linux-kernel - Re: [PATCH 3/3] perf: Allow kernel filters on cpu events

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Date:   Fri, 27 Jan 2017 14:31:22 +0200
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 3/3] perf: Allow kernel filters on cpu events

Mathieu Poirier <mathieu.poirier@...aro.org> writes:

> On Thu, Jan 26, 2017 at 11:40:57AM +0200, Alexander Shishkin wrote:
>> +	if (!ifh->nr_file_filters)
>> +		return;
>
> Is this mandatory or an optimisation to avoid circling through a list of filters
> that don't included user space files?

It's both. It stems from the fact that the remainder of this function
relies on ctx::task not being NULL, which is not the case with cpu
contexts and now that we've enabled address filters for such contexts,
it's a problem. So checking for !task would have done the trick here,
but this way we'll also avoid going down this path for task contexts
in the absence of file-based filters. In particular, grabbing the mmap
semaphore and filters spinlock we can do without.

Regards,
--
Alex