Re: [PATCH v2 2/2] perf/core: Remove perf_cpu_context::unique_pmu

[David Carrillo-Cisneros <davidcc@google.com>]

On Fri, Jan 20, 2017 at 1:20 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 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@google.com>
>> Acked-by: Mark Rutland <mark.rutland@arm.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.
>
What's a good way to solve this? Update the Changelog or add code to
update ctx->pmu?

This issue would go away cleanly if we were to remove the context
sharing across pmu's. Would you support work in that direction?

Thanks,
David