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Date: Mon, 11 Jan 2021 22:47:39 +0000 From: Valentin Schneider <valentin.schneider@....com> To: Peter Zijlstra <peterz@...radead.org> Cc: Thomas Gleixner <tglx@...utronix.de>, Lai Jiangshan <jiangshanlai@...il.com>, linux-kernel@...r.kernel.org, Qian Cai <cai@...hat.com>, Vincent Donnefort <vincent.donnefort@....com>, Dexuan Cui <decui@...rosoft.com>, Lai Jiangshan <laijs@...ux.alibaba.com>, Paul McKenney <paulmck@...nel.org>, Vincent Guittot <vincent.guittot@...aro.org>, Steven Rostedt <rostedt@...dmis.org>, Jens Axboe <axboe@...nel.dk> Subject: Re: [PATCH -tip V3 0/8] workqueue: break affinity initiatively On 11/01/21 21:23, Peter Zijlstra wrote: > On Mon, Jan 11, 2021 at 07:21:06PM +0000, Valentin Schneider wrote: >> I'm less fond of the workqueue pcpu flag toggling, but it gets us what >> we want: allow those threads to run on !active CPUs during online, but >> move them away before !online during offline. >> >> Before I get ahead of myself, do we *actually* require that first part >> for workqueue kthreads? I'm thinking (raise alarm) we could try another >> approach of making them pcpu kthreads that don't abide by the !active && >> online rule. > > There is code that really requires percpu workqueues to be percpu. Such > code will flush the percpu workqueue on hotplug and never hit the unbind > scenario. > > Other code uses those same percpu workqueues and only uses it as a > performance enhancer, it likes things to stay local, but if not, meh.. > And these users are what got us the weird ass semantics of workqueue. > > Sadly workqueue itself can't tell them apart. > Oh well... FWIW now that I've unconfused myself, that does look okay. >> > --- >> > include/linux/kthread.h | 3 +++ >> > kernel/kthread.c | 25 ++++++++++++++++++++++++- >> > kernel/sched/core.c | 2 +- >> > kernel/sched/sched.h | 4 ++-- >> > kernel/smpboot.c | 1 + >> > kernel/workqueue.c | 12 +++++++++--- >> > 6 files changed, 40 insertions(+), 7 deletions(-) >> > >> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> > index 15d2562118d1..e71f9e44789e 100644 >> > --- a/kernel/sched/core.c >> > +++ b/kernel/sched/core.c >> > @@ -7277,7 +7277,7 @@ static void balance_push(struct rq *rq) >> > * Both the cpu-hotplug and stop task are in this case and are >> > * required to complete the hotplug process. >> > */ >> > - if (is_per_cpu_kthread(push_task) || is_migration_disabled(push_task)) { >> > + if (rq->idle == push_task || is_per_cpu_kthread(push_task) || is_migration_disabled(push_task)) { >> >> I take it the p->set_child_tid thing you were complaining about on IRC >> is what prevents us from having the idle task seen as a pcpu kthread? > > Yes, to to_kthread() only tests PF_KTHREAD and then assumes > p->set_child_tid points to struct kthread, _however_ init_task has > PF_KTHREAD set, but a NULL ->set_child_tid. > > This then means the idle thread for the boot CPU will malfunction with > to_kthread() and will certainly not have KTHREAD_IS_PER_CPU set. Per > construction (fork_idle()) none of the other idle threads will have that > cured either. > > For fun and giggles, init (pid-1) will have PF_KTHREAD set for a while > as well, until we exec /sbin/init. > > Anyway, idle will fail kthread_is_per_cpu(), and hence without the > above, we'll try and push the idle task away, which results in much > fail. > Quite! >> Also, shouldn't this be done before the previous set_cpus_allowed_ptr() >> call (in the same function)? > > Don't see why; we need nr_cpus_allowed == 1, so best do it after, right? > Duh, yes. >> That is, if we patch >> __set_cpus_allowed_ptr() to also use kthread_is_per_cpu(). > > That seems wrong. > It is, apologies. >> > list_add_tail(&worker->node, &pool->workers); >> > worker->pool = pool;
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