linux-kernel - Re: [tip:perf/core] perf/x86/intel: Disable userspace RDPMC usage for large PEBS

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Date:   Mon, 12 Mar 2018 10:08:09 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Vince Weaver <vincent.weaver@...ne.edu>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, eranian@...gle.com, acme@...hat.com,
        hpa@...or.com, alexander.shishkin@...ux.intel.com,
        mingo@...nel.org, jolsa@...hat.com,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf/x86/intel: Disable userspace RDPMC usage for
 large PEBS



On 3/9/2018 2:10 PM, Vince Weaver wrote:
> On Fri, 9 Mar 2018, Peter Zijlstra wrote:
> 
>> On Fri, Mar 09, 2018 at 09:31:11AM -0500, Vince Weaver wrote:
>>> On Fri, 9 Mar 2018, tip-bot for Kan Liang wrote:
>>>
>>>> Commit-ID:  1af22eba248efe2de25658041a80a3d40fb3e92e
>>>> Gitweb:     https://git.kernel.org/tip/1af22eba248efe2de25658041a80a3d40fb3e92e
>>>> Author:     Kan Liang <kan.liang@...ux.intel.com>
>>>> AuthorDate: Mon, 12 Feb 2018 14:20:35 -0800
>>>> Committer:  Ingo Molnar <mingo@...nel.org>
>>>> CommitDate: Fri, 9 Mar 2018 08:22:23 +0100
>>>>
>>>> perf/x86/intel: Disable userspace RDPMC usage for large PEBS
>>>>
>>>
>>>
>>> So this whole commit log is about disabling RDPMC usage for "large PEBS"
>>> but the actual change disables RDPMC if "PERF_X86_EVENT_FREERUNNING"
>>>
>>> Either the commit log is really misleading, or else a poor name was chosen
>>> for this feature.
>>
>> Its the same thing, and yes that might want renaming I suppose.
> 
> I apologize for noticing these things so late in the game, but I haven't
> had time to keep up with a full lkml feed recently so I only see these
> things once I'm CC'd on them.
> 
> So to summarize this: rdpmc is only disabled on a per-event basis, and
> only if that event is doing multi-pebs sampling?
>

If the event can do multi-pebs sampling, the rdpmc will be disabled. 
Other events which cannot do multi-pebs will not be impacted.

To enable multi-pebs sampling for event, it requires a fixed period.
It doesn't support callgraph. For older platform (before SKL), it 
doesn't support time stamp.

Thanks,
Kan

> If that's true, then I don't think I have an issue with this.
> 
> We finally got rdpmc support in a released PAPI, and it is a massive
> improvement when self-monitoring (even moreso if KPTI is enabled) so I was
> just trying to make sure this wouldn't suddenly disable rdpmc out from
> under us.
> 
> Vince
>