"constand 2nd frame" feature is supposed to detect and workaround
incorrect cpu profile stack captures where parts of or whole cpu
profiling signal handler frames are not skipped.
I've seen programs where this feature incorrectly removes non-signal
frames.
Plus it actually hides bugs in stacktrace capturing which we want be
able to spot.
There is now --no-auto-signal-frm option for disabling it.
In cpu profiles that had parts of signal handler we could have
situation like that:
* PC
* signal handler frame
* PC
Specifically when capturing stacktraces via libunwind.
For such stacktraces pprof used to draw self-cycle in functions
confusing everybody. Given that me might have a number of such
profiles in the wild it makes sense to treat that duplicate PC issue.
Added two new configure flags, --with-tcmalloc-pagesize and
--with-tcmalloc-alignment, in order to set the tcmalloc internal page
size and tcmalloc allocation alignment without the need of a compiler
directive and to make the choice of the page size independent of the
allocation alignment.
Looks like even force_malloc trick was not enough to force clang to
actually call malloc. I'm now calling tc_malloc directly to prevent
that smartness.
It's not cheap at all when done in this way (i.e. without runtime
patching) and apparently useless.
It looks like Linux kernel never got this workaround at all. See
bugzilla ticket: https://bugzilla.kernel.org/show_bug.cgi?id=11305
And I see no traces of this workaround in glibc either.
On the other hand, opensolaris folks apparently still have it (or
something similar, based on comments on linux bugzilla) in their code:
32842aabdc/usr/src/uts/i86pc/os/mp_startup.c (L1136)
And affected CPUs (if any) are from year 2008 (that's 6 years now).
Plus even if somebody still uses those cpus (which is unlikely), they
won't have working kernel and glibc anyways.
Default mode of operation of cpu profiler uses itimer and
SIGPROF. This timer is by definition per-process and no spec defines
which thread is going to receive SIGPROF. And it provides correct
profiles only if we assume that probability of picking threads will be
proportional to cpu time spent by threads.
It is easy to see, that recent Linux (at least on common SMP hardware)
doesn't satisfy that assumption. Quite big skews of SIGPROF ticks
between threads is visible. I.e. I could see as big as 70%/20%
division instead of 50%/50% for pair of cpu-hog threads. (And I do see
it become 50/50 with new mode)
Fortunately POSIX provides mechanism to track per-thread cpu time via
posix timers facility. And even more fortunately, Linux also provides
mechanism to deliver timer ticks to specific threads.
Interestingly, it looks like FreeBSD also has very similar facility
and seems to suffer from same skew. But due to difference in a way
how threads are identified, I haven't bothered to try to support this
mode on FreeBSD.
This commit implements new profiling mode where every thread creates
posix timer which tracks thread's cpu time. Threads also also set up
signal delivery to itself on overflows of that timer.
This new mode requires every thread to be registered in cpu
profiler. Existing ProfilerRegisterThread function is used for that.
Because registering threads requires application support (or suitable
LD_PRELOAD-able wrapper for thread creation API), new mode is off by
default. And it has to be manually activated by setting environment
variable CPUPROFILE_PER_THREAD_TIMERS.
New mode also requires librt symbols to be available. Which we do not
link to due to librt's dependency on libpthread. Which we avoid due
to perf impact of bringing in libpthread to otherwise single-threaded
programs. So it has to be either already loaded by profiling program
or LD_PRELOAD-ed.
