linux-kernel - Re: [PATCH] [15/58] i386: Rewrite sched

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Message-Id: <200707201714.16811.ak@suse.de>
Date:	Fri, 20 Jul 2007 17:14:16 +0200
From:	Andi Kleen <ak@...e.de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	patches@...-64.org, linux-kernel@...r.kernel.org,
	Daniel Walker <dwalker@...sta.com>
Subject: Re: [PATCH] [15/58] i386: Rewrite sched_clock


> So if, after this, we run tsc_sched_clock() with an unstable TSC, we
> read a last_val containing the interrupt's MSB and the last_val LSB. It
> can particularity hurt if we are around a 32 bits overflow, because time
> could "jump" forward of about 1.43 seconds on a 3 GHz system.
> 
> So I guess we need synchronization on the fast path, and therefore using
> cmpxchg_local on x86_64

On x86-64 the 64bit write is atomic against interrupts.

You're right 32bit has a problem though. I'm not too happy about 
cmpxchg though because that wouldn't work on some CPUs.

I wonder if we can just get away with using a 32bit value on i386.
Just for the purpose of keeping the value monotonic it should be good
enough. Will think about it.

Thanks for the review.

-Andi

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