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

From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
To: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, vince@deater.net,
	eranian@google.com, Arnaldo Carvalho de Melo <acme@infradead.org>,
	Will Deacon <will.deacon@arm.com>,
	Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH 3/3] perf: Allow kernel filters on cpu events
Date: Fri, 27 Jan 2017 14:31:22 +0200	[thread overview]
Message-ID: <87efzobs6d.fsf@ashishki-desk.ger.corp.intel.com> (raw)
In-Reply-To: <20170126213855.GC1991@linaro.org>

Mathieu Poirier <mathieu.poirier@linaro.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