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Date:   Wed, 24 Oct 2018 13:32:08 -0300
From:   Arnaldo Carvalho de Melo <acme@kernel.org>
To:     Andi Kleen <ak@linux.intel.com>
Cc:     kan.liang@linux.intel.com, Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        bp@alien8.de, Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH 1/2] perf: Add munmap callback
Message-ID: <20181024163208.GC15106@kernel.org>
References: <20181024151116.30935-1-kan.liang@linux.intel.com>
 <20181024162334.GG6218@tassilo.jf.intel.com>
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Em Wed, Oct 24, 2018 at 09:23:34AM -0700, Andi Kleen escreveu:
> > +void perf_event_munmap(void)
> > +{
> > +	struct perf_cpu_context *cpuctx;
> > +	unsigned long flags;
> > +	struct pmu *pmu;
> > +
> > +	local_irq_save(flags);
> > +	list_for_each_entry(cpuctx, this_cpu_ptr(&sched_cb_list), sched_cb_entry) {
> 
> Would be good have a fast path here that checks for the list being empty
> without disabling the interrupts. munmap can be somewhat hot. I think
> it's ok to make it slower with perf running, but we shouldn't impact
> it without perf.

Right, look at how its counterpart, perf_event_mmap() works:

void perf_event_mmap(struct vm_area_struct *vma)
{
        struct perf_mmap_event mmap_event;

        if (!atomic_read(&nr_mmap_events))
                return;
<SNIP>
}

- Arnaldo