* [PATCH] perf/core: Avoid context switch overheads
@ 2017-08-04 8:50 石祤
2017-08-07 12:15 ` Peter Zijlstra
0 siblings, 1 reply; 3+ messages in thread
From: 石祤 @ 2017-08-04 8:50 UTC (permalink / raw)
To: yang_oliver, peterz, mingo, acme, alexander.shishkin,
linux-kernel, tglx, eranian, torvalds, jolsa, linxiulei
Cc: leilei.lin
From: "leilei.lin" <leilei.lin@alibaba-inc.com>
A performance issue caused by less strickly check in task
sched when these tasks were once attached by per-task perf_event.
A task will alloc task->perf_event_ctxp[ctxn] when it was called
by perf_event_open, and task->perf_event_ctxp[ctxn] would not
ever be freed to NULL.
__perf_event_task_sched_in()
if (task->perf_event_ctxp[ctxn]) // here is always true
perf_event_context_sched_in() // operate pmu
50% at most performance overhead was observed under some extreme
test case. Therefor, add a more strick check as to ctx->nr_events,
when ctx->nr_events == 0, it's no need to continue.
Signed-off-by: leilei.lin <leilei.lin@alibaba-inc.com>
---
kernel/events/core.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 426c2ff..f071013 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3179,6 +3179,9 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
if (cpuctx->task_ctx == ctx)
return;
+ if (!cpuctx->task_ctx && !ctx->nr_events)
+ return;
+
perf_ctx_lock(cpuctx, ctx);
perf_pmu_disable(ctx->pmu);
/*
--
2.8.4.31.g9ed660f
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] perf/core: Avoid context switch overheads
2017-08-04 8:50 [PATCH] perf/core: Avoid context switch overheads 石祤
@ 2017-08-07 12:15 ` Peter Zijlstra
2017-08-07 13:02 ` Peter Zijlstra
0 siblings, 1 reply; 3+ messages in thread
From: Peter Zijlstra @ 2017-08-07 12:15 UTC (permalink / raw)
To: 石祤
Cc: yang_oliver, mingo, acme, alexander.shishkin, linux-kernel, tglx,
eranian, torvalds, jolsa, leilei.lin
On Fri, Aug 04, 2017 at 04:50:54PM +0800, 石祤 wrote:
> From: "leilei.lin" <leilei.lin@alibaba-inc.com>
>
> A performance issue caused by less strickly check in task
> sched when these tasks were once attached by per-task perf_event.
>
> A task will alloc task->perf_event_ctxp[ctxn] when it was called
> by perf_event_open, and task->perf_event_ctxp[ctxn] would not
> ever be freed to NULL.
>
> __perf_event_task_sched_in()
> if (task->perf_event_ctxp[ctxn]) // here is always true
> perf_event_context_sched_in() // operate pmu
>
> 50% at most performance overhead was observed under some extreme
> test case. Therefor, add a more strick check as to ctx->nr_events,
> when ctx->nr_events == 0, it's no need to continue.
>
> Signed-off-by: leilei.lin <leilei.lin@alibaba-inc.com>
> ---
> kernel/events/core.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 426c2ff..f071013 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3179,6 +3179,9 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
> if (cpuctx->task_ctx == ctx)
> return;
>
> + if (!cpuctx->task_ctx && !ctx->nr_events)
> + return;
> +
> perf_ctx_lock(cpuctx, ctx);
> perf_pmu_disable(ctx->pmu);
> /*
I _think_ we must do this check after acquiring ctx->lock, because of
how perf_install_in_context() works. See commit:
63cae12bce98 ("perf/core: Fix sys_perf_event_open() vs. hotplug")
That is a giant bag of tricky, but the gist of it is that if
perf_event_ctxp[] is !NULL (as is the case in your scenario) then we
must acquire ctx->lock.
So I think we want something like the below.
---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 426c2ffba16d..bf40b7633cb0 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3180,6 +3180,13 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
return;
perf_ctx_lock(cpuctx, ctx);
+ /*
+ * We must check ctx->nr_events while holding ctx->lock, such
+ * that we serialize against perf_install_in_context().
+ */
+ if (!ctx->nr_events)
+ goto unlock;
+
perf_pmu_disable(ctx->pmu);
/*
* We want to keep the following priority order:
@@ -3193,6 +3200,7 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
perf_event_sched_in(cpuctx, ctx, task);
perf_pmu_enable(ctx->pmu);
+unlock:
perf_ctx_unlock(cpuctx, ctx);
}
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] perf/core: Avoid context switch overheads
2017-08-07 12:15 ` Peter Zijlstra
@ 2017-08-07 13:02 ` Peter Zijlstra
0 siblings, 0 replies; 3+ messages in thread
From: Peter Zijlstra @ 2017-08-07 13:02 UTC (permalink / raw)
To: 石祤
Cc: yang_oliver, mingo, acme, alexander.shishkin, linux-kernel, tglx,
eranian, torvalds, jolsa, leilei.lin
On Mon, Aug 07, 2017 at 02:15:51PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 04, 2017 at 04:50:54PM +0800, 石祤 wrote:
> > From: "leilei.lin" <leilei.lin@alibaba-inc.com>
> >
> > A performance issue caused by less strickly check in task
> > sched when these tasks were once attached by per-task perf_event.
> >
> > A task will alloc task->perf_event_ctxp[ctxn] when it was called
> > by perf_event_open, and task->perf_event_ctxp[ctxn] would not
> > ever be freed to NULL.
> >
> > __perf_event_task_sched_in()
> > if (task->perf_event_ctxp[ctxn]) // here is always true
> > perf_event_context_sched_in() // operate pmu
> >
> > 50% at most performance overhead was observed under some extreme
> > test case. Therefor, add a more strick check as to ctx->nr_events,
> > when ctx->nr_events == 0, it's no need to continue.
> >
> > Signed-off-by: leilei.lin <leilei.lin@alibaba-inc.com>
> > ---
> > kernel/events/core.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 426c2ff..f071013 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -3179,6 +3179,9 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
> > if (cpuctx->task_ctx == ctx)
> > return;
> >
> > + if (!cpuctx->task_ctx && !ctx->nr_events)
> > + return;
> > +
> > perf_ctx_lock(cpuctx, ctx);
> > perf_pmu_disable(ctx->pmu);
> > /*
>
> I _think_ we must do this check after acquiring ctx->lock, because of
> how perf_install_in_context() works. See commit:
>
> 63cae12bce98 ("perf/core: Fix sys_perf_event_open() vs. hotplug")
>
> That is a giant bag of tricky, but the gist of it is that if
> perf_event_ctxp[] is !NULL (as is the case in your scenario) then we
> must acquire ctx->lock.
The alternative is of course, trying to free the task ctx. I've not gone
through what would be required to make that happen.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2017-08-07 12:15 ` Peter Zijlstra
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