linux-kernel - Re: [tip:perf/core] perf: Per-pmu-per-cpu contexts

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Message-ID: <20100910145426.GA5618@nowhere>
Date:	Fri, 10 Sep 2010 16:54:29 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	mingo@...hat.com, hpa@...or.com, paulus@...ba.org,
	linux-kernel@...r.kernel.org, eranian@...glemail.com,
	a.p.zijlstra@...llo.nl, yanmin_zhang@...ux.intel.com,
	robert.richter@....com, ming.m.lin@...el.com, tglx@...utronix.de,
	mingo@...e.hu, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf: Per-pmu-per-cpu contexts

On Thu, Sep 09, 2010 at 07:51:53PM +0000, tip-bot for Peter Zijlstra wrote:
> @@ -3745,18 +3757,20 @@ static void perf_event_task_ctx(struct perf_event_context *ctx,
>  
>  static void perf_event_task_event(struct perf_task_event *task_event)
>  {
> -	struct perf_cpu_context *cpuctx;
>  	struct perf_event_context *ctx = task_event->task_ctx;
> +	struct perf_cpu_context *cpuctx;
> +	struct pmu *pmu;
>  
> -	rcu_read_lock();
> -	cpuctx = &get_cpu_var(perf_cpu_context);
> -	perf_event_task_ctx(&cpuctx->ctx, task_event);
> +	rcu_read_lock_sched();
> +	list_for_each_entry_rcu(pmu, &pmus, entry) {
> +		cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
> +		perf_event_task_ctx(&cpuctx->ctx, task_event);
> +	}
>  	if (!ctx)
>  		ctx = rcu_dereference(current->perf_event_ctxp);

So, you say below that it works because synchronize_srcu(), that
waits for qs after touching pmus, implies synchronize_sched(), right?

And I guess you picked rcu_read_lock_sched() here because that preempt_disable()
at the same time.

That looks complicated but I guess that works.

That said there is also this rcu_dereference(current->perf_event_ctxp).
Now, this ctx is released after srcu barrier right? So this should
be srcu_dereference(). But then you seem to actually use rcu_read_lock_sched()
as it's compatible, so this should be rcu_dereference_sched() ?

With the current state, rcu will whine.
Moreover there seem to be too much game between the different rcu
flavours here, and that breaks the reviewers parsing.

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