linux-kernel - Re: [PATCH 10/10] workqueue: Reimplement ordered workqueue using shared nr

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Date: Fri, 12 Jan 2024 14:18:02 -1000
From: Tejun Heo <tj@...nel.org>
To: jiangshanlai@...il.com
Cc: linux-kernel@...r.kernel.org, Naohiro.Aota@....com,
	kernel-team@...a.com
Subject: Re: [PATCH 10/10] workqueue: Reimplement ordered workqueue using
 shared nr_active

On Wed, Dec 20, 2023 at 04:24:41PM +0900, Tejun Heo wrote:
> Because nr_active used to be tied to pwq, an ordered workqueue had to have a
> single pwq to guarantee strict ordering. This led to several contortions to
> avoid creating multiple pwqs.
> 
> Now that nr_active can be shared across multiple pwqs, we can simplify
> ordered workqueue implementation. All that's necessary is ensuring that a
> single wq_node_nr_active is shared across all pwqs, which is achieved by
> making wq_node_nr_active() always return wq->node_nr_active[nr_node_ids] for
> ordered workqueues.
> 
> The new implementation is simpler and allows ordered workqueues to share
> locality aware worker_pools with other unbound workqueues which should
> improve execution locality.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>

This patch breaks ordered workqueues as the inactive pwq RR logic doesn't
follow work item queueing order. I could reproduce severe perf degradations
and outright hangs. I'm dropping this patch.

Thanks.

-- 
tejun