570 lines
18 KiB
TeX
570 lines
18 KiB
TeX
% vim: spell spelllang=en
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\documentclass[11pt,xcolor={usenames,dvipsnames}]{beamer}
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\usetheme{Warsaw}
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\usepackage[utf8]{inputenc}
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\usepackage[english]{babel}
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\usepackage[T1]{fontenc}
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\usepackage{amsmath}
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\usepackage{amsfonts}
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\usepackage{amssymb}
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\usepackage{booktabs}
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\usepackage{makecell}
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\usepackage{../shared/my_listings}
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%\usepackage{../shared/my_hyperref}
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\usepackage{../shared/specific}
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\usepackage{../shared/common}
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\usepackage{../shared/todo}
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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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\newcommand{\thenalert}[1]{\only<1>{#1}\only<2>{\alert{#1}}}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\author[Théophile Bastian]{Théophile \textsc{Bastian} \\
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\small{Under supervision of Francesco Zappa Nardelli}}
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\title{Internship defense, MPRI, M2}
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\subtitle{Speeding up stack unwinding by compiling DWARF debugging data}
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\date{March\ --\ August 2018}
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%\subject{}
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%\logo{}
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\institute{Team PARKAS, INRIA, Paris}
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\begin{document}
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\begin{frame}
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\addtocounter{framenumber}{-1}
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\titlepage{}
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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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\end{center}
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\end{frame}
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\begin{frame}
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\addtocounter{framenumber}{-1}
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\tableofcontents
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Stack unwinding data}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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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=, 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|
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(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|
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(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|
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(gdb) print some_fct_a_var
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$1 = 84
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\end{lstlisting}
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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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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Stack frames and unwinding}
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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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Common registers:
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\begin{itemize}
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\item \reg{rip}: program counter (PC)
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\item \reg{rsp}: stack pointer
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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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\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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\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]{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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\pause{}
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\vspace{-4cm}
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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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\begin{lstlisting}[numbers=none, language=]
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00009b30 48 009b34 FDE cie=0000 pc=0084950..0084b37
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DW_CFA_advance_loc: 2 to 0000000000084952
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DW_CFA_def_cfa_offset: 16
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DW_CFA_offset: r15 (r15) at cfa-16
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DW_CFA_advance_loc: 2 to 0000000000084954
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DW_CFA_def_cfa_offset: 24
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DW_CFA_offset: r14 (r14) at cfa-24
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DW_CFA_advance_loc: 2 to 0000000000084956
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DW_CFA_def_cfa_offset: 32
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DW_CFA_offset: r13 (r13) at cfa-32
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DW_CFA_advance_loc: 2 to 0000000000084958
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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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\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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\pause{}\item{} \alert{Profiling} with polling profilers
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\pause{}\item{} \alert{Exception handling} in C++
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\end{itemize}
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\vspace{2em}
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\begin{center}
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\textbf{Debug data is not only for debugging}
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\end{center}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Compiling DWARF}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Compilation Strategy}
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\begin{frame}[fragile]{Types}
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Generated data:
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\lstinputlisting[language=C]{src/unwind_context.c}
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\pause{}
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\vspace{1em}
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Function type:
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\begin{lstlisting}[language=C]
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unwind_context_t _eh_elf(
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unwind_context_t, instruction_pointer_t); \end{lstlisting}
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\end{frame}
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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$] gcc optimisations for free
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\end{itemize}
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\item Compiled as \alert{separate \texttt{.so} files}, called \ehelfs{}
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\bigskip{}
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\item Morally a \alert{monolithic switch} on IPs
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\item Each case fills the context structure
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\end{itemize}
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\end{frame}
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\begin{frame}{Compilation choices}
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\textbf{In order to keep the compiler \alert{simple} and \alert{easily
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testable}, the whole DWARF5 instruction set is not supported.}
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\begin{itemize}
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\item Tailored for \alert{x86\_64} (while DWARF is
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architecture-agnostic)
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\item Only supports \alert{unwinding registers}: \reg{rip}, \reg{rsp},
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\reg{rbp}, \reg{rbx}
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\begin{itemize}
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\item[$\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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\end{itemize}
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\end{frame}
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\begin{frame}[fragile]{A word on memory maps}
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\textbf{Various functions come from various ELFs}\\
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\pause{}
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\hfill{}\textbf{\ldots{}and thus various \ehelfs{}}
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\begin{lstlisting}[language=, numbers=none]
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55c81b11e000-55c81b126000 000k /usr/bin/cat
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55c81b325000-55c81b326000 007k /usr/bin/cat
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55c81b326000-55c81b327000 008k /usr/bin/cat
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55c81bcf3000-55c81bd14000 000k [heap]
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7f8a5b4ed000-7f8a5b50f000 000k /usr/lib/libc-2.28.so
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7f8a5b50f000-7f8a5b65a000 022k /usr/lib/libc-2.28.so
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7f8a5b65a000-7f8a5b6a6000 16dk /usr/lib/libc-2.28.so
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7f8a5b6ec000-7f8a5b6ee000 000k /usr/lib/ld-2.28.so
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7f8a5b6ee000-7f8a5b70d000 002k /usr/lib/ld-2.28.so
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7f8a5b70d000-7f8a5b715000 021k /usr/lib/ld-2.28.so
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7ffc8a66b000-7ffc8a68c000 000k [stack]
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7ffc8a74a000-7ffc8a74d000 000k [vvar]
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7ffc8a74d000-7ffc8a74f000 000k [vdso]
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\end{lstlisting}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Outlining}
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\begin{frame}{Size optimisation: outlining}
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\begin{itemize}
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\item This \alert{works}, but \alert{takes space}: about \alert{7 times
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heavier} than regular DWARF\@.
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\item DWARF optimisation strategy: \alert{alter previous row}. \\
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Causes slowness: we cannot do that.
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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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\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 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{}
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\bigskip{}
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\begin{center}
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$\leadsto$ only \textbf{2.5 times heavier than DWARF}
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\end{center}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Benchmarking}
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\begin{frame}{Benchmarking requirements}
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\begin{enumerate}
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\item Thousands of samples (single unwind: $10\,\mu{}s$)
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\item Interesting enough program to unwind: nested functions, complex
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FDEs
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\item Mitigate caching: don't always unwind from the \emph{same} point
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\item Yet be fair: don't always unwind from totally different places
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\item Distribute evenly: if possible, also from within libraries
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\end{enumerate}
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\pause{}\vspace{1em}
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\begin{itemize}
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\item 2 $\implies$ exit hand-crafted program, exit CSmith
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\item 5 $\implies$ cannot call the unwinding procedure by hand
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\end{itemize}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{Unwinding using perf}
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\begin{frame}{Presentation of perf}
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\textbf{A profiler is used to\ldots}
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\begin{itemize}
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\item get readings of the \alert{time spent in each function}
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\item detect ``hot paths'': functions you ought to optimize
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\item \ldots{}and \alert{benchmark \ehelfs{}}!
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\end{itemize}
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\vspace{1em}\pause{}
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\textbf{How does it work?}
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\begin{itemize}
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\item{} \alert{Polling profiler}: stops at regular intervals to perform
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analyses
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\item{} Upon polling, \alert{dumps the stack} to a file
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\item{} In the analysis phase (after the program terminated),
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\alert{unwinds all the stacks gathered} to get call paths
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\end{itemize}
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\end{frame}
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\begin{frame}{perf instrumentation}
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\textbf{Instrumenting perf matches all the requirements!}
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\vspace{1em}\pause{}
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\begin{itemize}
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\item{} \alert{Plug \ehelfs{} into perf}: use \ehelfs{} instead of
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DWARF to analyze stack dumps
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\item{} Implement \alert{unwinding performance counters} inside perf
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\bigskip{}
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\item{} Use perf on \alert{hackbench}, a kernel stress-test program
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\begin{itemize}
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\item Small program
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\item Lots of calls
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\item Relies on libc, libpthread
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\end{itemize}
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\end{itemize}
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\end{frame}
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\begin{frame}{libunwind implementation}
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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 \alert{Used by perf} as a backend for unwinding
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\pause{}\vspace{1em} \item{} Easiest way to use \ehelfs{} in perf:
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\alert{implement an alternative libunwind}
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\item{} Result: \alert{alternative implementation} of libunwind, nearly
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plug-and-play!
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\end{itemize}
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\end{frame}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Results}
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\begin{frame}{Time performance}
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\small
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\centering
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\begin{tabular}{l r r r r r}
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\toprule
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\thead{Unwinding method} & \thead{Frames \\ unwound}
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& \thead{Tot.\ time \\ ($\mu s$)}
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& \thead{Avg. \\ time / frame \\ ($ns$)}
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& \thead{Time \\ ratio} \\
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\midrule
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\alert{\ehelfs{}}
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& 23506 % Frames unwound
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& 14837 % Total time
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& 631 % Avg time
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& 1
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\\
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\prog{libunwind}, \alert{cached}
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& 27058 % Frames unwound
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& 441601 % Total time
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& 16320 % Avg time
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& \alert{25.9}
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\\
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\prog{libunwind}, \alert{uncached}
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& 27058 % Frames unwound
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& 671292 % Total time
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& 24809 % Avg time
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& \alert{39.3}
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\\
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\bottomrule
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\end{tabular}
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\end{frame}
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\begin{frame}{Space performance}
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\begin{center}
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\begin{tabular}{r r r r r r}
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\toprule
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\thead{Object}
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& \thead{Original \\ program size \\ (KiB)}
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& \thead{Original \\ \lstc{.eh\_frame} \\ (KiB)}
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& \thead{Generated \\ \ehelf{} \\ (KiB)}
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& \thead{\% \\ original \\ size}
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& \thead{Growth \\ factor} \\
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\midrule
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libc
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& 1\,434 & 130.1 & 313.2 & 21.88 & 2.41 \\
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libpthread
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& 58.1 & 11.6 & 25.4 & 43.71 & 2.19 \\
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ld
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& 129.6 & 9.6 & 28.6 & 22.09 & 2.97 \\
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hackbench
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& 2.9 & 0.555 & 2.8 & 93.87 & 4.99 \\
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Total
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& 1\,638 & 151.8 & 370.0 & 22.81 & \alert{2.44} \\
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\bottomrule
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\end{tabular}
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\end{center}
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\end{frame}
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\newcommand{\ofsupp}[1]{$\left[\text{#1}\right]$}
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\begin{frame}{Instructions coverage}
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\scriptsize\centering
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\begin{tabular}{r r r r r r}
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\toprule
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\thead{}
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& \thead{Undef}
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& \thead{Same \\ value}
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& \thead{Offset}
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& \thead{Val \\ offset}
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& \thead{Reg}
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\\
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\midrule
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\makecell{Only supp. \\ columns} \vspace{.4em}
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& 1698 (0.006\,\%)
|
|
& 0
|
|
& 30\,M (99.9\,\%)
|
|
& 0
|
|
& 14 (0\,\%)
|
|
\\
|
|
All columns
|
|
& 1698 (0.003\,\%)
|
|
& 0
|
|
& 55\,M (99.9\,\%)
|
|
& 0
|
|
& 22 (0\,\%)
|
|
\\
|
|
\bottomrule
|
|
\toprule
|
|
\thead{}
|
|
& \thead{Expr \\ \ofsupp{supported}}
|
|
& \thead{Val \\ expr}
|
|
& \thead{Archi.}
|
|
& & \thead{Total}
|
|
\\
|
|
\midrule
|
|
\makecell{Only supp. \\ columns} \vspace{.4em}
|
|
& \makecell{4475 (0.015\,\%) \\ \ofsupp{81.4\,\%}}
|
|
& 0
|
|
& 0
|
|
& & 30044442
|
|
\\
|
|
All columns
|
|
& \makecell{12367 (0.02\,\%) \\ \ofsupp{91.7\,\%}}
|
|
& 0
|
|
& 0
|
|
& & 54680492
|
|
\\
|
|
|
|
\bottomrule
|
|
\end{tabular}
|
|
|
|
\vspace{2em}\pause{}
|
|
|
|
\textbf{Among \alert{4000} randomly sampled filed, only \alert{24}
|
|
containing unsupported instructions}
|
|
\end{frame}
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{A glance at future work}
|
|
|
|
\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!
|
|
\end{itemize}
|
|
\end{frame}
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section*{}
|
|
|
|
\begin{frame}
|
|
\begin{columns}[c]
|
|
\begin{column}{0.35\textwidth}
|
|
\includegraphics[width=\linewidth]{img/keep_breathing}
|
|
\end{column}
|
|
\begin{column}{0.65\textwidth}
|
|
\begin{center}
|
|
\huge
|
|
And remember\ldots{}
|
|
\smallskip
|
|
|
|
DWARF is slow!
|
|
\end{center}
|
|
\end{column}
|
|
\end{columns}
|
|
|
|
\begin{center}
|
|
\Huge\bfseries
|
|
Thank you!
|
|
\end{center}
|
|
|
|
|
|
\end{frame}
|
|
|
|
\end{document}
|