling profiler for Linux.}
|
||||
\begin{itemize}
|
||||
\item{} used to get readings of the time spent in each function
|
||||
\item{} works by regularly stopping the program, unwinding its stack,
|
||||
|
|
@ -448,6 +465,7 @@ $1 = 84
|
|||
& 22.09 & 2.97 \\
|
||||
hackbench
|
||||
& 93.87 & 4.99 \\
|
||||
\midrule
|
||||
Total
|
||||
& 22.81 & \alert{2.44} \\
|
||||
\bottomrule
|
||||
|
|
@ -458,20 +476,25 @@ $1 = 84
|
|||
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\section*{}
|
||||
\setcounter{section}{0}
|
||||
|
||||
\begin{frame}{What next?}
|
||||
\begin{itemize}
|
||||
\item \alert{Outlining} was super efficient for
|
||||
compactness\ldots{} Worth trying on standard DWARF\@?
|
||||
|
||||
\item Implement a release-ready, packageable, easy to use version of
|
||||
perf with \ehelfs{} and submit it for inclusion
|
||||
|
||||
\item{} Measure \alert{C++ exceptions overhead} precisely in common
|
||||
software
|
||||
|
||||
\item{} Implement \alert{\ehelfs{}} support for \alert{C++ runtime}
|
||||
exception handling
|
||||
\item{} \ldots{}and many more possibilities to explore!
|
||||
exception handling, and other systems where unwinding is a
|
||||
performance bottleneck
|
||||
|
||||
\medskip
|
||||
|
||||
\item \alert{Outlining} was effective for
|
||||
compactness\ldots{} Try outlining DWARF bytecode\@?
|
||||
|
||||
\end{itemize}
|
||||
\end{frame}
|
||||
|
||||
|
|
@ -493,11 +516,14 @@ $1 = 84
|
|||
\end{columns}
|
||||
|
||||
\vspace{1.5em}
|
||||
\begin{center}
|
||||
\Huge\bfseries
|
||||
Thank you!
|
||||
\end{center}
|
||||
|
||||
\begin{center}
|
||||
\large
|
||||
\begin{align*}
|
||||
\textbf{Slides: } &\text{\url{https://tobast.fr/m2/slides.pdf}} \\
|
||||
\textbf{Report: } &\text{\url{https://tobast.fr/m2/report.pdf}}
|
||||
\end{align*}
|
||||
\end{center}
|
||||
|
||||
\end{frame}
|
||||
|
||||
|
|
|
|||
|
|
@ -1,13 +1,11 @@
|
|||
#include <stdio.h> DWARF
|
||||
DWARF
|
||||
CFA ra
|
||||
void fib7() { rsp+8 c-8
|
||||
int fibo[8]; rsp+48 c-8
|
||||
void fib7() { 0x615 rsp+8 c-8
|
||||
int fibo[8]; 0x620 rsp+48 c-8
|
||||
fibo[0] = 1;
|
||||
fibo[1] = 1;
|
||||
for(int pos = 2; pos < 8; ++pos)
|
||||
fibo[pos] =
|
||||
fibo[pos - 1]
|
||||
+ fibo[pos - 2];
|
||||
for(...)
|
||||
...
|
||||
printf("%d\n", fibo[7]);
|
||||
rsp+8 c-8
|
||||
0x659 rsp+8 c-8
|
||||
}
|
||||
|
|
|
|||
|
|
@ -3,16 +3,13 @@ unwind_context_t _eh_elf(
|
|||
{
|
||||
unwind_context_t out_ctx;
|
||||
switch(pc) {
|
||||
// [...] Previous FDEs redacted
|
||||
...
|
||||
case 0x615 ... 0x618:
|
||||
out_ctx.rsp = ctx.rsp + (8);
|
||||
out_ctx.rsp = ctx.rsp + 8;
|
||||
out_ctx.rip =
|
||||
*((uintptr_t*)(out_ctx.rsp + (-8)));
|
||||
*((uintptr_t*)(out_ctx.rsp - 8));
|
||||
out_ctx.flags = 3u;
|
||||
return out_ctx;
|
||||
// [...] Further lines and FDEs redacted
|
||||
default:
|
||||
out_ctx.flags = 128u;
|
||||
return out_ctx;
|
||||
}
|
||||
...
|
||||
}
|
||||
}
|
||||
|
|
|
|||
|
|
@ -2,15 +2,20 @@ unwind_context_t _eh_elf(
|
|||
unwind_context_t ctx, uintptr_t pc)
|
||||
{
|
||||
unwind_context_t out_ctx;
|
||||
if(pc < 0x619) { /* [...] */ } else {
|
||||
if(pc < 0x619) { ... }
|
||||
else {
|
||||
if(pc < 0x659) { // IP=0x619 ... 0x658
|
||||
goto _factor_4;
|
||||
} // [...]
|
||||
goto _factor_1;
|
||||
}
|
||||
...
|
||||
}
|
||||
|
||||
_factor_4:
|
||||
_factor_1:
|
||||
out_ctx.rsp = ctx.rsp + (48);
|
||||
out_ctx.rip = *((uintptr_t*)(out_ctx.rsp + (-8)));
|
||||
out_ctx.flags = 3u;
|
||||
|
||||
...
|
||||
|
||||
return out_ctx;
|
||||
}
|
||||
|
|
|
|||
Loading…
Reference in New Issue