linux-kernel - Re: [perfmon] Re: conflict between tickless and perfmon2

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Message-ID: <20071114163401.GN6557@frankl.hpl.hp.com>
Date:	Wed, 14 Nov 2007 08:34:01 -0800
From:	Stephane Eranian <eranian@....hp.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>, linux-ia64@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: [perfmon] Re: conflict between tickless and perfmon2

Thomas,

On Fri, Nov 09, 2007 at 07:40:31PM +0100, Thomas Gleixner wrote:
> On Fri, 9 Nov 2007, Peter Zijlstra wrote:
> 
> 
> > > It looks like a solution would be to change the implementation of
> > > timeout-based switching to use HR timers instead. Similar to what is
> > > done for ITIMER_REAL and ITIMER_VIRTUAL.
> 
> Using a hrtimer is perfrectly fine, I'd say it's preferred over hooks in 
> some code which has absoluty no guarantee of being executed periodically 
> or even executed at all. OTOH it seems rather stupid to measure stuff 
> while the system is idle and doing nothing.
> 
I managed to switch the perfmon2 code to use hrtimer(CLOCK_MONOTONIC)
for system-wide (per-cpu) measurements. The code is simple and this allowed
me to do some more cleanups. I think this was a good suggestion and I made
the change rapidly.

Now, I must admit I don't quite understand how to make this work for per-thread
measurements where the timer would have to operate like ITIMER_VIRTUAL,i.e., only
run when the thread runs. I looked at the setitimer() code and I admit it is
not clear to me. What about CLOCK_THREAD_CPUTIME_ID, would it do what I need from
inside the kernel?

Thanks.

-- 
-Stephane
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