From: Valentin Schneider <valentin.schneider@arm.com>
To: Peter Zijlstra <peterz@infradead.org>,
tglx@linutronix.de, mingo@kernel.org, bigeasy@linutronix.de,
swood@redhat.com, juri.lelli@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
bristot@redhat.com, vincent.donnefort@arm.com,
qais.yousef@arm.com
Cc: linux-kernel@vger.kernel.org, peterz@infradead.org
Subject: Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback
Date: Thu, 11 Mar 2021 15:13:04 +0000 [thread overview]
Message-ID: <871rclu3jz.mognet@e113632-lin.i-did-not-set--mail-host-address--so-tickle-me> (raw)
In-Reply-To: <20210310150109.259726371@infradead.org>
Peter Zijlstra <peterz@infradead.org> writes:
> @@ -7883,14 +7889,6 @@ int sched_cpu_deactivate(unsigned int cp
> set_cpu_active(cpu, false);
>
> /*
> - * From this point forward, this CPU will refuse to run any task that
> - * is not: migrate_disable() or KTHREAD_IS_PER_CPU, and will actively
> - * push those tasks away until this gets cleared, see
> - * sched_cpu_dying().
> - */
> - balance_push_set(cpu, true);
> -
> - /*
> * We've cleared cpu_active_mask / set balance_push, wait for all
> * preempt-disabled and RCU users of this state to go away such that
> * all new such users will observe it.
> @@ -7910,6 +7908,14 @@ int sched_cpu_deactivate(unsigned int cp
> }
> rq_unlock_irqrestore(rq, &rf);
>
> + /*
> + * From this point forward, this CPU will refuse to run any task that
> + * is not: migrate_disable() or KTHREAD_IS_PER_CPU, and will actively
> + * push those tasks away until this gets cleared, see
> + * sched_cpu_dying().
> + */
> + balance_push_set(cpu, true);
> +
AIUI with cpu_dying_mask being flipped before even entering
sched_cpu_deactivate(), we don't need this to be before the
synchronize_rcu() anymore; is there more than that to why you're punting it
back this side of it?
> #ifdef CONFIG_SCHED_SMT
> /*
> * When going down, decrement the number of cores with SMT present.
> @@ -8206,7 +8212,7 @@ void __init sched_init(void)
> rq->sd = NULL;
> rq->rd = NULL;
> rq->cpu_capacity = rq->cpu_capacity_orig = SCHED_CAPACITY_SCALE;
> - rq->balance_callback = NULL;
> + rq->balance_callback = &balance_push_callback;
> rq->active_balance = 0;
> rq->next_balance = jiffies;
> rq->push_cpu = 0;
> @@ -8253,6 +8259,7 @@ void __init sched_init(void)
>
> #ifdef CONFIG_SMP
> idle_thread_set_boot_cpu();
> + balance_push_set(smp_processor_id(), false);
> #endif
> init_sched_fair_class();
>
I don't get what these two changes do - the end result is the same as
before, no?
Also, AIUI this patch covers the cpu_dying -> !cpu_dying rollback case
since balance_push gets numbed down by !cpu_dying. What about the other way
around (hot-plug failure + rollback)? We may have allowed !pcpu tasks on the
now-dying CPU, and we'd need to re-install the balance_push callback.
I'm starting to think we'd need to have
rq->balance_callback = &balance_push_callback
for any CPU with hotplug state < CPUHP_AP_ACTIVE. Thus we would
need:
balance_push_set(cpu, true) in sched_init() and sched_cpu_deactivate()
balance_push_set(cpu, false) in sched_cpu_activate()
and the rest would be driven by the cpu_dying_mask.
next prev parent reply other threads:[~2021-03-11 15:14 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-10 14:52 [PATCH 0/3] sched: Fix remaining balance_push vs hotplug hole Peter Zijlstra
2021-03-10 14:52 ` [PATCH 1/3] cpumask: Make cpu_{online,possible,present,active}() inline Peter Zijlstra
2021-04-16 15:53 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-03-10 14:53 ` [PATCH 2/3] cpumask: Introduce DYING mask Peter Zijlstra
2021-03-21 19:30 ` Qais Yousef
2021-03-22 15:07 ` Steven Rostedt
2021-04-12 10:55 ` Peter Zijlstra
2021-04-12 11:16 ` Qais Yousef
2021-04-16 15:53 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-03-10 14:53 ` [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback Peter Zijlstra
2021-03-11 15:13 ` Valentin Schneider [this message]
2021-03-11 16:42 ` Peter Zijlstra
2021-04-12 12:03 ` Peter Zijlstra
2021-04-12 17:22 ` Valentin Schneider
2021-04-13 6:51 ` Peter Zijlstra
2021-04-15 8:59 ` Peter Zijlstra
2021-04-15 14:32 ` Valentin Schneider
2021-04-15 15:29 ` Peter Zijlstra
2021-04-15 15:34 ` Valentin Schneider
2021-04-19 10:56 ` Vincent Donnefort
2021-04-20 9:46 ` Vincent Donnefort
2021-04-20 14:20 ` Peter Zijlstra
2021-04-20 14:39 ` Peter Zijlstra
2021-04-20 14:58 ` Peter Zijlstra
2021-04-20 16:53 ` Vincent Donnefort
2021-04-20 18:07 ` Peter Zijlstra
2021-04-21 9:32 ` Valentin Schneider
2021-04-22 7:36 ` [tip: sched/core] cpumask/hotplug: Fix cpu_dying() state tracking tip-bot2 for Peter Zijlstra
2021-04-16 15:53 ` [tip: sched/core] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback tip-bot2 for Peter Zijlstra
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