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

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Date:   Wed, 20 Dec 2017 13:41:47 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     kan.liang@...ux.intel.com
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, jolsa@...hat.com,
        eranian@...gle.com
Subject: Re: [PATCH V2 4/4] perf/x86: fix: disable userspace RDPMC usage for
 large PEBS

On Wed, Dec 20, 2017 at 11:42:51AM -0800, kan.liang@...ux.intel.com wrote:
> From: Kan Liang <kan.liang@...ux.intel.com>
> 
> 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