linux-kernel - Re: [PATCH] hrtimer: Fix extra wakeups from __remove

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Message-ID: <20111119040836.GA1885@zhy>
Date:	Sat, 19 Nov 2011 12:08:37 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Jeff Ohlstein <johlstei@...eaurora.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hrtimer: Fix extra wakeups from __remove_hrtimer()

On Fri, Nov 18, 2011 at 03:47:10PM -0800, Jeff Ohlstein wrote:
> __remove_hrtimer() attempts to reprogram the clockevent device when
> the timer being removed is the next to expire. However,
> __remove_hrtimer() reprograms the clockevent *before* removing the
> timer from the timerqueue and thus when hrtimer_force_reprogram()
> finds the next timer to expire it finds the timer we're trying to
> remove.
> 
> This is especially noticeable when the system switches to NOHz mode
> and the system tick is removed. The timer tick is removed from the
> system but the clockevent is programmed to wakeup in another HZ
> anyway.

Reduced one needless call of hrtimer_interrupt() indeed.

> 
> Silence the extra wakeup by removing the timer from the timerqueue
> before calling hrtimer_force_reprogram() so that we actually program
> the clockevent for the next timer to expire.
> 
> This was broken by 998adc3 "hrtimers: Convert hrtimers to use
> timerlist infrastructure".

The issue also exists before 998adc3.

Apart from that.

Reviewed-by: Yong Zhang <yong.zhang0@...il.com>

> 
> Signed-off-by: Jeff Ohlstein <johlstei@...eaurora.org>
> ---
>  kernel/hrtimer.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
> index 422e567..ae34bf5 100644
> --- a/kernel/hrtimer.c
> +++ b/kernel/hrtimer.c
> @@ -885,10 +885,13 @@ static void __remove_hrtimer(struct hrtimer *timer,
>  			     struct hrtimer_clock_base *base,
>  			     unsigned long newstate, int reprogram)
>  {
> +	struct timerqueue_node *next_timer;
>  	if (!(timer->state & HRTIMER_STATE_ENQUEUED))
>  		goto out;
>  
> -	if (&timer->node == timerqueue_getnext(&base->active)) {
> +	next_timer = timerqueue_getnext(&base->active);
> +	timerqueue_del(&base->active, &timer->node);
> +	if (&timer->node == next_timer) {
>  #ifdef CONFIG_HIGH_RES_TIMERS
>  		/* Reprogram the clock event device. if enabled */
>  		if (reprogram && hrtimer_hres_active()) {
> @@ -901,7 +904,6 @@ static void __remove_hrtimer(struct hrtimer *timer,
>  		}
>  #endif
>  	}
> -	timerqueue_del(&base->active, &timer->node);
>  	if (!timerqueue_getnext(&base->active))
>  		base->cpu_base->active_bases &= ~(1 << base->index);
>  out:
> -- 
> Sent by an employee of the Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
> 
> --
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