linux-kernel - Re: [BUG] perf: sharing of cpuctx between core and ibs PMU causes problems

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Date:	Mon, 13 Aug 2012 14:53:18 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Yan, Zheng" <zheng.z.yan@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Robert Richter <robert.richter@....com>, mingo@...e.hu
Subject: Re: [BUG] perf: sharing of cpuctx between core and ibs PMU causes problems

Peter,

Ok, that should fix the problem that IBS would not work correctly in
per-thread mode.
I realized I was looking at an older kernel which did not have the
split between ibs
op and fetch. And there, the .task_nr_context was not initialized at all.

Your proposal solves the problem, though it is not that pretty because
you're exposing
IBS stuff in sched.h aside for the 2 new iterations. But I don't see
another way around
this at this point.

Thanks.

On Mon, Aug 13, 2012 at 12:28 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> OK,.. so the AMD IBS PMUs actually have perf_invalid_context.
>
> Lemme have a proper look...
>
>
> Weirdness.. perf_pmu_register() will allocate a pmu->pmu_cpu_context for
> each PMU. find_pmu_context() even special cases the perf_invalid_context
> to return NULL to force the allocation instead of sharing it.
>
> So both IBS PMUs should have their own cpuctx.
>
>
>
> In any case, I was talking about something like the below.. I hate
> growing the per-task ctx array with two entries, esp. since we'll mostly
> add two NULL pointer checks on every perf operation for everybody not
> using IBS.
>
> ---
> --- a/arch/x86/kernel/cpu/perf_event_amd_ibs.c
> +++ b/arch/x86/kernel/cpu/perf_event_amd_ibs.c
> @@ -436,7 +436,7 @@ static void perf_ibs_read(struct perf_ev
>
>  static struct perf_ibs perf_ibs_fetch = {
>         .pmu = {
> -               .task_ctx_nr    = perf_invalid_context,
> +               .task_ctx_nr    = perf_hw2_context,
>
>                 .event_init     = perf_ibs_init,
>                 .add            = perf_ibs_add,
> @@ -459,7 +459,7 @@ static struct perf_ibs perf_ibs_fetch =
>
>  static struct perf_ibs perf_ibs_op = {
>         .pmu = {
> -               .task_ctx_nr    = perf_invalid_context,
> +               .task_ctx_nr    = perf_hw3_context,
>
>                 .event_init     = perf_ibs_init,
>                 .add            = perf_ibs_add,
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1237,6 +1237,8 @@ enum perf_event_task_context {
>         perf_invalid_context = -1,
>         perf_hw_context = 0,
>         perf_sw_context,
> +       perf_hw2_context, /* AMD IBS (fetch) */
> +       perf_hw3_context, /* AMD IBS (ops)   */
>         perf_nr_task_contexts,
>  };
>
>
--
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