From: Peter Zijlstra <peterz@infradead.org> To: Alexey Klimov <aklimov@redhat.com> Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, yury.norov@gmail.com, tglx@linutronix.de, jobaker@redhat.com, audralmitchel@gmail.com, arnd@arndb.de, gregkh@linuxfoundation.org, rafael@kernel.org, tj@kernel.org, lizefan@huawei.com, qais.yousef@arm.com, hannes@cmpxchg.org, klimov.linux@gmail.com Subject: Re: [RFC][PATCH] cpu/hotplug: wait for cpuset_hotplug_work to finish on cpu online Date: Mon, 7 Dec 2020 09:38:27 +0100 [thread overview] Message-ID: <20201207083827.GD3040@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <20201203171431.256675-1-aklimov@redhat.com> On Thu, Dec 03, 2020 at 05:14:31PM +0000, Alexey Klimov wrote: > When a CPU offlined and onlined via device_offline() and device_online() > the userspace gets uevent notification. If, after receiving uevent, > userspace executes sched_setaffinity() on some task trying to move it > to a recently onlined CPU, then it will fail with -EINVAL. Userspace needs > to wait around 5..30 ms before sched_setaffinity() will succeed for > recently onlined CPU after receiving uevent. Right. > Unfortunately, the execution time of > echo 1 > /sys/devices/system/cpu/cpuX/online roughly doubled with this > change (on my test machine). Nobody cares, it's hotplug, it's supposed to be slow :-) That is, we fundamentally shift the work _to_ the hotplug path, so as to keep everybody else fast. > The nature of this bug is also described here (with different consequences): > https://lore.kernel.org/lkml/20200211141554.24181-1-qais.yousef@arm.com/ Yeah, pesky deadlocks.. someone was going to try again. > kernel/cpu.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/kernel/cpu.c b/kernel/cpu.c > index 6ff2578ecf17..f39a27a7f24b 100644 > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -15,6 +15,7 @@ > #include <linux/sched/smt.h> > #include <linux/unistd.h> > #include <linux/cpu.h> > +#include <linux/cpuset.h> > #include <linux/oom.h> > #include <linux/rcupdate.h> > #include <linux/export.h> > @@ -1275,6 +1276,8 @@ static int cpu_up(unsigned int cpu, enum cpuhp_state target) > } > > err = _cpu_up(cpu, 0, target); > + if (!err) > + cpuset_wait_for_hotplug(); > out: > cpu_maps_update_done(); > return err; My only consideration is if doing that flush under cpu_add_remove_lock() is wise.
WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org> To: Alexey Klimov <aklimov-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> Cc: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, yury.norov-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org, jobaker-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, audralmitchel-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, arnd-r2nGTMty4D4@public.gmane.org, gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org, rafael-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org, qais.yousef-5wv7dgnIgG8@public.gmane.org, hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org, klimov.linux-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org Subject: Re: [RFC][PATCH] cpu/hotplug: wait for cpuset_hotplug_work to finish on cpu online Date: Mon, 7 Dec 2020 09:38:27 +0100 [thread overview] Message-ID: <20201207083827.GD3040@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <20201203171431.256675-1-aklimov-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> On Thu, Dec 03, 2020 at 05:14:31PM +0000, Alexey Klimov wrote: > When a CPU offlined and onlined via device_offline() and device_online() > the userspace gets uevent notification. If, after receiving uevent, > userspace executes sched_setaffinity() on some task trying to move it > to a recently onlined CPU, then it will fail with -EINVAL. Userspace needs > to wait around 5..30 ms before sched_setaffinity() will succeed for > recently onlined CPU after receiving uevent. Right. > Unfortunately, the execution time of > echo 1 > /sys/devices/system/cpu/cpuX/online roughly doubled with this > change (on my test machine). Nobody cares, it's hotplug, it's supposed to be slow :-) That is, we fundamentally shift the work _to_ the hotplug path, so as to keep everybody else fast. > The nature of this bug is also described here (with different consequences): > https://lore.kernel.org/lkml/20200211141554.24181-1-qais.yousef-5wv7dgnIgG8@public.gmane.org/ Yeah, pesky deadlocks.. someone was going to try again. > kernel/cpu.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/kernel/cpu.c b/kernel/cpu.c > index 6ff2578ecf17..f39a27a7f24b 100644 > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -15,6 +15,7 @@ > #include <linux/sched/smt.h> > #include <linux/unistd.h> > #include <linux/cpu.h> > +#include <linux/cpuset.h> > #include <linux/oom.h> > #include <linux/rcupdate.h> > #include <linux/export.h> > @@ -1275,6 +1276,8 @@ static int cpu_up(unsigned int cpu, enum cpuhp_state target) > } > > err = _cpu_up(cpu, 0, target); > + if (!err) > + cpuset_wait_for_hotplug(); > out: > cpu_maps_update_done(); > return err; My only consideration is if doing that flush under cpu_add_remove_lock() is wise.
next prev parent reply other threads:[~2020-12-07 8:39 UTC|newest] Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-12-03 17:14 [RFC][PATCH] cpu/hotplug: wait for cpuset_hotplug_work to finish on cpu online Alexey Klimov 2020-12-03 17:14 ` Alexey Klimov 2020-12-07 8:38 ` Peter Zijlstra [this message] 2020-12-07 8:38 ` Peter Zijlstra 2020-12-09 2:40 ` Daniel Jordan 2020-12-09 2:40 ` Daniel Jordan 2021-01-15 3:21 ` Daniel Jordan 2021-01-15 3:21 ` Daniel Jordan 2021-01-19 16:52 ` Alexey Klimov 2021-01-19 16:52 ` Alexey Klimov 2021-01-21 2:09 ` Daniel Jordan 2020-12-09 2:09 ` Daniel Jordan
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