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Subject: Re: nbd: please don't spawn 16 threads when nbd is not even in use
To:     Pavel Machek <pavel@ucw.cz>, Eric Blake <eblake@redhat.com>
Cc:     kernel list <linux-kernel@vger.kernel.org>, josef@toxicpanda.com,
        linux-block@vger.kernel.org, nbd@other.debian.org,
        "yukuai (C)" <yukuai3@huawei.com>
References: <Y3y4+QqOlF00X9ET@duo.ucw.cz>
 <20221123200845.cuct5euvikqksojm@redhat.com> <Y36YHNVmbozzSdxH@duo.ucw.cz>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <ccee0ff5-309c-b430-09c7-8d2081c9981d@huaweicloud.com>
Date:   Thu, 24 Nov 2022 09:17:51 +0800
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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.

Thanks,
Kuai
> 
>        	       	    	  	  	    	     		Pavel
>