linux-kernel - Re: nbd: please don't spawn 16 threads when nbd is not even in use

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Message-ID: <Y39CFnRwNGBGIsCH@duo.ucw.cz>
Date:   Thu, 24 Nov 2022 11:06:14 +0100
From:   Pavel Machek <pavel@....cz>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     Eric Blake <eblake@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        josef@...icpanda.com, linux-block@...r.kernel.org,
        nbd@...er.debian.org, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: nbd: please don't spawn 16 threads when nbd is not even in use

On Thu 2022-11-24 09:17:51, Yu Kuai wrote:
> Hi,
> 
> 在 2022/11/24 6:01, Pavel Machek 写道:
> > Hi!
> > 
> > > > I see this... and it looks like there are 16 workqueues before nbd is
> > > > even used. Surely there are better ways to do that?
> > > 
> > > Yes, it would be nice to create a pool of workers that only spawns up
> > > threads when actual parallel requests are made.  Are you willing to
> > > help write the patch?
> > 
> > I was thinking more "only spawn a workqueue when nbd is opened" or so.
> > 
> > I have 16 of them, and I'm not using nbd. Workqueue per open device is
> > okay, current situation... not so much.
> 
> You can take a look at this commit:
> 
> e2daec488c57 ("nbd: Fix hungtask when nbd_config_put")
> 
> Allocation of recv_workq is moved from start device to alloc device to
> fix hungtask. You might need to be careful if you want to move this.

Can we get that reverted?

That is rather obscure bug (how many GFP_KERNEL failures do you
normally see?) and it costs, dunno, 100KB? of unswappable memory.

Best regards,
								Pavel
-- 
People of Russia, stop Putin before his war on Ukraine escalates.

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