linux-kernel - [PATCH] hrtimer: Only iterate over active bases in migrate

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Date:	Thu, 9 Apr 2015 08:39:47 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: [PATCH] hrtimer: Only iterate over active bases in migrate_hrtimers()


(fixed the subject to include the patch title. Patch quoted below.)

* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Ingo Molnar <mingo@...nel.org> wrote:
> 
> > This would speed up various hrtimer primitives like 
> > hrtimer_remove()/add and simplify the code. It would be a net code 
> > shrink as well.
> > 
> > Totally untested patch below. It gives:
> > 
> >    text    data     bss     dec     hex filename
> >    7502     427       0    7929    1ef9 hrtimer.o.before
> >    7422     427       0    7849    1ea9 hrtimer.o.after
> > 
> > and half of that code removal is from hot paths.
> > 
> > This would simplify the followup step of skipping over inactive bases 
> > as well.
> 
> The followup step is attached below (untested as well).
> 
> Note that all other iterations already had a check for active.next, so 
> the patch doesn't even bloat anything:
> 
>    text    data     bss     dec     hex filename
>    7422     427       0    7849    1ea9 hrtimer.o.before
>    7422     427       0    7849    1ea9 hrtimer.o.after
> 
> (I did a rename within migrate_hrtimers() because it used 'cpu_base' 
> vs 'clock_base' inconsistently in a confusing (to me) manner.)
> 
> I'd also suggest the removal of the timerqueue_getnext() obfuscation: 
> it 'sounds' complex but in reality it's a simple dereference to 
> active.next. I think this is what triggered this rather pointless 
> maintenance of active_bases.
> 
> Thanks,
> 
> 	Ingo
> 
> ---
>  kernel/time/hrtimer.c |   24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> Index: tip/kernel/time/hrtimer.c
> ===================================================================
> --- tip.orig/kernel/time/hrtimer.c
> +++ tip/kernel/time/hrtimer.c
> @@ -1660,29 +1660,35 @@ static void migrate_hrtimer_list(struct
>  
>  static void migrate_hrtimers(int scpu)
>  {
> -	struct hrtimer_cpu_base *old_base, *new_base;
> +	struct hrtimer_cpu_base *old_cpu_base, *new_cpu_base;
>  	int i;
>  
>  	BUG_ON(cpu_online(scpu));
>  	tick_cancel_sched_timer(scpu);
>  
>  	local_irq_disable();
> -	old_base = &per_cpu(hrtimer_bases, scpu);
> -	new_base = this_cpu_ptr(&hrtimer_bases);
> +	old_cpu_base = &per_cpu(hrtimer_bases, scpu);
> +	new_cpu_base = this_cpu_ptr(&hrtimer_bases);
>  	/*
>  	 * The caller is globally serialized and nobody else
>  	 * takes two locks at once, deadlock is not possible.
>  	 */
> -	raw_spin_lock(&new_base->lock);
> -	raw_spin_lock_nested(&old_base->lock, SINGLE_DEPTH_NESTING);
> +	raw_spin_lock(&new_cpu_base->lock);
> +	raw_spin_lock_nested(&old_cpu_base->lock, SINGLE_DEPTH_NESTING);
>  
>  	for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
> -		migrate_hrtimer_list(&old_base->clock_base[i],
> -				     &new_base->clock_base[i]);
> +		struct hrtimer_clock_base *old_base = old_cpu_base->clock_base + i;
> +		struct hrtimer_clock_base *new_base;
> +
> +		if (!old_base->active.next)
> +			continue;
> +
> +		new_base = new_cpu_base->clock_base + i;
> +		migrate_hrtimer_list(old_base, new_base);
>  	}
>  
> -	raw_spin_unlock(&old_base->lock);
> -	raw_spin_unlock(&new_base->lock);
> +	raw_spin_unlock(&old_cpu_base->lock);
> +	raw_spin_unlock(&new_cpu_base->lock);
>  
>  	/* Check, if we got expired work to do */
>  	__hrtimer_peek_ahead_timers();
--
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