linux-kernel - Re: round-robining per-cpu counters

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Date:	Tue, 5 May 2009 21:57:01 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Robert Richter <robert.richter@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Mike Galbraith <efault@....de>
Subject: Re: round-robining per-cpu counters

Ingo Molnar writes:

> The reservation mechanism really suffered from not being used by 
> anything or anyone, and it thus bit-rotted across 300 follow-on 
> commits.

Yeah.

> What would be the primary usecase? Allow admin to set aside (and 
> guarantee) space for task counters? Allow admin to 'force' 
> exclusivity of counter ownership?
> 
> I think a better general solution would be to have a single 
> round-robin list for all currently active counters (both percpu and 
> task counters) - and fairly round-robin all of them. The scaling 
> information makes it obvious when this is happening.

Sounds good, but I'm having trouble visualizing how we combine a
fairly static set of per-cpu counters with several sets of per-task
counters that come and go as their tasks get scheduled in and out.
Is there some clever scheduling technique that sorts all that out?

> If admin wants stronger ownership of counters then the 
> pinned/exclusive attribute can be used.

I agree.

> We really want to keep the counter-scheduler simple, and we also 
> want to make the default to be as permissive as possible.

Definitely.

Paul.
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