linux-kernel - Re: [PATCH RT] dont calculate hweight in update_migrate

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Message-ID: <20140324101800.3f978842@gandalf.local.home>
Date:	Mon, 24 Mar 2014 10:18:00 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Nicholas Mc Guire <der.herr@...r.at>
Cc:	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	tglx@...utronix.de,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RT] dont calculate hweight in update_migrate_disable

On Mon, 24 Mar 2014 13:18:48 +0100
Nicholas Mc Guire <der.herr@...r.at> wrote:

> Hi ! 
> 
> Proposal for a minor optimization in update_migrate_disable - its only a few 
> instructions saved but those are in the hot path of locks so it might be worth
> it
> 
> When being scheduled out while migrate_disable > 0 and migrate_disabled_updated
> is not yet set we end up here (kernel/sched/core.c):
> 
> static inline void update_migrate_disable(struct task_struct *p)
> {
>         ...
> 
>         mask = tsk_cpus_allowed(p);
> 
>         if (p->sched_class->set_cpus_allowed)
>                 p->sched_class->set_cpus_allowed(p, mask);
>         p->nr_cpus_allowed = cpumask_weight(mask);
> 
> 
> as we only can get here if migrate_disable > 0 there is no need to calculate
> the cpumask_weight(mask) as tsk_cpus_allowed in that case will return
> cpumask_of(task_cpu(p)) which only can have a hamming weight of 1 anyway.
> So we can simply do:
> 
>         p->nr_cpus_allowed = 1;
> 
> without changing the behavior.
> 
> Patch is on top of 3.12.13-rt21
> 

Reviewed-by: Steven Rostedt <rostedt@...dmis.org>

-- Steve

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