linux-kernel - Re: [RFC PATCH 16/22 -v2] add get_monotonic

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Message-ID: <Pine.LNX.4.58.0801162202160.20257@gandalf.stny.rr.com>
Date:	Wed, 16 Jan 2008 22:02:49 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
cc:	john stultz <johnstul@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Daniel Walker <dwalker@...sta.com>
Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles


On Wed, 16 Jan 2008, Mathieu Desnoyers wrote:

> It would imply the creation of a new vsyscall : vgetschedperiod
>
> It would read a counter that would increment each time the thread is
> scheduled out (or in). It would be a per thread counter (not a per cpu
> counter) so we can deal appropriately with a stopped thread that would
> happen to come back running a loooong time afterward (if we do per-cpu
> counters, we could get the same 32 bits counter value falsely if it is
> shared with other thread activity).
>
> Then, the clocksource read code would look like :
>
> int period;
>
> do {
>   period = vgetschedperiod();
>
>   perform the clocksource read..
>
> } while (period != vgetschedperiod());
>
> Therefore, we would be sure that we have not been scheduled out while
> reading the value. I think this new vsyscall could be useful for others.
> Actually, it would make implementation of RCU in user-space possible (as
> long as the read-side can retry the read operation).


This is something that I would agree is useful.

-- Steve

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