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

From: Mathieu Poirier <mathieu.poirier@linaro.org>
To: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Ingo Molnar <mingo@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Vince Weaver <vince@deater.net>,
	Stephane Eranian <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 10:38:01 -0700	[thread overview]
Message-ID: <CANLsYkzaQEc5TSmqS7rKtO-iZ2LjMjdxiUvt2XinJ=d=5xXGvw@mail.gmail.com> (raw)
In-Reply-To: <87efzobs6d.fsf@ashishki-desk.ger.corp.intel.com>

On 27 January 2017 at 05:31, Alexander Shishkin
<alexander.shishkin@linux.intel.com> wrote:
> 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.

Yes, I see it now.

Do you have bigger plans for ->nr_file_filters?  If the purpose is
only to indicate the presence of user space files then it should be a
'bool' type (and probably changed to ->file_filters).

Thanks,
Mathieu

>
> Regards,
> --
> Alex