report/slides/slides.tex

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\newcommand{\thenalert}[1]{\only<1>{#1}\only<2>{\alert{#1}}}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\author[Théophile Bastian]{Théophile \textsc{Bastian} \\
    \small{Under supervision of Francesco Zappa Nardelli}}
\title{Internship defense, MPRI, M2}
\subtitle{Speeding up stack unwinding by compiling DWARF debugging data}
\date{March\ --\ August 2018}
%\subject{}
%\logo{}
\institute{Team PARKAS, INRIA, Paris}

\begin{document}

\begin{frame}
    \addtocounter{framenumber}{-1}
	\titlepage{}

    \begin{center}
        Slides: \url{https://tobast.fr/m2/slides.pdf} \\
        Report: \url{https://tobast.fr/m2/report.pdf}
    \end{center}
\end{frame}

\begin{frame}
    \addtocounter{framenumber}{-1}
    \tableofcontents
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Stack unwinding data}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Introduction}

\begin{frame}[fragile]{We often use stack unwinding!}
    \begin{lstlisting}[language=, numbers=none, escapechar=|]
Program received signal SIGSEGV, Segmentation fault.
0x0000555555554625 in fct_b (m=0x5c) at segfault.c:5
5           printf("%l\n", *m);
|\pause|
(gdb) backtrace
#0  0x0000555555554625 in fct_b (m=0x5c) at segfault.c:5
#1  0x0000555555554663 in fct_a (n=42) at segfault.c:10
#2  0x0000555555554674 in main () at segfault.c:14
|\pause|
(gdb) frame 1
#1  0x0000555555554663 in fct_a (n=42) at segfault.c:10
10          fct_b((int*)(some_fct_a_var + 8));
|\pause|
(gdb) print some_fct_a_var
$1 = 84
    \end{lstlisting}

    \pause{}
    \begin{center}
        \textbf{\Large How does it work?!}
    \end{center}
    \vspace{1em}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Stack frames and unwinding}

\begin{frame}{Call stack and registers}
    \begin{columns}[c]
        \begin{column}{0.65\textwidth}
            \begin{itemize}
                \item Programs use a \alert{call stack}
                \item Organized in \alert{stack frames}
                    \begin{itemize}
                        \item Local variables
                        \item Function parameters
                        \item Keep track of nesting, registers and ``return
                            point''
                    \end{itemize}
            \end{itemize}

            Common registers:
            \begin{itemize}
                \item \reg{rip}: program counter (PC)

                \item \reg{rsp}: stack pointer

                \item \reg{rbp}: base pointer
                    \begin{itemize}
                        \item Saves \reg{rsp}
                        \item Easy access
                        \item Wastes a register
                        \item Not often used (x86\_64)
                    \end{itemize}
            \end{itemize}
        \end{column}
        \begin{column}{0.35\textwidth}
            \includegraphics[width=0.95\linewidth]{../shared/imgs/call_stack}
        \end{column}
    \end{columns}
\end{frame}

\begin{frame}{Isn't it as trivial as \texttt{pop()}?}
    \begin{itemize}
        \item This is only a \alert{blob of binary data} without mandatory
            structure
        \item We ignore \alert{which registers were saved}
        \item We ignore \alert{whether \reg{rbp} was used}
        \item We ignore \alert{where the return address is stored}
        \item We ignore \alert{where the previous frame begins}
    \end{itemize}

    \medskip

    But\ldots{} if we know how to \alert{unwind one}, we can \alert{recurse}!
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{DWARF tables}

\begin{frame}[fragile]{DWARF unwinding data}

    \begin{lstlisting}[numbers=none, language=]
00009b30 48 009b34 FDE cie=0000 pc=0084950..0084b37
 LOC    CFA      rbx   rbp   r12   r13   r14   r15   ra
0084950 rsp+8    u     u     u     u     u     u     c-8
0084952 rsp+16   u     u     u     u     u     c-16  c-8
0084954 rsp+24   u     u     u     u     c-24  c-16  c-8
0084956 rsp+32   u     u     u     c-32  c-24  c-16  c-8
0084958 rsp+40   u     u     c-40  c-32  c-24  c-16  c-8
0084959 rsp+48   u     c-48  c-40  c-32  c-24  c-16  c-8
008495a rsp+56   c-56  c-48  c-40  c-32  c-24  c-16  c-8
0084962 rsp+64   c-56  c-48  c-40  c-32  c-24  c-16  c-8
0084a19 rsp+56   c-56  c-48  c-40  c-32  c-24  c-16  c-8
0084a1d rsp+48   c-56  c-48  c-40  c-32  c-24  c-16  c-8
0084a1e rsp+40   c-56  c-48  c-40  c-32  c-24  c-16  c-8
0084a20 rsp+32   c-56  c-48  c-40  c-32  c-24  c-16  c-8
0084a22 rsp+24   c-56  c-48  c-40  c-32  c-24  c-16  c-8
0084a24 rsp+16   c-56  c-48  c-40  c-32  c-24  c-16  c-8
0084a26 rsp+8    c-56  c-48  c-40  c-32  c-24  c-16  c-8
0084a30 rsp+64   c-56  c-48  c-40  c-32  c-24  c-16  c-8
    \end{lstlisting}

    \pause{}

    \vspace{-4cm}
    \hfill\includegraphics[height=3cm, angle=45, origin=c]{img/dwarf_logo}
    \hspace{-1cm}

\end{frame}

\begin{frame}[fragile]{The real DWARF}
    \begin{lstlisting}[numbers=none, language=]
00009b30 48 009b34 FDE cie=0000 pc=0084950..0084b37
  DW_CFA_advance_loc: 2 to 0000000000084952
  DW_CFA_def_cfa_offset: 16
  DW_CFA_offset: r15 (r15) at cfa-16
  DW_CFA_advance_loc: 2 to 0000000000084954
  DW_CFA_def_cfa_offset: 24
  DW_CFA_offset: r14 (r14) at cfa-24
  DW_CFA_advance_loc: 2 to 0000000000084956
  DW_CFA_def_cfa_offset: 32
  DW_CFA_offset: r13 (r13) at cfa-32
  DW_CFA_advance_loc: 2 to 0000000000084958
  DW_CFA_def_cfa_offset: 40
  DW_CFA_offset: r12 (r12) at cfa-40
  DW_CFA_advance_loc: 1 to 0000000000084959
  DW_CFA_def_cfa_offset: 48
  DW_CFA_offset: r6 (rbp) at cfa-48
  DW_CFA_advance_loc: 1 to 000000000008495a
  [...]
    \end{lstlisting}
\end{frame}

\begin{frame}{Why does slow matter?}
    \textbf{Do we really care about speed for unwinding?}
    \begin{itemize}

        \item{} After all, we're talking about \alert{debugging procedures} ran
            by a \alert{human being} (slower than the machine).

            \ldots{}or are we?

        \pause{}\item{} \alert{Profiling} with polling profilers

        \pause{}\item{} \alert{Exception handling} in C++

    \end{itemize}

    \vspace{2em}

    \begin{center}
        \textbf{Debug data is not only for debugging}
    \end{center}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Compiling DWARF}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Compilation Strategy}

\begin{frame}[fragile]{Types}
    Generated data:
    \lstinputlisting[language=C]{src/unwind_context.c}
    \pause{}

    \vspace{1em}
    Function type:
    \begin{lstlisting}[language=C]
unwind_context_t _eh_elf(
    unwind_context_t, instruction_pointer_t); \end{lstlisting}
\end{frame}

\begin{frame}{Compilation overview}
    \begin{itemize}
        \item Compiled to \alert{C code}
        \item C code then \alert{compiled to native binary} (gcc)
            \begin{itemize}
                \item[$\leadsto$] gcc optimisations for free
            \end{itemize}
        \item Compiled as \alert{separate \texttt{.so} files}, called \ehelfs{}
            \bigskip{}
        \item Morally a \alert{monolithic switch} on IPs
        \item Each case fills the context structure
    \end{itemize}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Outlining}

\begin{frame}{Size optimisation: outlining}
    \begin{itemize}
        \item This \alert{works}, but \alert{takes space}: about \alert{7 times
            heavier} than regular DWARF\@.

        \item DWARF optimisation strategy: \alert{alter previous row}. \\
            Causes slowness: we cannot do that.

        \item Remark: a lot of lines appear often.
            \begin{itemize}
                \item[$\leadsto$] \emph{outline} them!
            \end{itemize}
    \end{itemize}

    \pause{}

    \textbf{Outlining:}
    \begin{itemize}
        \item Turn the big switch into a binary search \alert{if/else tree}
        \item \alert{Extract} the conditional bodies, put them afterwards
        \item Jump to them using a \alert{label/goto}
    \end{itemize}

    \pause{}

    \bigskip{}
    \begin{center}
        $\leadsto$ only \textbf{2.5 times heavier than DWARF}
    \end{center}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Benchmarking}

\begin{frame}{Benchmarking requirements}
    \begin{enumerate}
        \item Thousands of samples (single unwind: $10\,\mu{}s$)
        \item Interesting enough program to unwind: nested functions, complex
            FDEs
        \item Mitigate caching: don't always unwind from the \emph{same} point
        \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, exit CSmith
        \item 5 $\implies$ cannot call the unwinding procedure by hand
    \end{itemize}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Unwinding using perf}

\begin{frame}{Presentation of perf}
    \textbf{A profiler is used to\ldots}
    \begin{itemize}
        \item get readings of the \alert{time spent in each function}
        \item detect ``hot paths'': functions you ought to optimize
        \item \ldots{}and \alert{benchmark \ehelfs{}}!
    \end{itemize}

    \vspace{1em}\pause{}
    \textbf{How does it work?}
    \begin{itemize}
        \item{} \alert{Polling profiler}: stops at regular intervals to perform
            analyses
        \item{} Upon polling, \alert{dumps the stack} to a file
        \item{} In the analysis phase (after the program terminated),
            \alert{unwinds all the stacks gathered} to get call paths
    \end{itemize}
\end{frame}

\begin{frame}{perf instrumentation}
    \textbf{Instrumenting perf matches all the requirements!}
    \vspace{1em}\pause{}

    \begin{itemize}
        \item{} \alert{Plug \ehelfs{} into perf}: use \ehelfs{} instead of
            DWARF to analyze stack dumps
        \item{} Implement \alert{unwinding performance counters} inside perf
            \bigskip{}

        \item{} Use perf on \alert{hackbench}, a kernel stress-test program
            \begin{itemize}
                \item Small program
                \item Lots of calls
                \item Relies on libc, libpthread
            \end{itemize}
    \end{itemize}
\end{frame}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{A glance at future work}

\end{document}