linux-kernel - Re: [RFC PATCH v1 0/6] Introducing `wq_cpu

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Date:   Mon, 27 Feb 2023 01:26:16 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Filipe Manana <fdmanana@...e.com>,
        Linux Btrfs Mailing List <linux-btrfs@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Fsdevel Mailing List <linux-fsdevel@...r.kernel.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>
Subject: Re: [RFC PATCH v1 0/6] Introducing `wq_cpu_set` mount option for
 btrfs

Hello,

On Sun, 26 Feb 2023 07:01:41 -1000, Tejun Heo wrote:
> Hmm... the allowed cpumasks for unbounded workqueues can already be set
> through /sys/devices/virtual/workqueue/cpumask and also each individual
> workqueue can be exposed in the matching subdirectory by setting WQ_SYSFS.
> Wouldn't the proposed btrfs option be a bit reduandant?

Thank you for the comment. I just realized the sysfs facility for this.
So I will take a look into it deeper. For now, I have several reasons to
use the `wq_cpu_set` option:

1. Currently, there are 15 btrfs workqueues. It would not be convenient
   to let the user manage each of them via the sysfs. Using `wq_cpu_set`
   option at mounting time allows the user to set all of them in one
   shot.

   (for btrfs maintainers):
   I am also not sure if the number of btrfs workqueues is stable so
   that the user can rely on the WQ_SYSFS facility.

2. I looked at /sys/devices/virtual/workqueue/ and saw its
   subdirectories. The directory name is taken from the wq->name. But
   how to distinguish multiple workqueues with the same name?

Each btrfs mount will at least do this:

	alloc_workqueue("btrfs-compressed-write", flags, max_active);

When we do:

	mount -t -o rw btrfs /dev/sda1 a;
	mount -t -o rw btrfs /dev/sda2 b;
	mount -t -o rw btrfs /dev/sda3 c;
	mount -t -o rw btrfs /dev/sda4 d;

Is there a way to identify which sysfs devices correspond to a specific
mounted btrfs fs workqueues? Let's say I want each mount to have a
different CPU mask.

-- 
Ammar Faizi