linux-kernel - Re: [PATCH RFC] rcu: Permit limiting of force_quiescent

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Message-ID: <1332128832.7283.34.camel@marge.simpson.net>
Date:	Mon, 19 Mar 2012 04:47:12 +0100
From:	Mike Galbraith <efault@....de>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	sivanich@....com
Subject: Re: [PATCH RFC] rcu: Permit limiting of force_quiescent_state()
 latency

On Sun, 2012-03-18 at 17:10 -0700, Paul E. McKenney wrote:

> Thank you very much for your efforts on this!!!

No problem, irq holdoff troubles are worth effort.

> Given that you were seeing about 200-microsecond latency spikes on
> grace-period initialization, I would expect that you would need about
> 200 dyntick-idle CPUs for force_quiescent_state() to give you a
> ten-microsecond spike, so I am not surprised that you could not see
> the difference on 60 CPUs, which probably have given you something
> like 3 microseconds..

Crawling over fewer locks should still save cycles, so I'll measure.
Big box rt needs every little usec we can scrape together.

-Mike

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