linux-kernel - Re: [patch] remove put_cpu_no

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Date:	Sun, 3 May 2009 10:30:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tony Luck <tony.luck@...el.com>,
	Trond Myklebust <Trond.Myklebust@...app.com>
Subject: Re: [patch] remove put_cpu_no_resched() [ Resend ]


* Thomas Gleixner <tglx@...utronix.de> wrote:

> 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@...utronix.de>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Tony Luck <tony.luck@...el.com>
> Cc: Trond Myklebust <Trond.Myklebust@...app.com>
> ---
>  arch/ia64/kernel/perfmon.c |    2 +-
>  fs/nfs/iostat.h            |    4 ++--
>  include/linux/smp.h        |    1 -
>  3 files changed, 3 insertions(+), 4 deletions(-)

Good catch!

Acked-by: Ingo Molnar <mingo@...e.hu>

	Ingo
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