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

[Liang, Kan]

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@xxxxxxxxxxxxxxx>
> > > > AuthorDate: Mon, 12 Feb 2018 14:20:35 -0800
> > > > Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
> > > > 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