From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id ACB5CC433B4 for ; Thu, 15 Apr 2021 09:22:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 72D5D61249 for ; Thu, 15 Apr 2021 09:22:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231862AbhDOJWW (ORCPT ); Thu, 15 Apr 2021 05:22:22 -0400 Received: from out30-43.freemail.mail.aliyun.com ([115.124.30.43]:51457 "EHLO out30-43.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229820AbhDOJWV (ORCPT ); Thu, 15 Apr 2021 05:22:21 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R471e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04400;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0UVdSSqP_1618478516; Received: from admindeMacBook-Pro-2.local(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0UVdSSqP_1618478516) by smtp.aliyun-inc.com(127.0.0.1); Thu, 15 Apr 2021 17:21:56 +0800 Subject: Re: [PATCH V5 11/12] block: add poll_capable method to support bio-based IO polling To: Ming Lei Cc: Christoph Hellwig , Jens Axboe , linux-block@vger.kernel.org, Mike Snitzer , dm-devel@redhat.com, Hannes Reinecke References: <20210401021927.343727-1-ming.lei@redhat.com> <20210401021927.343727-12-ming.lei@redhat.com> <20210412093856.GA978201@infradead.org> <5f30059d-6650-8268-b681-d8567ac1c509@linux.alibaba.com> From: JeffleXu Message-ID: <0ceb3060-bce4-c39d-26cf-8c715ebbfd51@linux.alibaba.com> Date: Thu, 15 Apr 2021 17:21:56 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 4/15/21 3:43 PM, Ming Lei wrote: > On Thu, Apr 15, 2021 at 09:34:36AM +0800, JeffleXu wrote: >> >> >> On 4/14/21 7:24 PM, Ming Lei wrote: >>> On Wed, Apr 14, 2021 at 04:38:25PM +0800, JeffleXu wrote: >>>> >>>> >>>> On 4/12/21 5:38 PM, Christoph Hellwig wrote: >>>>> On Thu, Apr 01, 2021 at 10:19:26AM +0800, Ming Lei wrote: >>>>>> From: Jeffle Xu >>>>>> >>>>>> This method can be used to check if bio-based device supports IO polling >>>>>> or not. For mq devices, checking for hw queue in polling mode is >>>>>> adequate, while the sanity check shall be implementation specific for >>>>>> bio-based devices. For example, dm device needs to check if all >>>>>> underlying devices are capable of IO polling. >>>>>> >>>>>> Though bio-based device may have done the sanity check during the >>>>>> device initialization phase, cacheing the result of this sanity check >>>>>> (such as by cacheing in the queue_flags) may not work. Because for dm >>>>>> devices, users could change the state of the underlying devices through >>>>>> '/sys/block//io_poll', bypassing the dm device above. In this case, >>>>>> the cached result of the very beginning sanity check could be >>>>>> out-of-date. Thus the sanity check needs to be done every time 'io_poll' >>>>>> is to be modified. >>>>> >>>>> I really don't think thi should be a method, and I really do dislike >>>>> how we have all this "if (is_mq)" junk. Why can't we have a flag on >>>>> the gendisk that signals if the device can support polling that >>>>> is autoamtically set for blk-mq and as-needed by bio based drivers? >>>> >>>> That would consume one more bit of queue->queue_flags. >>>> >>>> Besides, DM/MD is somehow special here that when one of the underlying >>>> devices is disabled polling through '/sys/block//io_poll', >>>> currently there's no mechanism notifying the above MD/DM to clear the >>>> previously set queue_flags. Thus the outdated queue_flags still >>>> indicates this DM/MD is capable of polling, while in fact one of the >>>> underlying device has been disabled for polling. >>> >>> Right, just like there isn't queue limit progagation. >>> >>> Another blocker could be that bio based queue doesn't support queue >>> freezing. >> >> Do you mean the queue freezing is called in the following code snippet? >> >> ``` >> static ssize_t queue_poll_store(struct request_queue *q, const char >> *page, size_t count) >> { >> ... >> if (poll_on) { >> blk_queue_flag_set(QUEUE_FLAG_POLL, q); >> } else { >> blk_mq_freeze_queue(q); >> blk_queue_flag_clear(QUEUE_FLAG_POLL, q); >> blk_mq_unfreeze_queue(q); >> } >> ``` > > Yes, if it is a bio based queue. Or bio queued queue(DM, MD or others) may > use freeze_queue to do similar thing. > >> >> And I can't understand how bio-based queue doesn't support queue freezing. >> >> ``` >> submit_bio_noacct >> __submit_bio_noacct >> bio_queue_enter >> ``` >> >> Every time submitting a bio, bio_queue_enter() will be called, and once >> the queue has been frozen, bio_queue_enter() will wait there until the >> queue is unfrozen. > > Not like blk-mq, the refcount is just grabbed during submission for bio based > queue. Could you please explain it more detailed .... I will research a bit and see if we can extend freeze queue for > covering bio based queue. One trouble is that bio is ended before > freeing request. > >> >>> >>>> >>>> Mike had ever suggested that we can trust the queue_flag, and clear the >>>> outdated queue_flags when later the IO submission or polling routine >>>> finally finds that the device is not capable of polling. Currently >>>> submit_bio_checks() will silently clear the REQ_HIPRI flag and still >>>> submit the bio when the device is actually not capable of polling. To >>>> fix the issue, could we break the submission and return an error code in >>>> submit_bio_checks() if the device is not capable of polling when >>>> submitting HIPRI bio? >>> >>> I think we may just leave it alone, if underlying queue becomes not pollable, >>> the bio still can be submitted & completed via IRQ, just not efficient enough. >> >> Yes it still works. I agree if there's no better solution... >> >> And what about the issue Christoph originally concerned? Do we use one >> more flag bit indicating if the queue capable of polling, or the >> poll_capable() method way? > > Just wondering why we can't use QUEUE_FLAG_POLL simply? If user wants to > enable it, let's do it for them. And bio driver can start with default poll > state by checking underlying queues. > Consider the following scenario: QUEUE_FLAG_POLL is set after initialization, indicating the device capable of polling; then polling is turned off by '/sys/block//io_poll', thus QUEUE_FLAG_POLL is cleared. So we have to implement two semantics: 1. a flag indicating whether the device is **capable** of polling or not; (adding another flag bit, or calling poll_capable() every time polling is to be turned on) 2. a flag indicating whether polling is currently **turned on** or not (i.e., QUEUE_FLAG_POLL) -- Thanks, Jeffle