linux-kernel - btrfs thread_pool_size logic out of sync with workqueue

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Message-ID: <5ee9b1d7-4772-4404-b972-93b01ed1e033@cs.wisc.edu>
Date: Mon, 28 Apr 2025 18:23:39 -0500
From: Junxuan Liao <ljx@...wisc.edu>
To: Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: btrfs thread_pool_size logic out of sync with workqueue

Hi all,

Commit 636b927eba5bc63375 (workqueue: Make unbound workqueues to use
per-cpu pool_workqueues) makes max_active per CPU for unbounded
workqueues as well, but thread_pool_size in btrfs_fs_info and the mount
option thread_pool still assume the limit is global.

e.g. The default value of 8 allows a total of 64 workers on an 8-CPU
machine.

As far as I know, this means that on the Btrfs side we can no longer
control the concurrency level at the same granularity as before. We
should rewrite the auto-scaling logic, change the default
thread_pool_size value, and update the documentation. 

Am I missing something?

-- 
Thanks,
Junxuan