linux-kernel - Re: [PATCH 0/4] perf: Fix the ctx->pmu for a hybrid system

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Date:   Fri, 18 Jun 2021 09:54:18 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org, acme@...nel.org,
        mark.rutland@....com, ak@...ux.intel.com,
        alexander.shishkin@...ux.intel.com, namhyung@...nel.org,
        jolsa@...hat.com
Subject: Re: [PATCH 0/4] perf: Fix the ctx->pmu for a hybrid system



On 6/17/2021 7:33 AM, Peter Zijlstra wrote:
> On Thu, Jun 17, 2021 at 12:23:06PM +0200, Peter Zijlstra wrote:
>> On Wed, Jun 16, 2021 at 11:55:30AM -0700, kan.liang@...ux.intel.com wrote:
>>
>>> To fix the issue, the generic perf codes have to understand the
>>> supported CPU mask of a specific hybrid PMU. So it can update the
>>> ctx->pmu accordingly, when a task is scheduled on a CPU which has
>>> a different type of PMU from the previous CPU. The supported_cpus
>>> has to be moved to the struct pmu.
>>
>> Urghh.. I so hate this :-/
>>
>> I *did* point you to:
>>
>>    https://lore.kernel.org/lkml/20181010104559.GO5728@hirez.programming.kicks-ass.net/
>>
>> when you started this whole hybrid crud, and I think that's still the
>> correct thing to do.
>>
>> Still, let me consider if there's a workable short-term cludge I hate
>> less.
> 
> How's this? We already have x86_pmu_update_cpu_context() setting the
> 'correct' pmu in the cpuctx, so we can simply fold that back into the
> task context.
> 
> For normal use this is a no-op.
> 
> Now I need to go audit all ctx->pmu usage :-(
> 
> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index db4604c4c502..6a496c29ef00 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3822,9 +3822,16 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
>   					struct task_struct *task)
>   {
>   	struct perf_cpu_context *cpuctx;
> -	struct pmu *pmu = ctx->pmu;
> +	struct pmu *pmu;
>   
>   	cpuctx = __get_cpu_context(ctx);
> +
> +	/*
> +	 * HACK; for HETEROGENOUS the task context might have switched to a

%s/HETEROGENOUS/HETEROGENEOUS/

> +	 * different PMU, don't bother gating this.
> +	 */
> +	pmu = ctx->pmu = cpuctx->ctx.pmu;
> +
>   	if (cpuctx->task_ctx == ctx) {
>   		if (cpuctx->sched_cb_usage)
>   			__perf_pmu_sched_task(cpuctx, true);
> 

With the patch, the issue has gone.
We've also run a full test on ADL and SPR. There is no other issues 
found with the patch.

Tested-by: Kan Liang <kan.liang@...ux.intel.com>

Thanks,
Kan