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Message-Id: <D29JDHZSVCII.YCP01SIB9FYA@gmail.com>
Date: Wed, 26 Jun 2024 10:52:45 +1000
From: "Nicholas Piggin" <npiggin@...il.com>
To: "Tejun Heo" <tj@...nel.org>
Cc: "Paul E . McKenney" <paulmck@...nel.org>, "Peter Zijlstra"
<peterz@...radead.org>, "Lai Jiangshan" <jiangshanlai@...il.com>, "Srikar
Dronamraju" <srikar@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] workqueue: Improve scalability of workqueue
watchdog touch
On Wed Jun 26, 2024 at 2:57 AM AEST, Tejun Heo wrote:
> On Tue, Jun 25, 2024 at 09:42:45PM +1000, Nicholas Piggin wrote:
> > On a ~2000 CPU powerpc system, hard lockups have been observed in the
> > workqueue code when stop_machine runs (in this case due to CPU hotplug).
> > This is due to lots of CPUs spinning in multi_cpu_stop, calling
> > touch_nmi_watchdog() which ends up calling wq_watchdog_touch().
> > wq_watchdog_touch() writes to the global variable wq_watchdog_touched,
> > and that can find itself in the same cacheline as other important
> > workqueue data, which slows down operations to the point of lockups.
> >
> > In the case of the following abridged trace, worker_pool_idr was in
> > the hot line, causing the lockups to always appear at idr_find.
> >
> > watchdog: CPU 1125 self-detected hard LOCKUP @ idr_find
> > Call Trace:
> > get_work_pool
> > __queue_work
> > call_timer_fn
> > run_timer_softirq
> > __do_softirq
> > do_softirq_own_stack
> > irq_exit
> > timer_interrupt
> > decrementer_common_virt
> > * interrupt: 900 (timer) at multi_cpu_stop
> > multi_cpu_stop
> > cpu_stopper_thread
> > smpboot_thread_fn
> > kthread
> >
> > Fix this by having wq_watchdog_touch() only write to the line if the
> > last time a touch was recorded exceeds 1/4 of the watchdog threshold.
> >
> > Reported-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> > Signed-off-by: Nicholas Piggin <npiggin@...il.com>
>
> Applied 1-2 to wq/for-6.11.
Thanks Tejun.
> I think 3 and 4 should probably be routed
> through either tip or Andrew?
Yeah, let's see if it gets any comments.
Thanks,
Nick
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