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Date: Wed, 24 May 2017 09:55:49 -0700
From: Andi Kleen <ak@linux.intel.com>
To: Vince Weaver <vincent.weaver@maine.edu>
Cc: Stephane Eranian <eranian@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Liang, Kan" <kan.liang@intel.com>,
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        "alexander.shishkin@linux.intel.com" 
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        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>
Subject: Re: [PATCH 1/2] perf/x86/intel: enable CPU ref_cycles for GP counter
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> The whole point of the rdpmc() implementation is to be low overhead.
> If you have to parse 10 different mmap() fields it starts to defeat the 
> purpose.

You would only use it with ref-cycles of course. So for the normal
case there is no overhead.

> If the watchdog counter is constantly running, can't you just modify 
> perf_event to just grab start/stop values at context switch time and 
> provide the difference to the user?  Sort of like the "always running" 
> patchsets that float around? Though I guess that doesn't help much with 
> sampling.

This wouldn't work with ring filters unfortunately. 

-Andi