RE: [RFC][PATCH 2/6] perf, arch: Rework perf_event_index()

[Oleg Strikov]

> The user-readable clock will first appear in Cortex-A15, so the code for that still needs to hit mainline before I can look at doing this in perf.

This could be very useful. I think that the cycles counter covers the 90% of all the in-app profiler needs.

-----Original Message-----
From: Will Deacon [mailto:will.deacon@xxxxxxx] 
Sent: Tuesday, November 22, 2011 3:47 PM
To: Peter Zijlstra
Cc: mingo@xxxxxxx; William Cohen; linux-kernel@xxxxxxxxxxxxxxx; Michael Cree; Deng-Cheng Zhu; Anton Blanchard; Eric B Munson; Heiko Carstens; Paul Mundt; David S. Miller; Richard Kuo; Stephane Eranian; Arun Sharma; Vince Weaver; Oleg Strikov
Subject: Re: [RFC][PATCH 2/6] perf, arch: Rework perf_event_index()

On Tue, Nov 22, 2011 at 11:26:20AM +0000, Peter Zijlstra wrote:
> On Mon, 2011-11-21 at 22:43 +0000, Will Deacon wrote:
> > Perhaps we could disable it while per-cpu events are running, 
> > although I think this will probably just lead to SIGILL central for 
> > anybody trying to use the counters in userspace.
> 
> One possibility would be to do as I did in patch 4, except ARM has it 
> disabled by default and the folks who think they know WTF they're 
> doing can enable it or so.

The problem is that everybody thinks they know WTF they're doing!

> Ostrikov mentioned on #kernelnewbies he wanted to have this enabled 
> because apparently games use it. He mentioned toggling the user access 
> on/off depending on if the kernel was using perf or not, but that 
> would create very unreliable service.

Well we already have a reserve/release PMU thing which perf honours so we could conceivably do this. I still reckon this will just lead to SIGILLs in userspace though because we can't sanely notify tasks that they should leave the PMU alone for a bit.

> Best would be to use perf to program the thing and use the userspace 
> read and simply agree not to write to these registers (and pray people 
> don't)..

But you know that the first thing people will do is zero the registers.

> Also, for those ARMs that do have a user readable clock, you could 
> support the new time_{mult,shift,offset} from patch 5.

The user-readable clock will first appear in Cortex-A15, so the code for that still needs to hit mainline before I can look at doing this in perf.

Will
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