linux-kernel - Re: [PATCH 0/4] Fix scalability problem in workqueue watchdog touch caused by stop

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Date: Tue, 25 Jun 2024 07:53:59 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Nicholas Piggin <npiggin@...il.com>
Cc: Tejun Heo <tj@...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 0/4] Fix scalability problem in workqueue watchdog touch
 caused by stop_machine

On Tue, Jun 25, 2024 at 09:42:43PM +1000, Nicholas Piggin wrote:
> Here are a few patches to fix a lockup caused by very slow progress due
> to a scalability problem in workqueue watchdog touch being hammered by
> thousands of CPUs in multi_cpu_stop. Patch 2 is the fix.
> 
> I did notice when making a microbenchmark reproducer that the RCU call
> was actually also causing slowdowns. Not nearly so bad as the workqueue
> touch, but workqueue queueing of dummy jobs slowed down by a factor of
> several times when lots of other CPUs were making
> rcu_momentary_dyntick_idle() calls. So I did the stop_machine patches to
> reduce that. So those patches 3,4 are independent of the first two and
> can go in any order.

For the series:

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> Thanks,
> Nick
> 
> Nicholas Piggin (4):
>   workqueue: wq_watchdog_touch is always called with valid CPU
>   workqueue: Improve scalability of workqueue watchdog touch
>   stop_machine: Rearrange multi_cpu_stop state machine loop
>   stop_machine: Add a delay between multi_cpu_stop touching watchdogs
> 
>  kernel/stop_machine.c | 31 +++++++++++++++++++++++--------
>  kernel/workqueue.c    | 12 ++++++++++--
>  2 files changed, 33 insertions(+), 10 deletions(-)
> 
> -- 
> 2.45.1
>