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authorShaohua Li <shli@fb.com>2015-02-05 15:55:32 -0800
committerIngo Molnar <mingo@kernel.org>2015-02-18 17:01:45 +0100
commit6a694a607a97d58c042fb7fbd60ef1caea26950c (patch)
tree37647025d7f1eff5b16703c442e901e084008b4a /arch
parent72f669c0086fbbbbebc92ce7390125722c4c0ec5 (diff)
perf: Update userspace page info for software event
For hardware events, the userspace page of the event gets updated in context switches, so if we read the timestamp in the page, we get fresh info. For software events, this is missing currently. This patch makes the behavior consistent. With this patch, we can implement clock_gettime(THREAD_CPUTIME) with PERF_COUNT_SW_DUMMY in userspace as suggested by Andy and Peter. Code like this: if (pc->cap_user_time) { do { seq = pc->lock; barrier(); running = pc->time_running; cyc = rdtsc(); time_mult = pc->time_mult; time_shift = pc->time_shift; time_offset = pc->time_offset; barrier(); } while (pc->lock != seq); quot = (cyc >> time_shift); rem = cyc & ((1 << time_shift) - 1); delta = time_offset + quot * time_mult + ((rem * time_mult) >> time_shift); running += delta; return running; } I tried it on a busy system, the userspace page updating doesn't have noticeable overhead. Signed-off-by: Shaohua Li <shli@fb.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Arnaldo Carvalho de Melo <acme@kernel.org> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/aa2dd2e4f1e9f2225758be5ba00f14d6909a8ce1.1423180257.git.shli@fb.com [ Improved the changelog. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'arch')
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