Messages in this thread | | | Date | Mon, 11 Jan 2021 10:09:07 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH -tip V3 0/8] workqueue: break affinity initiatively |
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On Mon, Jan 11, 2021 at 06:16:39PM +0100, Peter Zijlstra wrote: > > While thinking more about this, I'm thinking a big part of the problem > is that we're not dinstinguishing between geniuine per-cpu kthreads and > kthreads that just happen to be per-cpu. > > Geniuine per-cpu kthreads are kthread_bind() and have PF_NO_SETAFFINITY, > but sadly a lot of non-per-cpu kthreads, that might happen to still be > per-cpu also have that -- again workqueue does that even to it's unbound > workers :-( > > Now, anything created by smpboot, is created through > kthread_create_on_cpu() and that additionally sets to_kthread(p)->flags > KTHREAD_IS_PER_CPU. > > And I'm thinking that might be sufficient, if we modify > is_per_cpu_kthread() to check that, then we only match smpboot threads > (which include the hotplug and stopper threads, but notably not the idle > thread) > > Sadly it appears like io_uring() uses kthread_create_on_cpu() without > then having any hotplug crud on, so that needs additinoal frobbing. > > Also, init_task is PF_KTHREAD but doesn't have a struct kthread on.. and > I suppose bound workqueues don't go through this either. > > Let me rummage around a bit... > > This seems to not insta-explode... opinions?
It passes quick tests on -rcu both with and without the rcutorture fixes, which is encouraging. I will start a more vigorous test in about an hour.
Thanx, Paul
> --- > 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/include/linux/kthread.h b/include/linux/kthread.h > index 65b81e0c494d..fdd5a52e35d8 100644 > --- a/include/linux/kthread.h > +++ b/include/linux/kthread.h > @@ -33,6 +33,9 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data), > unsigned int cpu, > const char *namefmt); > > +void kthread_set_per_cpu(struct task_struct *k, bool set); > +bool kthread_is_per_cpu(struct task_struct *k); > + > /** > * kthread_run - create and wake a thread. > * @threadfn: the function to run until signal_pending(current). > diff --git a/kernel/kthread.c b/kernel/kthread.c > index a5eceecd4513..7f081530e459 100644 > --- a/kernel/kthread.c > +++ b/kernel/kthread.c > @@ -493,11 +493,34 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data), > return p; > kthread_bind(p, cpu); > /* CPU hotplug need to bind once again when unparking the thread. */ > - set_bit(KTHREAD_IS_PER_CPU, &to_kthread(p)->flags); > to_kthread(p)->cpu = cpu; > return p; > } > > +void kthread_set_per_cpu(struct task_struct *k, bool set) > +{ > + struct kthread *kthread = to_kthread(k); > + if (!kthread) > + return; > + > + if (set) { > + WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY)); > + WARN_ON_ONCE(k->nr_cpus_allowed != 1); > + set_bit(KTHREAD_IS_PER_CPU, &kthread->flags); > + } else { > + clear_bit(KTHREAD_IS_PER_CPU, &kthread->flags); > + } > +} > + > +bool kthread_is_per_cpu(struct task_struct *k) > +{ > + struct kthread *kthread = to_kthread(k); > + if (!kthread) > + return false; > + > + return test_bit(KTHREAD_IS_PER_CPU, &kthread->flags); > +} > + > /** > * kthread_unpark - unpark a thread created by kthread_create(). > * @k: thread created by kthread_create(). > 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)) { > /* > * If this is the idle task on the outgoing CPU try to wake > * up the hotplug control thread which might wait for the > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index 12ada79d40f3..3679f63e0aa2 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -2697,10 +2697,10 @@ static inline bool is_per_cpu_kthread(struct task_struct *p) > if (!(p->flags & PF_KTHREAD)) > return false; > > - if (p->nr_cpus_allowed != 1) > + if (!(p->flags & PF_NO_SETAFFINITY)) > return false; > > - return true; > + return kthread_is_per_cpu(p); > } > #endif > > diff --git a/kernel/smpboot.c b/kernel/smpboot.c > index 2efe1e206167..b0abe575a524 100644 > --- a/kernel/smpboot.c > +++ b/kernel/smpboot.c > @@ -188,6 +188,7 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu) > kfree(td); > return PTR_ERR(tsk); > } > + kthread_set_per_cpu(tsk, true); > /* > * Park the thread so that it could start right on the CPU > * when it is available. > diff --git a/kernel/workqueue.c b/kernel/workqueue.c > index 9880b6c0e272..824276e4fb2e 100644 > --- a/kernel/workqueue.c > +++ b/kernel/workqueue.c > @@ -1861,6 +1861,8 @@ static void worker_attach_to_pool(struct worker *worker, > */ > if (pool->flags & POOL_DISASSOCIATED) > worker->flags |= WORKER_UNBOUND; > + else > + kthread_set_per_cpu(worker->task, true); > > list_add_tail(&worker->node, &pool->workers); > worker->pool = pool; > @@ -4919,8 +4921,10 @@ static void unbind_workers(int cpu) > > raw_spin_unlock_irq(&pool->lock); > > - for_each_pool_worker(worker, pool) > - WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_active_mask) < 0); > + for_each_pool_worker(worker, pool) { > + kthread_set_per_cpu(worker->task, false); > + WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, cpu_possible_mask) < 0); > + } > > mutex_unlock(&wq_pool_attach_mutex); > > @@ -4972,9 +4976,11 @@ static void rebind_workers(struct worker_pool *pool) > * of all workers first and then clear UNBOUND. As we're called > * from CPU_ONLINE, the following shouldn't fail. > */ > - for_each_pool_worker(worker, pool) > + for_each_pool_worker(worker, pool) { > WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, > pool->attrs->cpumask) < 0); > + kthread_set_per_cpu(worker->task, true); > + } > > raw_spin_lock_irq(&pool->lock); >
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