linux-kernel - Re: [PATCH 1/7] sched: Update rq clock on nohz CPU before migrating tasks

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Date:	Mon, 8 Apr 2013 13:48:51 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Mike Galbraith <efault@....de>
Subject: Re: [PATCH 1/7] sched: Update rq clock on nohz CPU before migrating
 tasks


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> @@ -1115,6 +1116,12 @@ static inline void dec_nr_running(struct rq *rq)
>  
>  extern void update_rq_clock(struct rq *rq);
>  
> +static inline void update_nohz_rq_clock(struct rq *rq)
> +{
> +	if (tick_nohz_extended_cpu(cpu_of(rq)))
> +		update_rq_clock(rq);
> +}

A minor comment: instead of implicitly knowing that full nohz CPUs mean a stale 
rq_clock, how about adding this information to the rq-> itself?

Something like introducing rq->clock_valid, initializing it to 1, and setting it 
to 0 when a CPU stops the tick.

(This would also allow the debug detection of sched_clock() use of stale values.)

We already have a similar flag: rq->skip_clock_update. I'd suggest to introduce a 
'struct sched_clock' helper structure and add the flags and scheduler clock fields 
as:

   rq->clock                 ->       rq->clock.cpu
   rq->clock_task            ->       rq->clock.task
   rq->clock_valid           ->       rq->clock.valid
   rq->clock_skip_uipdate    ->       rq->clock.skip_update
   rq->hrtick_timer          ->       rq->clock.hrtick_timer

   rq->prev_irq_time         ->       rq->clock.prev_irq_time
   rq->prev_steal_time       ->       rq->clock.prev_steal_time
   rq->prev_steal_time_rq    ->       rq->clock.prev_steal_time_rq

Thanks,

	ngo
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