linux-kernel - Re: [PATCH V3]hrtimer: Fix a performance regression by disable reprogramming in remove

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Date:	Thu, 8 Aug 2013 23:02:06 +0800
From:	"ethan.zhao" <ethan.kernel@...il.com>
To:	Mike Galbraith <bitbucket@...ine.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, johlstei@...eaurora.org,
	Yinghai Lu <yinghai@...nel.org>, Jin Feng <joe.jin@...cle.com>
Subject: Re: [PATCH V3]hrtimer: Fix a performance regression by disable reprogramming in remove_hrtimer


在 2013-8-6，下午3:29，Mike Galbraith <bitbucket@...ine.de> 写道：

> +int sched_needs_cpu(int cpu)
> +{
> +	return  cpu_rq(cpu)->avg_idle < sysctl_sched_migration_cost;
> +}
> +
> #else /* CONFIG_NO_HZ_COMMON */
> 
> static inline bool got_nohz_idle_kick(void)
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -548,7 +548,7 @@ static ktime_t tick_nohz_stop_sched_tick
> 		time_delta = timekeeping_max_deferment();
> 	} while (read_seqretry(&jiffies_lock, seq));
> 
> -	if (rcu_needs_cpu(cpu, &rcu_delta_jiffies) ||
> +	if (sched_needs_cpu(cpu) || rcu_needs_cpu(cpu, &rcu_delta_jiffies) ||
> 	    arch_needs_cpu(cpu) || irq_work_needs_cpu()) {
> 		next_jiffies = last_jiffies + 1;
> 		delta_jiffies = 1;

If the performace regression was caused by too much expensive clock device reprogramming and too frequent entering /exiting of C-states…   this patch should work.
except the following result is almost always false under 3.11-rc3 code.

> return  cpu_rq(cpu)->avg_idle < sysctl_sched_migration_cost;



Ethan

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