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Use patchable rdtsc sequence to avoid slowdowns under rr

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We (Julia) ship both support for using tracy to trace julia applications,
as well as using `rr` (https://github.com/rr-debugger/rr) for record-replay debugging.
After our most recent rebuild of tracy, users have been reporting signfificant performance
slowdowns when `rr` recording a session that happens to also load the tracy library
(even if tracing is not enabled). Upon further examination, the recompile happened
to trigger a protective heuristic that disabled rr's patching of tracy's use of
`rdtsc` because an earlier part of the same function happened to look like a
conditional branch into the patch region. See https://github.com/rr-debugger/rr/pull/3580
for details. To avoid this issue occurring again in future rebuilds of tracy,
adjust tracy's `rdtsc` sequence to be `nopl; rdtsc`, which (as of of the
linked PR) is a sequence that is guaranteed to bypass this heuristic
and not incur the additional overhead when run under rr.

This functionality is kept behind a compile-time flag `TRACY_PATCHABLE_NOPSLEDS`
in order to avoid polluting the instruction cache unnecessarily.
This commit is contained in:
Keno Fischer 2023-08-19 01:40:18 +00:00 committed by Cody Tapscott
parent 60a3a85069
commit 5417227e83
3 changed files with 20 additions and 0 deletions
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1
CMakeLists.txt
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@ -78,6 +78,7 @@ set_option(TRACY_NO_VERIFY "Disable zone validation for C API" OFF)
set_option(TRACY_NO_VSYNC_CAPTURE "Disable capture of hardware Vsync events" OFF)
set_option(TRACY_NO_FRAME_IMAGE "Disable the frame image support and its thread" OFF)
set_option(TRACY_NO_SYSTEM_TRACING "Disable systrace sampling" OFF)
set_option(TRACY_PATCHABLE_NOPSLEDS "Enable nopsleds for efficient patching by system-level tools (e.g. rr)" OFF)
set_option(TRACY_DELAYED_INIT "Enable delayed initialization of the library (init on first call)" OFF)
set_option(TRACY_MANUAL_LIFETIME "Enable the manual lifetime management of the profile" OFF)
set_option(TRACY_FIBERS "Enable fibers support" OFF)

4
meson.build
View File

@ -64,6 +64,10 @@ if get_option('tracy_no_system_tracing')
add_project_arguments('-DTRACY_NO_SYSTEM_TRACING', language : 'cpp')
endif
if get_option('tracy_no_extra_nopsleds')
add_project_arguments('-DTRACY_PATCHABLE_NOPSLEDS', language : 'cpp')
endif
if get_option('tracy_delayed_init')
add_project_arguments('-DTRACY_DELAYED_INIT', language : 'cpp')
endif

15
public/client/TracyProfiler.hpp
View File

@ -209,7 +209,22 @@ public:
if( HardwareSupportsInvariantTSC() )
{
uint64_t rax, rdx;
#ifdef TRACY_PATCHABLE_NOPSLEDS
// Some external tooling (such as rr) wants to patch our rdtsc and replace it by a
// branch to control the external input seen by a program. This kind of patching is
// not generally possible depending on the surrounding code and can lead to significant
// slowdowns if the compiler generated unlucky code and rr and tracy are used together.
// To avoid this, use the rr-safe `nopl 0(%rax, %rax, 1); rdtsc` instruction sequence,
// which rr promises will be patchable independent of the surrounding code.
asm volatile (
// This is nopl 0(%rax, %rax, 1), but assemblers are inconsistent about whether
// they emit that as a 4 or 5 byte sequence and we need to be guaranteed to use
// the 5 byte one.
".byte 0x0f, 0x1f, 0x44, 0x00, 0x00\n\t"
"rdtsc" : "=a" (rax), "=d" (rdx) );
#else
asm volatile ( "rdtsc" : "=a" (rax), "=d" (rdx) );
#endif
return (int64_t)(( rdx << 32 ) + rax);
}
# else