linux-kernel - Re: [PATCH v1 5/9] block: loop: convert to blk-mq

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <CACVXFVNEuEOXphJK5XGbAGRC9tL7iTv=PE_v+Dnw3CReAEkonw@mail.gmail.com>
Date:	Thu, 21 Aug 2014 11:13:39 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Zach Brown <zab@...bo.net>, Benjamin LaHaise <bcrl@...ck.org>,
	Kent Overstreet <kmo@...erainc.com>,
	"open list:AIO <linux-aio@...ck.org>, Linux FS Devel
	<linux-fsdevel@...r.kernel.org>, Dave Chinner <david@...morbit.com>,
	Tejun Heo" <tj@...nel.org>
Subject: Re: [PATCH v1 5/9] block: loop: convert to blk-mq

On Thu, Aug 21, 2014 at 10:58 AM, Jens Axboe <axboe@...nel.dk> wrote:
> On 2014-08-20 21:54, Ming Lei wrote:
>>>>
>>>>   From my investigation, context switch increases almost 50% with
>>>> workqueue compared with kthread in loop in a quad-core VM. With
>>>> kthread, requests may be handled as batch in cases which won't be
>>>> blocked in read()/write()(like null_blk, tmpfs, ...), but it is
>>>> impossible
>>>> with
>>>> workqueue any more.  Also block plug&unplug should have been used
>>>> with kthread to optimize the case, especially when kernel AIO is
>>>> applied,
>>>> still impossible with work queue too.
>>>
>>>
>>>
>>> OK, that one is actually a good point, since one need not do per-item
>>> queueing. We could handle different units, though. And we should have
>>> proper
>>> marking of the last item in a chain of stuff, so we might even be able to
>>> offload based on that instead of doing single items. It wont help the
>>> sync
>>> case, but for that, workqueue and kthread would be identical.
>>
>>
>> We may do that by introducing callback of queue_rq_list in blk_mq_ops,
>> and I will figure out one patch today to see if it can help the case.
>
>
> I don't think we should add to the interface, I prefer keeping it clean like
> it is right now. At least not if we can get around it. My point is that the
> driver already knows when the chain is complete, when REQ_LAST is set. So
> before that event triggers, it need not kick off IO, or at least i could do
> it in batches before that. That may not be fully reliable in case of
> queueing errors, but if REQ_LAST or 'error return' is used as the way to
> kick off pending IO, then that should be good enough. Haven't audited this
> in a while, but at least that is the intent of REQ_LAST.

Yes, I thought of too, but driver need another context for handling that,
either workqueue or kthread, which may cause the introduced per-device
workqueue useless.

thanks,
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/