linux-kernel - Re: [PATCH v2 2/5] perf: Free aux pages in unmap path

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Date:	Mon, 14 Mar 2016 13:38:37 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	vince@...ter.net, eranian@...gle.com,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH v2 2/5] perf: Free aux pages in unmap path

On Fri, Mar 04, 2016 at 03:42:46PM +0200, Alexander Shishkin wrote:
> @@ -4649,10 +4679,22 @@ static void perf_mmap_close(struct vm_area_struct *vma)
>  	 */
>  	if (rb_has_aux(rb) && vma->vm_pgoff == rb->aux_pgoff &&
>  	    atomic_dec_and_mutex_lock(&rb->aux_mmap_count, &event->mmap_mutex)) {
> +		/*
> +		 * Stop all aux events that are writing to this here buffer,
> +		 * so that we can free its aux pages and corresponding pmu
> +		 * data. Note that after rb::aux_mmap_count dropped to zero,
> +		 * they won't start any more (see perf_aux_output_begin()).
> +		 */
> +		perf_pmu_output_stop(event);

So to me it seems like we're interested in rb, we don't particularly
care about @event in this case.

> +
> +		/* now it's safe to free the pages */
>  		atomic_long_sub(rb->aux_nr_pages, &mmap_user->locked_vm);
>  		vma->vm_mm->pinned_vm -= rb->aux_mmap_locked;
>  
> +		/* this has to be the last one */
>  		rb_free_aux(rb);
> +		WARN_ON_ONCE(atomic_read(&rb->aux_refcount));
> +
>  		mutex_unlock(&event->mmap_mutex);
>  	}
>  

> +static void __perf_event_output_stop(struct perf_event *event, void *data)
> +{
> +	struct perf_event *parent = event->parent;
> +	struct remote_output *ro = data;
> +	struct ring_buffer *rb = ro->rb;
> +
> +	if (!has_aux(event))
> +		return;
> +

	if (!parent)
		parent = event;

> +	if (rcu_dereference(event->rb) == rb)
s/event/parent/

> +		ro->err = __perf_event_stop(event);

> +	else if (parent && rcu_dereference(parent->rb) == rb)
> +		ro->err = __perf_event_stop(event);

and these can go.. However..

> +}
> +
> +static int __perf_pmu_output_stop(void *info)
> +{
> +	struct perf_event *event = info;
> +	struct pmu *pmu = event->pmu;
> +	struct perf_cpu_context *cpuctx = get_cpu_ptr(pmu->pmu_cpu_context);
> +	struct remote_output ro = {
> +		.rb	= event->rb,
> +	};
> +
> +	rcu_read_lock();
> +	perf_event_aux_ctx(&cpuctx->ctx, __perf_event_output_stop, &ro);
> +	if (cpuctx->task_ctx)
> +		perf_event_aux_ctx(cpuctx->task_ctx, __perf_event_output_stop,
> +				   &ro);
> +	rcu_read_unlock();
> +
> +	return ro.err;
> +}
> +
> +static void perf_pmu_output_stop(struct perf_event *event)
> +{
> +	int cpu, err;
> +
> +	/* better be thorough */
> +	get_online_cpus();
> +restart:
> +	for_each_online_cpu(cpu) {
> +		err = cpu_function_call(cpu, __perf_pmu_output_stop, event);
> +		if (err)
> +			goto restart;
> +	}
> +	put_online_cpus();
> +}

This seems wildly overkill, could we not iterate rb->event_list like we
do for the normal buffer?

Sure, we need to IPI for each event found, but that seems better than
unconditionally sending IPIs to all CPUs.