Messages in this thread | | | Subject | Re: [PATCH] trace: reset sleep/block start time on task switch | From | Peter Zijlstra <> | Date | Mon, 23 Jan 2012 12:34:09 +0100 |
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On Thu, 2012-01-19 at 18:20 -0800, Arun Sharma wrote: > Without this patch, the first sample we get on a > task might be bad because of a stale sleep_start > value that wasn't reset at the last task switch > because the tracepoint was not active. > > The problem can be worked around via perf record > --filter "sleeptime < some-large-number" in practice > and it's not clear if the added code to the context > switch path is worth it. > > I'm posting this patch regardless, just in case > more people start noticing this and start wondering > where the bogus numbers came from. > > Signed-off-by: Arun Sharma <asharma@fb.com> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> > Cc: Arnaldo Carvalho de Melo <acme@infradead.org> > Cc: Andrew Vagin <avagin@openvz.org> > Cc: Frederic Weisbecker <fweisbec@gmail.com> > Cc: Ingo Molnar <mingo@elte.hu> > Cc: linux-kernel@vger.kernel.org > --- > include/trace/events/sched.h | 3 --- > kernel/sched/core.c | 3 +++ > 2 files changed, 3 insertions(+), 3 deletions(-) > > diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h > index 6ba596b..814cdf1 100644 > --- a/include/trace/events/sched.h > +++ b/include/trace/events/sched.h > @@ -378,9 +378,6 @@ static inline u64 trace_get_sleeptime(struct task_struct *tsk) > > block = tsk->se.statistics.block_start; > sleep = tsk->se.statistics.sleep_start; > - tsk->se.statistics.block_start = 0; > - tsk->se.statistics.sleep_start = 0; > - > return block ? block : sleep ? sleep : 0; > #else > return 0; > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 457c881..6349cee 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -1937,7 +1937,10 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev) > local_irq_enable(); > #endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */ > finish_lock_switch(rq, prev); > + > trace_sched_stat_sleeptime(current, rq->clock); > + current->se.statistics.block_start = 0; > + current->se.statistics.sleep_start = 0; > > fire_sched_in_preempt_notifiers(current); > if (mm)
Its not just your tracepoint data being wrong, it'll wreck all related stats :/
This'll fail to compile for !CONFIG_SCHEDSTAT I guess.. I should have paid more attention to the initial patch, that tracepoint having side-effects is a big no-no.
Having unconditional writes there is somewhat sad, but I suspect putting a conditional around it isn't going to help much.. bah can we restructure things so we don't need this?
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