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Subject: Re: [patch] remove put_cpu_no_resched()
From: Trond Myklebust <Trond.Myklebust@netapp.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
       Tony Luck <tony.luck@intel.com>
In-Reply-To: <20090204001840.664546943@linutronix.de>
References: <20090204001840.664546943@linutronix.de>
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Organization: NetApp
Date: Tue, 03 Feb 2009 19:33:59 -0500
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On Wed, 2009-02-04 at 00:27 +0000, Thomas Gleixner wrote:
> plain text document attachment (remove-put-cpu-no-resched.patch)
> Impact: remove a high latency source
> 
> put_cpu_no_resched() is an optimization of put_cpu() which
> unfortunately can cause high latencies.
> 
> The nfs iostats code uses put_cpu_no_resched() in a code sequence
> where a reschedule request caused by an interrupt between the
> get_cpu() and the put_cpu_no_resched() can delay the reschedule for at
> least HZ.
> 
> The other users of put_cpu_no_resched() optimize correctly in
> interrupt code, but there is no real harm in using the put_cpu()
> function which is an alias for preempt_enable(). The extra check of the
> preemmpt count is not as critical as the potential source of missing a
> reschedule.
> 
> Debugged in the preempt-rt tree and verified in mainline.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Reviewed-by: Ingo Molnar <mingo@elte.hu>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: Trond Myklebust <Trond.Myklebust@netapp.com>

Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com