Messages in this thread | | | Date | Mon, 15 Aug 2016 16:01:34 +0100 | From | Morten Rasmussen <> | Subject | Re: [PATCH v3 09/13] sched/fair: Let asymmetric cpu configurations balance at wake-up |
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On Mon, Aug 15, 2016 at 03:39:49PM +0200, Peter Zijlstra wrote: > On Mon, Jul 25, 2016 at 02:34:26PM +0100, Morten Rasmussen wrote: > > > Because I forgot _again_, I added: > > /* > * Disable WAKE_AFFINE in the case where task @p doesn't fit in the > * capacity of either the waking CPU @cpu or the previous CPU @prev_cpu. > * > * In that case WAKE_AFFINE doesn't make sense and we'll let > * BALANCE_WAKE sort things out. > */
Thanks.
> > > +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?
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.
However, as you say, I'm not sure if we care that much.
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?
> > > + /* Minimum capacity is close to max, no need to abort wake_affine */ > > + if (max_cap - min_cap < max_cap >> 3) > > + return 0; > > + > > + return min_cap * 1024 < task_util(p) * capacity_margin; > > +} > > + > > /* > > * select_task_rq_fair: Select target runqueue for the waking task in domains > > * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE, > > @@ -5389,7 +5414,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f > > > > if (sd_flag & SD_BALANCE_WAKE) { > > record_wakee(p); > > - want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p)); > > + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) > > + && cpumask_test_cpu(cpu, tsk_cpus_allowed(p)); > > } > > > > rcu_read_lock(); > > -- > > 1.9.1 > >
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