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From: Peter Zijlstra <peterz@infradead.org>
To: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: mingo@redhat.com, dietmar.eggemann@arm.com, yuyang.du@intel.com,
	vincent.guittot@linaro.org, mgalbraith@suse.de,
	sgurrappadi@nvidia.com, freedom.tan@mediatek.com,
	keita.kobayashi.ym@renesas.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 09/13] sched/fair: Let asymmetric cpu configurations balance at wake-up
Date: Mon, 15 Aug 2016 17:10:06 +0200	[thread overview]
Message-ID: <20160815151006.GD30192@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20160815150132.GC3391@e105550-lin.cambridge.arm.com>

On Mon, Aug 15, 2016 at 04:01:34PM +0100, Morten Rasmussen wrote:
> On Mon, Aug 15, 2016 at 03:39:49PM +0200, Peter Zijlstra wrote:
> > > +static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
> > > +{
> > > +	long min_cap, max_cap;
> > > +
> > > +	min_cap = min(capacity_orig_of(prev_cpu), capacity_orig_of(cpu));
> > > +	max_cap = cpu_rq(cpu)->rd->max_cpu_capacity;
> > 
> > There's a tiny hole here, which I'm fairly sure we don't care about. If
> > @p last ran on @prev_cpu before @prev_cpu was split from @rd this
> > doesn't 'work' right.
> 
> I hadn't considered that. What is 'working right' in this scenario?
> Ignoring @prev_cpu as it isn't a valid option anymore?

Probably, yeah.

> In that case, since @prev_cpu is only used as part the min() it should
> only cause min_cap to be potentially smaller than it should be, not
> larger. It could lead us to let BALANCE_WAKE take over in scenarios
> where select_idle_sibling() would have been sufficient, but it should
> harm.

+not, right?

> However, as you say, I'm not sure if we care that much.

Yeah, don't think so, its extremely unlikely to happen, almost nobody
mucks about with root_domains anyway. And those that do, do so once to
setup things and then leave them be.

> Talking about @rd, I discussed with Juri and Dietmar the other week
> whether the root_domain is RCU protected, and if we therefore have to
> move the call to wake_cap() after the rcu_read_lock() below. I haven't
> yet done thorough investigation to find the answer. Should it be
> protected?

Yeah, I think either RCU or RCU-sched, I forever forget.

  reply	other threads:[~2016-08-15 15:10 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-07-25 13:34 [PATCH v3 00/13] sched: Clean-ups and asymmetric cpu capacity support Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 01/13] sched: Fix power to capacity renaming in comment Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 02/13] sched/fair: Consistent use of prev_cpu in wakeup path Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 03/13] sched/fair: Optimize find_idlest_cpu() when there is no choice Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 04/13] sched/core: Remove unnecessary null-pointer check Morten Rasmussen
2016-08-18 10:56   ` [tip:sched/core] sched/core: Remove unnecessary NULL-pointer check tip-bot for Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 05/13] sched: Introduce SD_ASYM_CPUCAPACITY sched_domain topology flag Morten Rasmussen
2016-08-15 10:54   ` Peter Zijlstra
2016-08-15 11:43     ` Morten Rasmussen
2016-08-18 10:56     ` [tip:sched/core] sched/core: Clarify SD_flags comment tip-bot for Peter Zijlstra
2016-08-17  8:42   ` [PATCH v3 05/13] sched: Introduce SD_ASYM_CPUCAPACITY sched_domain topology flag Wanpeng Li
2016-08-17  9:23     ` Morten Rasmussen
2016-08-17  9:26       ` Wanpeng Li
2016-08-18 10:56   ` [tip:sched/core] sched/core: " tip-bot for Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 06/13] sched/core: Pass child domain into sd_init Morten Rasmussen
2016-08-18 10:57   ` [tip:sched/core] sched/core: Pass child domain into sd_init() tip-bot for Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 07/13] sched: Enable SD_BALANCE_WAKE for asymmetric capacity systems Morten Rasmussen
2016-08-18 10:57   ` [tip:sched/core] sched/core: " tip-bot for Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 08/13] sched: Store maximum per-cpu capacity in root domain Morten Rasmussen
2016-08-01 18:53   ` Dietmar Eggemann
2016-08-16 12:24     ` Vincent Guittot
2016-08-18 10:58     ` [tip:sched/core] sched/core: Store maximum per-CPU " tip-bot for Dietmar Eggemann
2016-07-25 13:34 ` [PATCH v3 09/13] sched/fair: Let asymmetric cpu configurations balance at wake-up Morten Rasmussen
2016-08-15 13:39   ` Peter Zijlstra
2016-08-15 15:01     ` Morten Rasmussen
2016-08-15 15:10       ` Peter Zijlstra [this message]
2016-08-15 15:30         ` Morten Rasmussen
2016-08-18 10:58   ` [tip:sched/core] sched/fair: Let asymmetric CPU " tip-bot for Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 10/13] sched/fair: Compute task/cpu utilization at wake-up more correctly Morten Rasmussen
2016-08-15 14:23   ` Peter Zijlstra
2016-08-15 15:42     ` Morten Rasmussen
2016-08-18  8:40       ` Morten Rasmussen
2016-08-18 10:24         ` Morten Rasmussen
2016-08-18 11:46           ` Wanpeng Li
2016-08-18 13:45             ` Morten Rasmussen
2016-08-19  1:43               ` Wanpeng Li
2016-08-19 14:03                 ` Morten Rasmussen
2016-08-22  1:48                   ` Wanpeng Li
2016-08-22 11:29                     ` Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 11/13] sched/fair: Consider spare capacity in find_idlest_group() Morten Rasmussen
2016-08-16 13:57   ` Vincent Guittot
2016-08-18 11:16     ` Morten Rasmussen
2016-08-18 12:28       ` Peter Zijlstra
2016-07-25 13:34 ` [PATCH v3 12/13] sched: Add per-cpu min capacity to sched_group_capacity Morten Rasmussen
2016-07-25 13:34 ` [PATCH v3 13/13] sched/fair: Avoid pulling tasks from non-overloaded higher capacity groups Morten Rasmussen

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