m2-internship/talk-2018-11-uppsala
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Théophile Bastian 2018-11-08 19:37:04 +01:00
parent e74efb43d5
commit b6197a7d7c
7 changed files with 150 additions and 46 deletions
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7
Makefile
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@ -1,5 +1,10 @@
all:
all: build
build:
latexmk -xelatex -pdf slides.tex
clean:
rm -f *aux *bbl *bcf *blg *_latexmk *fls *log *out *.run.xml
upload: build
scp slides.pdf www.tobast:~/files/sweden18.pdf

157
slides.tex
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@ -42,7 +42,7 @@
\newcommand{\cmark}{\color{OliveGreen}\ding{52}}
\newcommand{\xmark}{\color{BrickRed}\ding{56}}
\AtBeginSection[]{
\AtBeginSection{
\begin{frame}
\vfill
\centering
@ -86,10 +86,8 @@
\vspace{-2em}
\begin{center}
\todo{}
\begin{align*}
\text{Slides: } &\text{\url{https://tobast.fr/m2/slides.pdf}} \\
\text{Report: } &\text{\url{https://tobast.fr/m2/report.pdf}}
\text{Slides: } &\text{\url{https://tobast.fr/files/sweden18.pdf}} \\
\end{align*}
\end{center}
\end{frame}
@ -292,25 +290,28 @@ $1 = 84
\end{center}
\end{frame}
\begin{frame}{A debugging hell: Linux kernel}
\newcommand{\LinusMailOne}{
``Sorry, but last time was too f\dots painful. The whole (and
only) point of unwinders is to make debugging easy
when a bug occurs. But \alert{the dwarf unwinder had bugs}
itself, or \alert{our dwarf information had bugs}, and in either
case it actually turned several trivial bugs into a \alert{total
undebuggable hell}.''
\vspace{1em}
\only<1>{\hfill ---~Linus Torvalds, Kernel mailing list, 2012}
\pause{}
}
\newcommand{\LinusMailTwo}{
``If you can \alert{mathematically prove that the unwinder is
correct} — even in the presence of bogus and actively
incorrect unwinding information — and never ever
follows a bad pointer, \alert{I’ll reconsider}.''
}
\newcommand{\LinusSource}{
\hfill ---~Linus Torvalds, Kernel mailing list, 2012
}
\begin{frame}{A debugging hell: Linux kernel}
\LinusMailOne{}
\vspace{1em}
\hfill ---~Linus Torvalds, Kernel mailing list, 2012
\LinusSource{}
\pause{}\vspace{1em}
\begin{center}
@ -552,9 +553,92 @@ $1 = 84
\begin{itemize}
\item[$\leadsto$] only \cfa{} tracking matters (for unwinding)
\end{itemize}
\item We had a working strategy for a \alert{linear execution}
\item We still have to handle
\begin{itemize}
\item \alert{\cfa{} expression}
\item \alert{control flow graph}
\end{itemize}
\end{itemize}
\end{frame}
\begin{frame}{\cfa{} expression}
Two possibilities:
\begin{itemize}
\item Either \reg{rbp} is used as base pointer \pause{}
\item Or we must track \cfa{} wrt. \reg{rsp}
\begin{itemize}
\item And update it after each instruction if needed
\end{itemize}
\end{itemize}
\end{frame}
\begin{frame}{Control flow graph}
\begin{columns}[c]
\column{0.4\textwidth}
\lstinputlisting[language=C]{src/cfg/cfg.c}
\pause{}
\column{0.30\textwidth}
\begin{figure}
\centering
\includegraphics[width=\textwidth]{src/cfg/cfg.png}
\end{figure}
\end{columns}
\pause{}
\begin{itemize}
\item \alert{Upon split} (eg. \texttt{X})\alert{:} nothing special,
propagate end state of X to children nodes A and B
\item \alert{Upon join} (eg.\texttt{while\_end})\alert{:} check
consistency of both input states
\begin{itemize}
\item If tricky, \texttt{gcc} will have used \reg{rbp}, even
with \texttt{-fomit-frame-pointer}.
\end{itemize}
\end{itemize}
\end{frame}
\renewcommand{\tblrowval}[4]{#1 & #2 & #3 & #4 \\}
\begin{frame}{Trust the compiler to avoid tricky unwinding}
\begin{columns}[c]
\column{0.55\textwidth}
\lstinputlisting[language=C,firstline=3,lastline=7]
{src/use_rbp/use_rbp.c}
\pause{}
\begin{itemize}
\item At each loop cycle, \alert{\texttt{y} is larger} and
\alert{allocated on the stack}
\item Thus, \reg{rsp} is \alert{farther from \cfa} at each cycle: no
constant rule $\cfa = \reg{rsp} + k$.
\item A complex DWARF expression is possible, but
\alert{the compiler won't}.
\end{itemize}
\pause{}
\column{0.45\textwidth}
\lstbash{\$ gcc -O0 -g -c src.c -fomit-frame-pointer}
\vspace{1em}
\begin{center}
\tt
\begin{tabular}{l l l l}
\tblrowval{LOC} {CFA} {rbp} {ra}
\tblrowval{000} {rsp+8} {u} {c-8}
\tblrowval{001} {rsp+16} {c-16} {c-8}
\tblrowval{004} {\alert{rbp}+16} {c-16} {c-8}
\tblrowval{010} {\alert{rbp}+16} {c-16} {c-8}
\tblrowval{0ce} {rsp+8} {c-16} {c-8}
\end{tabular}
\end{center}
\end{columns}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Unwinding data compilation}
@ -664,23 +748,6 @@ $1 = 84
\end{frame}
\begin{frame}[t]{A word on formalization}
\begin{itemize}
\item First task: \alert{writing semantics} for DWARF, written as
mapping to C code.
\item DWARF5 specification: \alert{plain English}, no proper semantics
\item Compiled code is in substance equivalent to semantics
\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}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Benchmarking}
@ -697,7 +764,7 @@ $1 = 84
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}{perf instrumentation}
\textbf{\alert{perf} is the state-of-the-art polling profiler for Linux.}
\textbf{\alert{perf} is a 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,
@ -782,27 +849,28 @@ $1 = 84
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section*{}
\section*{Conclusion}
\setcounter{section}{0}
\begin{frame}{What next?}
\begin{itemize}
\item Implement a release-ready, packageable, easy to use version of
perf with \ehelfs{} and submit it for inclusion
\begin{frame}{}
\LinusMailOne{}
\item{} Measure \alert{C++ exceptions overhead} precisely in common
software
\only<1>{\LinusSource{}}
\only<2->\vspace{1em}
\pause{}
\item{} Implement \alert{\ehelfs{}} support for \alert{C++ runtime}
exception handling, and other systems where unwinding is a
performance bottleneck
\LinusMailTwo{}
\medskip
\LinusSource{}
\item \alert{Outlining} was effective for
compactness\ldots{} Try outlining DWARF bytecode\@?
\pause{}
\vspace{1em}
\begin{center}
\bf
Give us a few months to make Linus reconsider \texttt{;)}
\end{center}
\end{itemize}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
@ -814,8 +882,7 @@ $1 = 84
\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}}
\textbf{Slides: } &\text{\url{https://tobast.fr/files/sweden18.pdf}} \\
\end{align*}
\end{center}

8
src/cfg/cfg.c Normal file
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while(/* ... */) {
X;
if(/* ... */) {
A;
} else {
B;
}
}

8
src/cfg/cfg.dot Normal file
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digraph {
while_beg -> X;
X -> A;
X -> B;
A -> while_end;
B -> while_end;
while_end -> while_beg;
}

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8
src/use_rbp/use_rbp.c Normal file
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#include <stdlib.h>
int main() {
int z = rand();
for(int x=1; x < z; ++x) {
int y[x]; // Variable size
/* do something */
}
}

8
src/use_rbp/use_rbp.fde Normal file
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@ -0,0 +1,8 @@
\begin{tabular}{l l l l}
\tblrowval{LOC} {CFA} {rbp} {ra}
\tblrowval{000} {rsp+8} {u} {c-8}
\tblrowval{001} {rsp+16} {c-16} {c-8}
\tblrowval{004} {rbp+16} {c-16} {c-8}
\tblrowval{010} {rbp+16} {c-16} {c-8}
\tblrowval{0ce} {rsp+8} {c-16} {c-8}
\end{tabular}