Re: [PATCH 1/2] perf/x86/intel: enable CPU ref_cycles for GP counter

From: Vince Weaver <vincent.weaver@maine.edu>
To: Andi Kleen <ak@linux.intel.com>
Cc: Stephane Eranian <eranian@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Liang, Kan" <kan.liang@intel.com>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"alexander.shishkin@linux.intel.com" 
	<alexander.shishkin@linux.intel.com>,
	"acme@redhat.com" <acme@redhat.com>,
	"jolsa@redhat.com" <jolsa@redhat.com>,
	"torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"vincent.weaver@maine.edu" <vincent.weaver@maine.edu>
Subject: Re: [PATCH 1/2] perf/x86/intel: enable CPU ref_cycles for GP counter
Date: Wed, 24 May 2017 12:01:50 -0400 (EDT)	[thread overview]
Message-ID: <alpine.DEB.2.20.1705241158160.23659@macbook-air> (raw)
In-Reply-To: <20170524154518.GA24144@tassilo.jf.intel.com>

On Wed, 24 May 2017, Andi Kleen wrote:

> > Right, I did not even consider the rdpmc, but yeah, you will get a count that
> > is not relevant to the user visible event. Unless you fake it using the time
> > scaling fields there but that's ugly.
> 
> Could add another scaling field to the mmap page for this.

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.

I already have people really grumpy that you have to have one mmap() page 
per event, meaning you sacrifice one TLB entry for each event you are 
measuring.

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.

Vince