linux-kernel - Re: [PATCH v2 2/2] perf/core: Remove perf_cpu_context::unique

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Date:   Fri, 20 Jan 2017 10:20:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Carrillo-Cisneros <davidcc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, "x86@...nel.org" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>, Borislav Petkov <bp@...e.de>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Vince Weaver <vince@...ter.net>, Paul Turner <pjt@...gle.com>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2 2/2] perf/core: Remove perf_cpu_context::unique_pmu

On Wed, Jan 18, 2017 at 11:24:54AM -0800, David Carrillo-Cisneros wrote:
> cpuctx->unique_pmu was originally introduced as a way to identify cpuctxs
> with shared pmus in order to avoid visiting the same cpuctx more than once
> in a for_each_pmu loop.
> 
> cpuctx->unique_pmu == cpuctx->pmu in non-software task contexts since they
> have only one pmu per cpuctx. Since perf_pmu_sched_task is only called in
> hw contexts, this patch replaces cpuctx->unique_pmu by cpuctx->pmu in it.
> 
> The change above, together with the previous patch in this series, removed
> the remaining uses of cpuctx->unique_pmu, so we remove it altogether.
> 
> Signed-off-by: David Carrillo-Cisneros <davidcc@...gle.com>
> Acked-by: Mark Rutland <mark.rutland@....com>


> @@ -8572,37 +8572,10 @@ static struct perf_cpu_context __percpu *find_pmu_context(int ctxn)
>  	return NULL;
>  }
>  
> -static void update_pmu_context(struct pmu *pmu, struct pmu *old_pmu)
> -{
> -	int cpu;
> -
> -	for_each_possible_cpu(cpu) {
> -		struct perf_cpu_context *cpuctx;
> -
> -		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
> -
> -		if (cpuctx->unique_pmu == old_pmu)
> -			cpuctx->unique_pmu = pmu;
> -	}
> -}
> -
>  static void free_pmu_context(struct pmu *pmu)
>  {
> -	struct pmu *i;
> -
>  	mutex_lock(&pmus_lock);
> -	/*
> -	 * Like a real lame refcount.
> -	 */
> -	list_for_each_entry(i, &pmus, entry) {
> -		if (i->pmu_cpu_context == pmu->pmu_cpu_context) {
> -			update_pmu_context(i, pmu);
> -			goto out;
> -		}
> -	}
> -
>  	free_percpu(pmu->pmu_cpu_context);
> -out:
>  	mutex_unlock(&pmus_lock);
>  }

This very much relies on us never calling perf_pmu_unregister() on the
software PMUs afaict. A condition not mention in the Changelog.