Re: Patch "tick/nohz: Fix possible missing clock reprog after tick soft restart" has been added to the 4.9-stable tree

[Greg KH <gregkh@linuxfoundation.org>]

Oops, nope, this breaks the build on 4.9 and 4.4-stable kernels, so I'm
dropping it there :(

greg k-h


On Sun, Feb 12, 2017 at 02:15:28PM -0800, gregkh@linuxfoundation.org wrote:
> 
> This is a note to let you know that I've just added the patch titled
> 
>     tick/nohz: Fix possible missing clock reprog after tick soft restart
> 
> to the 4.9-stable tree which can be found at:
>     http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> 
> The filename of the patch is:
>      tick-nohz-fix-possible-missing-clock-reprog-after-tick-soft-restart.patch
> and it can be found in the queue-4.9 subdirectory.
> 
> If you, or anyone else, feels it should not be added to the stable tree,
> please let <stable@vger.kernel.org> know about it.
> 
> 
> >From 7bdb59f1ad474bd7161adc8f923cdef10f2638d1 Mon Sep 17 00:00:00 2001
> From: Frederic Weisbecker <fweisbec@gmail.com>
> Date: Tue, 7 Feb 2017 17:44:54 +0100
> Subject: tick/nohz: Fix possible missing clock reprog after tick soft restart
> 
> From: Frederic Weisbecker <fweisbec@gmail.com>
> 
> commit 7bdb59f1ad474bd7161adc8f923cdef10f2638d1 upstream.
> 
> ts->next_tick keeps track of the next tick deadline in order to optimize
> clock programmation on irq exit and avoid redundant clock device writes.
> 
> Now if ts->next_tick missed an update, we may spuriously miss a clock
> reprog later as the nohz code is fooled by an obsolete next_tick value.
> 
> This is what happens here on a specific path: when we observe an
> expired timer from the nohz update code on irq exit, we perform a soft
> tick restart which simply fires the closest possible tick without
> actually exiting the nohz mode and restoring a periodic state. But we
> forget to update ts->next_tick accordingly.
> 
> As a result, after the next tick resulting from such soft tick restart,
> the nohz code sees a stale value on ts->next_tick which doesn't match
> the clock deadline that just expired. If that obsolete ts->next_tick
> value happens to collide with the actual next tick deadline to be
> scheduled, we may spuriously bypass the clock reprogramming. In the
> worst case, the tick may never fire again.
> 
> Fix this with a ts->next_tick reset on soft tick restart.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Reviewed: Wanpeng Li <wanpeng.li@hotmail.com>
> Acked-by: Rik van Riel <riel@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Link: http://lkml.kernel.org/r/1486485894-29173-1-git-send-email-fweisbec@gmail.com
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> ---
>  kernel/time/tick-sched.c |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -730,6 +730,11 @@ static ktime_t tick_nohz_stop_sched_tick
>  		 */
>  		if (delta == 0) {
>  			tick_nohz_restart(ts, now);
> +			/*
> +			 * Make sure next tick stop doesn't get fooled by past
> +			 * clock deadline
> +			 */
> +			ts->next_tick = 0;
>  			goto out;
>  		}
>  	}
> 
> 
> Patches currently in stable-queue which might be from fweisbec@gmail.com are
> 
> queue-4.9/tick-nohz-fix-possible-missing-clock-reprog-after-tick-soft-restart.patch
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