linux-kernel - Re: [PATCH 1/2] perf/x86: update Haswell PEBS event constraints

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Message-ID: <CABPqkBSvzo3kNrR1AS1RfZNoYQPuaKWyShfpjNKNrMAs6YqQPw@mail.gmail.com>
Date:	Fri, 20 Jun 2014 15:44:56 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>, Joe Mario <jmario@...hat.com>,
	Don Zickus <dzickus@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH 1/2] perf/x86: update Haswell PEBS event constraints

On Thu, Jun 19, 2014 at 10:45 PM, Stephane Eranian <eranian@...gle.com> wrote:
> On Thu, Jun 19, 2014 at 10:40 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>> On Thu, Jun 19, 2014 at 10:31:29PM +0200, Stephane Eranian wrote:
>>> On Thu, Jun 19, 2014 at 10:18 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>>> >> I don't quite understand that.
>>> >> You need to know which events support PEBS. You need a table
>>> >
>>> > We're talking about the kernel allowing things here.
>>> > Yes the user still needs to know what supports PEBS, but
>>> > that doesn't concern the kernel.
>>> >
>>> Just need to make sure you don't return bogus information.
>>
>> GIGO. We only need to prevent security issues.
>>
>>> > You can just allow it for all, it's a nop if the event doesn't
>>> > support it. And also the fields like DataLA are simply 0 when
>>> > not supported.
>>> >
>>>
>>> Let's take a example. If I do resource_stalls:pp, the kernel
>>> will let it go through and clear the PMI bit on the config as
>>> is required for PEBS mode. The counter will count normally
>>> and never fire an interrupt, even when it overflows. It would
>>> never execute the PMI handler and thus never look at the
>>> PEBS content. You'd never get any samples.
>>
>> Yes if the user specifies a bogus raw event it will not count.
>> That's fine. The important part is just that nothing ever crashes.
>>
> That would certainly avoid the problem of missing events in pebs table.
> I had a problem with that just today. It also speed up scheduling
> as well by avoid the table lookups.
>
I can take of writing the patch to do this, if you want.

> Note that I will soon post a patch to speed up scheduling for all x86
> processors.

I'll put that one in too.
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