linux-kernel - Re: [PATCH 1/2] Allow a kthread to declare that it calls task_work

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Date:   Tue, 5 Dec 2023 17:41:46 +1100
From:   Dave Chinner <david@...morbit.com>
To:     NeilBrown <neilb@...e.de>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, Oleg Nesterov <oleg@...hat.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jeff Layton <jlayton@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org
Subject: Re: [PATCH 1/2] Allow a kthread to declare that it calls
 task_work_run()

On Mon, Dec 04, 2023 at 12:36:41PM +1100, NeilBrown wrote:
> User-space processes always call task_work_run() as needed when
> returning from a system call.  Kernel-threads generally do not.
> Because of this some work that is best run in the task_works context
> (guaranteed that no locks are held) cannot be queued to task_works from
> kernel threads and so are queued to a (single) work_time to be managed
> on a work queue.
> 
> This means that any cost for doing the work is not imposed on the kernel
> thread, and importantly excessive amounts of work cannot apply
> back-pressure to reduce the amount of new work queued.
> 
> I have evidence from a customer site when nfsd (which runs as kernel
> threads) is being asked to modify many millions of files which causes
> sufficient memory pressure that some cache (in XFS I think) gets cleaned
> earlier than would be ideal.  When __dput (from the workqueue) calls
> __dentry_kill, xfs_fs_destroy_inode() needs to synchronously read back
> previously cached info from storage.

We fixed that specific XFS problem in 5.9.

https://lore.kernel.org/linux-xfs/20200622081605.1818434-1-david@fromorbit.com/

Can you reproduce these issues on a current TOT kernel?

If not, there's no bugs to fix in the upstream kernel. If you can,
then we've got more XFS issues to work through and fix. 

Fundamentally, though, we should not be papering over an XFS issue
by changing how core task_work infrastructure is used. So let's deal
with the XFS issue first....

-Dave.
-- 
Dave Chinner
david@...morbit.com