Messages in this thread | | | Date | Tue, 15 Dec 2020 09:49:14 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 00/10] workqueue: break affinity initiatively |
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On Tue, Dec 15, 2020 at 04:14:26PM +0800, Lai Jiangshan wrote: > On Tue, Dec 15, 2020 at 3:50 PM Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Tue, Dec 15, 2020 at 01:44:53PM +0800, Lai Jiangshan wrote: > > > I don't know how the scheduler distinguishes all these > > > different cases under the "new assumption". > > > > The special case is: > > > > (p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1 > > > > > > So unbound per-node workers can possibly match this test. So there is code > needed to handle for unbound workers/pools which is done by this patchset.
Curious; how could a per-node worker match this? Only if the node is a single CPU, or otherwise too?
> Is this the code of is_per_cpu_kthread()? I think I should have also > used this function in workqueue and don't break affinity for unbound > workers have more than 1 cpu.
Yes, that function captures it. If you want to use it, feel free to move it to include/linux/sched.h.
This class of threads is 'special', since it needs to violate the regular hotplug rules, and migrate_disable() made it just this little bit more special. It basically comes down to how we need certain per-cpu kthreads to run on a CPU while it's brought up, before userspace is allowed on, and similarly they need to run on the CPU after userspace is no longer allowed on in order to bring it down.
(IOW, they must be allowed to violate the active mask)
Due to migrate_disable() we had to move the migration code from the very last cpu-down stage, to earlier. This in turn brought the expectation (which is normally met) that per-cpu kthreads will stop/park or otherwise make themselves scarce when the CPU goes down. We can no longer force migrate them.
Workqueues are the sole exception to that, they've got some really 'dodgy' hotplug behaviour.
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