linux-kernel - Re: [PATCH for-6.9] workqueue: Drain BH work items on hot-unplugged CPUs

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Date: Thu, 29 Feb 2024 13:07:21 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Tejun Heo <tj@...nel.org>
Cc: torvalds@...ux-foundation.org, mpatocka@...hat.com,
	linux-kernel@...r.kernel.org, dm-devel@...ts.linux.dev,
	msnitzer@...hat.com, ignat@...udflare.com, damien.lemoal@....com,
	bob.liu@...cle.com, houtao1@...wei.com, peterz@...radead.org,
	mingo@...nel.org, netdev@...r.kernel.org, allen.lkml@...il.com,
	kernel-team@...a.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH for-6.9] workqueue: Drain BH work items on hot-unplugged
 CPUs

On Thu, Feb 29, 2024 at 10:37:28AM -1000, Tejun Heo wrote:
> On Mon, Feb 26, 2024 at 03:38:55PM -1000, Tejun Heo wrote:
> > Boqun pointed out that workqueues aren't handling BH work items on offlined
> > CPUs. Unlike tasklet which transfers out the pending tasks from
> > CPUHP_SOFTIRQ_DEAD, BH workqueue would just leave them pending which is
> > problematic. Note that this behavior is specific to BH workqueues as the
> > non-BH per-CPU workers just become unbound when the CPU goes offline.
> > 
> > This patch fixes the issue by draining the pending BH work items from an
> > offlined CPU from CPUHP_SOFTIRQ_DEAD. Because work items carry more context,
> > it's not as easy to transfer the pending work items from one pool to
> > another. Instead, run BH work items which execute the offlined pools on an
> > online CPU.
> > 
> > Note that this assumes that no further BH work items will be queued on the
> > offlined CPUs. This assumption is shared with tasklet and should be fine for
> > conversions. However, this issue also exists for per-CPU workqueues which
> > will just keep executing work items queued after CPU offline on unbound
> > workers and workqueue should reject per-CPU and BH work items queued on
> > offline CPUs. This will be addressed separately later.
> > 
> > Signed-off-by: Tejun Heo <tj@...nel.org>
> > Reported-by: Boqun Feng <boqun.feng@...il.com>
> > Link: http://lkml.kernel.org/r/Zdvw0HdSXcU3JZ4g@boqun-archlinux
> 
> Applying this to wq/for-6.9.
> 

FWIW,

Reviewed-by: Boqun Feng <boqun.feng@...il.com>

(I took a look yesterday, but hasn't gotten the time to reply..)

Regards,
Boqun

> Thanks.
> 
> -- 
> tejun