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

Peter Zijlstra
2017-01-20T12:24:20Z

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.