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

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Jan 20, 2017 at 10:20:33AM +0100, Peter Zijlstra 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.

Ah; I did not consider that that could leave perf_pmu_sched_task()
seeing a stale cpuctx->ctx.pmu.

That said, is this not already a problem elsewhere? We don't update
ctx->pmu in perf_pmu_unregister, so this would be a problem for any path
using ctx->pmu today (e.g. perf_event_context_sched_in()).

Thanks,
Mark.