From: Will Deacon <will@kernel.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
linux-kernel@vger.kernel.org,
Catalin Marinas <catalin.marinas@arm.com>,
Marc Zyngier <maz@kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Morten Rasmussen <morten.rasmussen@arm.com>,
Qais Yousef <qais.yousef@arm.com>,
Suren Baghdasaryan <surenb@google.com>,
Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>,
Johannes Weiner <hannes@cmpxchg.org>,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
"Rafael J. Wysocki" <rjw@rjwysocki.net>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
kernel-team@android.com
Subject: Re: [PATCH v7 01/22] sched: Favour predetermined active CPU as migration destination
Date: Wed, 26 May 2021 17:03:18 +0100 [thread overview]
Message-ID: <20210526160317.GB19691@willie-the-truck> (raw)
In-Reply-To: <877djlhhmb.mognet@arm.com>
On Wed, May 26, 2021 at 12:14:20PM +0100, Valentin Schneider wrote:
> On 25/05/21 16:14, Will Deacon wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 5226cc26a095..1702a60d178d 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1869,6 +1869,7 @@ static struct rq *move_queued_task(struct rq *rq, struct rq_flags *rf,
> > struct migration_arg {
> > struct task_struct *task;
> > int dest_cpu;
> > + const struct cpumask *dest_mask;
> > struct set_affinity_pending *pending;
> > };
> >
> > @@ -1917,6 +1918,7 @@ static int migration_cpu_stop(void *data)
> > struct set_affinity_pending *pending = arg->pending;
> > struct task_struct *p = arg->task;
> > int dest_cpu = arg->dest_cpu;
> > + const struct cpumask *dest_mask = arg->dest_mask;
> > struct rq *rq = this_rq();
> > bool complete = false;
> > struct rq_flags rf;
> > @@ -1956,12 +1958,8 @@ static int migration_cpu_stop(void *data)
> > complete = true;
> > }
> >
> > - if (dest_cpu < 0) {
> > - if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask))
> > - goto out;
> > -
> > - dest_cpu = cpumask_any_distribute(&p->cpus_mask);
> > - }
> > + if (dest_mask && (cpumask_test_cpu(task_cpu(p), dest_mask)))
> > + goto out;
> >
>
> IIRC the reason we deferred the pick to migration_cpu_stop() was because of
> those insane races involving multiple SCA calls the likes of:
>
> p->cpus_mask = [0, 1]; p on CPU0
>
> CPUx CPUy CPU0
>
> SCA(p, [2])
> __do_set_cpus_allowed();
> queue migration_cpu_stop()
> SCA(p, [3])
> __do_set_cpus_allowed();
> migration_cpu_stop()
>
> The stopper needs to use the latest cpumask set by the second SCA despite
> having an arg->pending set up by the first SCA. Doesn't this break here?
Yes, well spotted. I was so caught up with the hotplug race that I didn't
even consider a straightforward SCA race. Hurumph.
> I'm not sure I've paged back in all of the subtleties laying in ambush
> here, but what about the below?
I can't break it, but I'm also not very familiar with this code. Please can
you post it as a proper patch so that I drop this from my series?
Thanks,
Will
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next prev parent reply other threads:[~2021-05-26 17:14 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-25 15:14 [PATCH v7 00/22] Add support for 32-bit tasks on asymmetric AArch32 systems Will Deacon
2021-05-25 15:14 ` [PATCH v7 01/22] sched: Favour predetermined active CPU as migration destination Will Deacon
2021-05-26 11:14 ` Valentin Schneider
2021-05-26 12:32 ` Peter Zijlstra
2021-05-26 12:36 ` Valentin Schneider
2021-05-26 16:03 ` Will Deacon [this message]
2021-05-26 17:46 ` Valentin Schneider
2021-05-25 15:14 ` [PATCH v7 02/22] arm64: cpuinfo: Split AArch32 registers out into a separate struct Will Deacon
2021-05-25 15:14 ` [PATCH v7 03/22] arm64: Allow mismatched 32-bit EL0 support Will Deacon
2021-05-25 15:14 ` [PATCH v7 04/22] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched " Will Deacon
2021-05-25 15:14 ` [PATCH v7 05/22] arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs Will Deacon
2021-05-25 15:14 ` [PATCH v7 06/22] arm64: Advertise CPUs capable of running 32-bit applications in sysfs Will Deacon
2021-05-25 15:14 ` [PATCH v7 07/22] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection Will Deacon
2021-05-25 15:14 ` [PATCH v7 08/22] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1 Will Deacon
2021-05-26 15:02 ` Peter Zijlstra
2021-05-26 16:07 ` Will Deacon
2021-05-25 15:14 ` [PATCH v7 09/22] cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus() Will Deacon
2021-05-25 15:14 ` [PATCH v7 10/22] sched: Reject CPU affinity changes based on task_cpu_possible_mask() Will Deacon
2021-05-26 15:15 ` Peter Zijlstra
2021-05-26 16:12 ` Will Deacon
2021-05-26 17:56 ` Peter Zijlstra
2021-05-26 18:59 ` Will Deacon
2021-05-25 15:14 ` [PATCH v7 11/22] sched: Introduce task_struct::user_cpus_ptr to track requested affinity Will Deacon
2021-05-25 15:14 ` [PATCH v7 12/22] sched: Split the guts of sched_setaffinity() into a helper function Will Deacon
2021-05-25 15:14 ` [PATCH v7 13/22] sched: Allow task CPU affinity to be restricted on asymmetric systems Will Deacon
2021-05-26 16:20 ` Peter Zijlstra
2021-05-26 16:35 ` Will Deacon
2021-05-26 16:30 ` Peter Zijlstra
2021-05-26 17:02 ` Will Deacon
2021-05-27 7:56 ` Peter Zijlstra
2021-05-25 15:14 ` [PATCH v7 14/22] sched: Introduce task_cpus_dl_admissible() to check proposed affinity Will Deacon
2021-05-25 15:14 ` [PATCH v7 15/22] freezer: Add frozen_or_skipped() helper function Will Deacon
2021-05-25 15:14 ` [PATCH v7 16/22] sched: Defer wakeup in ttwu() for unschedulable frozen tasks Will Deacon
2021-05-27 14:10 ` Peter Zijlstra
2021-05-27 14:31 ` Peter Zijlstra
2021-05-27 14:44 ` Will Deacon
2021-05-27 14:55 ` Peter Zijlstra
2021-05-27 14:50 ` Peter Zijlstra
2021-05-28 10:49 ` Peter Zijlstra
2021-05-27 14:36 ` Will Deacon
2021-06-01 8:21 ` [RFC][PATCH] freezer,sched: Rewrite core freezer logic Peter Zijlstra
2021-06-01 11:27 ` Peter Zijlstra
2021-06-02 12:54 ` Will Deacon
2021-06-03 10:35 ` Peter Zijlstra
2021-06-03 10:58 ` Will Deacon
2021-06-03 11:26 ` Peter Zijlstra
2021-06-03 11:36 ` Will Deacon
2021-05-25 15:14 ` [PATCH v7 17/22] arm64: Implement task_cpu_possible_mask() Will Deacon
2021-05-25 15:14 ` [PATCH v7 18/22] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0 Will Deacon
2021-05-25 15:14 ` [PATCH v7 19/22] arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system Will Deacon
2021-05-25 15:14 ` [PATCH v7 20/22] arm64: Hook up cmdline parameter to allow mismatched 32-bit EL0 Will Deacon
2021-05-25 15:14 ` [PATCH v7 21/22] arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores Will Deacon
2021-05-25 15:14 ` [PATCH v7 22/22] Documentation: arm64: describe asymmetric 32-bit support Will Deacon
2021-05-25 17:13 ` Marc Zyngier
2021-05-25 17:27 ` Will Deacon
2021-05-25 18:11 ` Marc Zyngier
2021-05-26 16:00 ` Will Deacon
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