linux-kernel - Re: [PATCH] i386 Time: Avoid PIT SMP lockups

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Date:	Wed, 11 Oct 2006 14:26:46 -0700
From:	Andrew Morton <akpm@...l.org>
To:	john stultz <johnstul@...ibm.com>
Cc:	Andi Kleen <ak@...e.de>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] i386 Time: Avoid PIT SMP lockups

On Wed, 11 Oct 2006 12:54:21 -0700
john stultz <johnstul@...ibm.com> wrote:

> Andrew: I think this is 2.6.19 material, but probably should go through an -mm or two.
> 
> thanks
> -john
> 
> 
> This patch avoids possible PIT livelock issues seen on SMP systems (and
> reported by Andi), by not allowing it as a clocksource on SMP boxes.
> 
> However, since the PIT may no longer be present, we have to properly
> handle the cases where SMP systems have TSC skew and fall back from the
> TSC. Since the PIT isn't there, it would "fall back" to the TSC again.
> So this changes the jiffies rating to 1, and the TSC-bad rating value to
> 0.
> 
> Thus you will get the following behavior priority on i386 systems:
> 
> tsc		[if present & stable]
> hpet		[if present]
> cyclone		[if present]
> acpi_pm		[if present]
> pit		[if UP]
> jiffies
> 
> Rather then the current more complicated:
> tsc		[if present & stable]
> hpet		[if present]
> cyclone		[if present]
> acpi_pm		[if present]
> pit		[if cpus < 4]

Actually <=4, and that matters: there are a lot of 4-ways.

> tsc		[if present & unstable]
> jiffies
> 

So this patch has the potential to screw up people who have 2-way or 4-way,
no hpet/pm-timer and dodgy TSCs.

Wouldn't it be better to fix the livelock?  What's causing it?
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