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

[Andi Kleen]

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

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

-Andi