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e803cf97a4
When perf creates a new child to profile, the events are enabled on
exec(). And in this case, it doesn't synthesize any event for the
child since they'll be generated during exec(). But there's an window
between the enabling and the event generation.
It used to be overcome since samples are only in kernel (so we always
have the map) and the comm is overridden by a later COMM event.
However it won't work if events are processed and displayed before the
COMM event overrides like in 'perf script'. This leads to those early
samples (like native_write_msr_safe) not having a comm but pid (like
':15328').
So it needs to synthesize COMM event for the child explicitly before
enabling so that it can have a correct comm. But at this time, the
comm will be "perf" since it's not exec-ed yet.
Committer note:
Before this patch:
# perf record usleep 1
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.017 MB perf.data (7 samples) ]
# perf script --show-task-events
:4429 4429 27909.079372: 1 cycles: ffffffff8105f45a native_write_msr_safe (/lib/modules/4.
:4429 4429 27909.079375: 1 cycles: ffffffff8105f45a native_write_msr_safe (/lib/modules/4.
:4429 4429 27909.079376: 10 cycles: ffffffff8105f45a native_write_msr_safe (/lib/modules/4.
:4429 4429 27909.079377: 223 cycles: ffffffff8105f45a native_write_msr_safe (/lib/modules/4.
:4429 4429 27909.079378: 6571 cycles: ffffffff8105f45a native_write_msr_safe (/lib/modules/4.
usleep 4429 27909.079380: PERF_RECORD_COMM exec: usleep:4429/4429
usleep 4429 27909.079381: 185403 cycles: ffffffff810a72d3 flush_signal_handlers (/lib/modules/4.
usleep 4429 27909.079444:
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