Re: [PATCH V2 4/4] perf/x86: fix: disable userspace RDPMC usage for large PEBS

[Liang, Kan]

On 12/20/2017 4:41 PM, Andi Kleen wrote:
> On Wed, Dec 20, 2017 at 11:42:51AM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
> > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> >
> > The userspace RDPMC usage never works for large PEBS since the large
> > PEBS is introduced by
> > commit b8241d20699e ("perf/x86/intel: Implement batched PEBS interrupt
> > handling (large PEBS interrupt threshold)")
> >
> > When the PEBS interrupt threshold is larger than one, there is no way to
> > get exact auto-reload times and value for userspace RDPMC.
> >
> > Disable the userspace RDPMC usage when large PEBS is enabled.
>
> The only drawback is that the event wraps quickly. 

It's broken. The value read from RDPMC command will always between 
-reload_val and 0. Without reload times, I think it's impossible to 
calculate the total count number.

> I suspect in many
> cases it's still usable for measuring short regions.

Unless the short region is less than reload_val.
Otherwise, reload times is needed.

Thanks,
Kan

> I wouldn't force disable it, just let the users deal with it.
>
> -Andi