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From: "NeilBrown" <neilb@suse.de>
To: "Oleg Nesterov" <oleg@redhat.com>
Cc: "Al Viro" <viro@zeniv.linux.org.uk>,
 "Christian Brauner" <brauner@kernel.org>, "Jens Axboe" <axboe@kernel.dk>,
 "Chuck Lever" <chuck.lever@oracle.com>, "Jeff Layton" <jlayton@kernel.org>,
 "Ingo Molnar" <mingo@redhat.com>, "Peter Zijlstra" <peterz@infradead.org>,
 "Juri Lelli" <juri.lelli@redhat.com>,
 "Vincent Guittot" <vincent.guittot@linaro.org>, linux-fsdevel@vger.kernel.org,
 linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org
Subject: Re: [PATCH/RFC] core/nfsd: allow kernel threads to use task_work.
In-reply-to: <20231128140156.GC22743@redhat.com>
References: <170112272125.7109.6245462722883333440@noble.neil.brown.name>,
 <20231128140156.GC22743@redhat.com>
Date: Wed, 29 Nov 2023 11:14:22 +1100
Message-id: <170121686264.7109.13475581089284671405@noble.neil.brown.name>

On Wed, 29 Nov 2023, Oleg Nesterov wrote:
> On 11/28, NeilBrown wrote:
> >
> > I have evidence from a customer site of 256 nfsd threads adding files to
> > delayed_fput_lists nearly twice as fast they are retired by a single
> > work-queue thread running delayed_fput().  As you might imagine this
> > does not end well (20 million files in the queue at the time a snapshot
> > was taken for analysis).
> 
> On a related note... Neil, Al, et al, can you look at
> 
> 	[PATCH 1/3] fput: don't abuse task_work_add() when possible
> 	https://lore.kernel.org/all/20150908171446.GA14589@redhat.com/
> 

Would it make sense to create a separate task_struct->delayed_fput
llist?
fput() adds the file to this llist and if it was the first item on the
list, it then adds the task_work.  That would probably request adding a
callback_head to struct task_struct as well.

NeilBrown