linux-kernel - Re: [PATCH -rt] fix setting of preempt_max_latency and preempt

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Message-ID: <20070327191548.GC29929@elte.hu>
Date:	Tue, 27 Mar 2007 21:15:48 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Roland Dreier <rdreier@...co.com>
Subject: Re: [PATCH -rt] fix setting of preempt_max_latency and preempt_thresh on 64 bit systems


* Steven Rostedt <rostedt@...dmis.org> wrote:

> Ingo,
> 
> I wasn't able to turn on latency tracing on a x86_64 box. Using logdev 
> to see what was happening, I found that echoing 0 into 
> /proc/sys/kernel/preempt_max_latency would only change the max from 
> -1ULL to 0xffffffff00000000. Which would keep the max pretty high 
> still.

ah! This should explain Roland Dreier's report about tracing not working 
on x86_64.

> The problem is in sysctl.c where preempt_max_latency can be modified. 
> On systems where cycle_t is greater than the size of int (x86_64) we 
> only write to the 4 LSB.
> 
> Here's the patch:

thanks, applied.

	Ingo
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