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	<1300130283-10466-2-git-send-email-fweisbec@gmail.com>
Date: Tue, 15 Mar 2011 22:36:18 +0800
Message-ID: <AANLkTim4gFdxn3pHZuxXe-AwTCYrnUHHeFkEK6wisYiL@mail.gmail.com>
Subject: Re: [RFC PATCH 1/4] perf: Starter and stopper events
From: Lin Ming <minggr@gmail.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Paul Mackerras <paulus@samba.org>,
        Stephane Eranian <eranian@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
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On Tue, Mar 15, 2011 at 3:18 AM, Frederic Weisbecker <fweisbec@gmail.com> wrote:

>
> +static void perf_event_pause_resume(struct perf_event *event, int nmi,
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? struct perf_sample_data *data,
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? struct pt_regs *regs)
> +{
> + ? ? ? struct perf_event *iter;
> + ? ? ? unsigned long flags;
> +
> + ? ? ? /*
> + ? ? ? ?* Ensure the targets can't be sched in/out concurrently.
> + ? ? ? ?* Disabling irqs is sufficient for that because starters/stoppers
> + ? ? ? ?* are on the same cpu/task.
> + ? ? ? ?*/
> + ? ? ? local_irq_save(flags);

Could you explain this more detail?

> +
> +
> + ? ? ? /* Prevent the targets from being removed under us. */
> + ? ? ? rcu_read_lock();
> +
> + ? ? ? list_for_each_entry_rcu(iter, &event->starter_list, starter_entry) {
> + ? ? ? ? ? ? ? /*
> + ? ? ? ? ? ? ? ?* There is a small race window here, between the state
> + ? ? ? ? ? ? ? ?* gets set to active and the event is actually ->add().
> + ? ? ? ? ? ? ? ?* We need to find a way to ensure the starter/stopper
> + ? ? ? ? ? ? ? ?* can't trigger in between.

Can we set the state to active after the event is actually ->add()?

> + ? ? ? ? ? ? ? ?*/
> + ? ? ? ? ? ? ? if (iter->state == PERF_EVENT_STATE_ACTIVE) {
> + ? ? ? ? ? ? ? ? ? ? ? if (iter->paused) {
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? iter->pmu->start(iter, PERF_EF_RELOAD);
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? iter->paused = 0;
> + ? ? ? ? ? ? ? ? ? ? ? }
> + ? ? ? ? ? ? ? }
> + ? ? ? }
> +

Thanks,
Lin Ming
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