linux-kernel - Re: perf: is it possible to userspace rdpmc but only on a certain core type

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Message-ID: <Z5MivTsyHedobQGY@tassilo>
Date: Thu, 23 Jan 2025 21:18:53 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: Vince Weaver <vincent.weaver@...ne.edu>
Cc: Peter Zijlstra <peterz@...radead.org>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com
Subject: Re: perf: is it possible to userspace rdpmc but only on a certain
 core type

On Thu, Jan 23, 2025 at 02:45:33PM -0500, Vince Weaver wrote:
> On Thu, 23 Jan 2025, Andi Kleen wrote:
> 
> > > I have a student working on this for PAPI.  If we get it working we can 
> > > see if perf could use support too if it doesn't have it already.
> > 
> > perf user space doesn't have a ring 3 self access library.
> 
> what happens if you're doing top-down measurments with perf on a hybrid 
> system and perf gets migrated to an E-core?
> 
> or are you saying perf always uses a syscall to read the top-down values 
> and doesn't use rdpmc in that case? 

perf always uses system calls or mmap.

-Andi