Messages in this thread Patch in this message | | | Date | Fri, 1 Jul 2016 11:40:54 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC] sched: Make wake_up_nohz_cpu() handle CPUs going offline |
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On Fri, Jul 01, 2016 at 01:29:59AM +0200, Frederic Weisbecker wrote: > On Thu, Jun 30, 2016 at 10:58:45AM -0700, Paul E. McKenney wrote: > > Both timers and hrtimers are maintained on the outgoing CPU until > > CPU_DEAD time, at which point they are migrated to a surviving CPU. If a > > mod_timer() executes between CPU_DYING and CPU_DEAD time, x86 systems > > will splat in native_smp_send_reschedule() when attempting to wake up > > the just-now-offlined CPU, as shown below from a NO_HZ_FULL kernel: > > > > [ 7976.741556] WARNING: CPU: 0 PID: 661 at /home/paulmck/public_git/linux-rcu/arch/x86/kernel/smp.c:125 native_smp_send_reschedule+0x39/0x40 > > [ 7976.741595] Modules linked in: > > [ 7976.741595] CPU: 0 PID: 661 Comm: rcu_torture_rea Not tainted 4.7.0-rc2+ #1 > > [ 7976.741595] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 > > [ 7976.741595] 0000000000000000 ffff88000002fcc8 ffffffff8138ab2e 0000000000000000 > > [ 7976.741595] 0000000000000000 ffff88000002fd08 ffffffff8105cabc 0000007d1fd0ee18 > > [ 7976.741595] 0000000000000001 ffff88001fd16d40 ffff88001fd0ee00 ffff88001fd0ee00 > > [ 7976.741595] Call Trace: > > [ 7976.741595] [<ffffffff8138ab2e>] dump_stack+0x67/0x99 > > [ 7976.741595] [<ffffffff8105cabc>] __warn+0xcc/0xf0 > > [ 7976.741595] [<ffffffff8105cb98>] warn_slowpath_null+0x18/0x20 > > [ 7976.741595] [<ffffffff8103cba9>] native_smp_send_reschedule+0x39/0x40 > > [ 7976.741595] [<ffffffff81089bc2>] wake_up_nohz_cpu+0x82/0x190 > > [ 7976.741595] [<ffffffff810d275a>] internal_add_timer+0x7a/0x80 > > [ 7976.741595] [<ffffffff810d3ee7>] mod_timer+0x187/0x2b0 > > [ 7976.741595] [<ffffffff810c89dd>] rcu_torture_reader+0x33d/0x380 > > [ 7976.741595] [<ffffffff810c66f0>] ? sched_torture_read_unlock+0x30/0x30 > > [ 7976.741595] [<ffffffff810c86a0>] ? rcu_bh_torture_read_lock+0x80/0x80 > > [ 7976.741595] [<ffffffff8108068f>] kthread+0xdf/0x100 > > [ 7976.741595] [<ffffffff819dd83f>] ret_from_fork+0x1f/0x40 > > [ 7976.741595] [<ffffffff810805b0>] ? kthread_create_on_node+0x200/0x200 > > > > However, in this case, the wakeup is redundant, because the timer > > migration will reprogram timer hardware as needed. Note that the fact > > that preemption is disabled does not avoid the splat, as the offline > > operation has already passed both the synchronize_sched() and the > > stop_machine() that would be blocked by disabled preemption. > > > > This commit therefore modifies wake_up_nohz_cpu() to avoid attempting > > to wake up offline CPUs. It also adds a comment stating that the > > caller must tolerate lost wakeups when the target CPU is going offline, > > and suggesting the CPU_DEAD notifier as a recovery mechanism. > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > Cc: Peter Zijlstra <peterz@infradead.org> > > Cc: Frederic Weisbecker <fweisbec@gmail.com> > > Cc: Thomas Gleixner <tglx@linutronix.de> > > --- > > core.c | 7 ++++++- > > 1 file changed, 6 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 7f2cae4620c7..08502966e7df 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -590,9 +590,14 @@ static bool wake_up_full_nohz_cpu(int cpu) > > return false; > > } > > > > +/* > > + * Wake up the specified CPU. If the CPU is going offline, it is the > > + * caller's responsibility to deal with the lost wakeup, for example, > > + * by hooking into the CPU_DEAD notifier like timers and hrtimers do. > > + */ > > void wake_up_nohz_cpu(int cpu) > > { > > - if (!wake_up_full_nohz_cpu(cpu)) > > + if (cpu_online(cpu) && !wake_up_full_nohz_cpu(cpu)) > > So at this point, as we passed CPU_DYING, I believe the CPU isn't visible in the domains > anymore (correct me if I'm wrong), therefore get_nohz_timer_target() can't return it, > unless smp_processor_id() is the only alternative.
Right, but the timers have been posted long before even CPU_UP_PREPARE. From what I can see, they are left alone until CPU_DEAD. Which means that if you try to mod_timer() them between CPU_DYING and CPU_DEAD, you can get the above splat.
Or am I missing somthing subtle here?
> Hence, that call to wake_up_nohz_cpu() can only happen to online CPUs or the current > one (pinned). And wake_up_idle_cpu() on the current CPU is a no-op. So only > wake_up_full_nohz_cpu() is concerned. Then perhaps it would be better to move that > cpu_online() check to wake_up_full_nohz_cpu() ?
As in the patch shown below? Either way works for me.
> BTW, it seems that rcutorture stops its kthreads after CPU_DYING, is it expected that > it queues timers at this stage?
Hmmm... From what I can see, rcutorture cleans up its priority-boost kthreads at CPU_DOWN_PREPARE time. The other threads are allowed to migrate wherever the scheduler wants, give or take the task shuffling. The task shuffling only excludes one CPU at a time, and I have seen this occur when multiple CPUs were running, e.g., 0, 2, and 3 while offlining 1.
Besides which, doesn't the scheduler prevent anything but the idle thread from running after CPU_DYING time?
Thanx, Paul
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diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 7f2cae4620c7..08502966e7df 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -590,9 +590,14 @@ static bool wake_up_full_nohz_cpu(int cpu) return false; } +/* + * Wake up the specified CPU. If the CPU is going offline, it is the + * caller's responsibility to deal with the lost wakeup, for example, + * by hooking into the CPU_DEAD notifier like timers and hrtimers do. + */ void wake_up_nohz_cpu(int cpu) { - if (!wake_up_full_nohz_cpu(cpu)) + if (cpu_online(cpu) && !wake_up_full_nohz_cpu(cpu)) wake_up_idle_cpu(cpu); }
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