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From: Sagi Grimberg <sagi@grimberg.me>
To: Christoph Hellwig <hch@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	David Runge <dave@sleepmap.de>,
	linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Daniel Wagner <dwagner@suse.de>, Mike Galbraith <efault@gmx.de>
Subject: Re: [PATCH 3/3] blk-mq: Use llist_head for blk_cpu_done
Date: Wed, 4 Nov 2020 11:15:27 -0800
Message-ID: <75970f9d-7e59-5fba-280a-d0d935fc4d2f@grimberg.me> (raw)
In-Reply-To: <20201102181238.GA17806@infradead.org>


>>> There really aren't any rules for this, and it's perfectly legit to
>>> complete from process context. Maybe you're a kthread driven driver and
>>> that's how you handle completions. The block completion path has always
>>> been hard IRQ safe, but possible to call from anywhere.
>>
>> I'm not trying to put restrictions and forbidding completions from a
>> kthread. I'm trying to avoid the pointless softirq dance for no added
>> value. We could:
> 
>> to not break that assumption you just mentioned and provide
>> |static inline void blk_mq_complete_request_local(struct request *rq)
>> |{
>> |                 rq->q->mq_ops->complete(rq);
>> |}
>>
>> so that completion issued from from process context (like those from
>> usb-storage) don't end up waking `ksoftird' (running at SCHED_OTHER)
>> completing the requests but rather performing it right away. The softirq
>> dance makes no sense here.
> 
> Agreed.  But I don't think your above blk_mq_complete_request_local
> is all that useful either as ->complete is defined by the caller,
> so we could just do a direct call.  Basically we should just
> return false from blk_mq_complete_request_remote after updating
> the state when called from process context.

Agreed.

> But given that IIRC
> we are not supposed to check what state we are called from
> we'll need a helper just for updating the state instead and
> ensure the driver uses the right helper.  Now of course we might
> have process context callers that still want to bounce to the
> submitting CPU, but in that case we should go directly to a
> workqueue or similar.

This would mean that it may be suboptimal for nvme-tcp to complete
requests in softirq context from the network context (determined by
NIC steering). Because in this case, this would trigger workqueue
schedule on a per-request basis rather than once per .data_ready
call like we do today. Is that correct?

It has been observed that completing commands in softirq context
(network determined cpu) because basically the completion does
IPI + local complete, not IPI + softirq or IPI + workqueue.

> Either way doing this properly will probabl involve an audit of all
> drivers, but I think that is worth it.

Agree.

  reply index

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-21 17:50 5.9.1-rt18: issues with Firewire card on AMD hardware David Runge
2020-10-23 11:04 ` [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT Sebastian Andrzej Siewior
2020-10-23 11:21   ` Christoph Hellwig
2020-10-23 13:52     ` Sebastian Andrzej Siewior
2020-10-27  9:26       ` Christoph Hellwig
2020-10-27 10:11         ` Sebastian Andrzej Siewior
2020-10-27 16:07           ` Christoph Hellwig
2020-10-27 17:05             ` Thomas Gleixner
2020-10-27 17:23               ` Christoph Hellwig
2020-10-27 17:59                 ` Sebastian Andrzej Siewior
2020-10-27 20:58                 ` Sebastian Andrzej Siewior
2020-10-28  6:56                   ` Christoph Hellwig
2020-10-28 14:12                     ` [PATCH 1/3] blk-mq: Don't complete on a remote CPU in force threaded mode Sebastian Andrzej Siewior
2020-10-28 14:12                       ` [PATCH 2/3] blk-mq: Always complete remote completions requests in softirq Sebastian Andrzej Siewior
2020-10-28 14:12                       ` [PATCH 3/3] blk-mq: Use llist_head for blk_cpu_done Sebastian Andrzej Siewior
2020-10-28 14:44                         ` Christoph Hellwig
2020-10-28 14:47                           ` Sebastian Andrzej Siewior
2020-10-29 13:12                         ` Sebastian Andrzej Siewior
2020-10-29 14:05                           ` Christoph Hellwig
2020-10-29 14:56                             ` Sebastian Andrzej Siewior
2020-10-29 14:57                               ` Christoph Hellwig
2020-10-29 20:03                                 ` Sagi Grimberg
2020-10-29 21:01                                   ` Sebastian Andrzej Siewior
2020-10-29 21:07                                     ` Sagi Grimberg
2020-10-31 10:41                                       ` Sebastian Andrzej Siewior
2020-10-31 15:00                                         ` Jens Axboe
2020-10-31 15:01                                           ` Jens Axboe
2020-10-31 18:09                                             ` Christoph Hellwig
2020-11-02  9:55                                           ` Sebastian Andrzej Siewior
2020-11-02 18:12                                             ` Christoph Hellwig
2020-11-04 19:15                                               ` Sagi Grimberg [this message]
2020-11-06 15:23                                               ` Sebastian Andrzej Siewior
2020-10-28 10:04                 ` [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT Peter Zijlstra
2020-10-26  0:37 ` 5.9.1-rt18: issues with Firewire card on AMD hardware David Runge

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