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

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Date:   Fri, 9 Mar 2018 14:10:30 -0500 (EST)
From:   Vince Weaver <vincent.weaver@...ne.edu>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Vince Weaver <vincent.weaver@...ne.edu>, kan.liang@...ux.intel.com,
        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 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 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