From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from esa6.hgst.iphmx.com ([216.71.154.45]:35664 "EHLO esa6.hgst.iphmx.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751119AbdE3RCZ (ORCPT ); Tue, 30 May 2017 13:02:25 -0400 From: Bart Van Assche To: "ming.lei@redhat.com" CC: "hch@infradead.org" , "linux-block@vger.kernel.org" , "axboe@fb.com" Subject: Re: [PATCH v2 6/8] blk-mq: don't stop queue for quiescing Date: Tue, 30 May 2017 17:02:23 +0000 Message-ID: <1496163742.2627.11.camel@sandisk.com> References: <20170527142126.26079-1-ming.lei@redhat.com> <20170527142126.26079-7-ming.lei@redhat.com> <1495921766.13651.4.camel@sandisk.com> <20170528105047.GC6488@ming.t460p> <1495987400.2849.3.camel@sandisk.com> <20170530002703.GB29253@ming.t460p> In-Reply-To: <20170530002703.GB29253@ming.t460p> Content-Type: text/plain; charset="iso-8859-1" MIME-Version: 1.0 Sender: linux-block-owner@vger.kernel.org List-Id: linux-block@vger.kernel.org On Tue, 2017-05-30 at 08:27 +0800, Ming Lei wrote: > On Sun, May 28, 2017 at 04:03:20PM +0000, Bart Van Assche wrote: > > On Sun, 2017-05-28 at 18:50 +0800, Ming Lei wrote: > > > On Sat, May 27, 2017 at 09:49:27PM +0000, Bart Van Assche wrote: > > > > On Sat, 2017-05-27 at 22:21 +0800, Ming Lei wrote: > > > > > diff --git a/block/blk-mq.c b/block/blk-mq.c > > > > > index 032045841856..84cce67caee3 100644 > > > > > --- a/block/blk-mq.c > > > > > +++ b/block/blk-mq.c > > > > > @@ -168,8 +168,6 @@ void blk_mq_quiesce_queue(struct request_queu= e *q) > > > > > unsigned int i; > > > > > bool rcu =3D false; > > > > > =20 > > > > > - __blk_mq_stop_hw_queues(q, true); > > > > > - > > > > > spin_lock_irq(q->queue_lock); > > > > > queue_flag_set(QUEUE_FLAG_QUIESCED, q); > > > > > spin_unlock_irq(q->queue_lock); > > > > > @@ -198,7 +196,12 @@ void blk_mq_unquiesce_queue(struct request_q= ueue *q) > > > > > queue_flag_clear(QUEUE_FLAG_QUIESCED, q); > > > > > spin_unlock_irq(q->queue_lock); > > > > > =20 > > > > > - blk_mq_start_stopped_hw_queues(q, true); > > > > > + /* > > > > > + * During quiescing, requests can be inserted > > > > > + * to scheduler's queue or sw queue, so we run > > > > > + * queues for dispatching these requests. > > > > > + */ > > > > > + blk_mq_start_hw_queues(q); > > > > > } > > > > > EXPORT_SYMBOL_GPL(blk_mq_unquiesce_queue); > > > >=20 > > > > This looks really weird to me. If blk_mq_quiesce_queue() doesn't st= op any > > > > hardware queues, why should blk_mq_unquiesce_queue() start any hard= ware > > > > queues? > > >=20 > > > Please see the above comment, especially in direct issue path, we hav= e to > > > insert the request into sw/scheduler queue when queue is quiesced, > > > and these queued requests have to be dispatched out during unquiescin= g. > > >=20 > > > I considered to sleep the current context under this situation, but > > > turns out it is more complicated to handle, given we have very limite= d > > > queue depth, just let applications consumes all, then wait. > >=20 > > Hello Ming, > >=20 > > The above seems to me to be an argument to *run* the queue from inside > > blk_mq_unquiesce_queue() instead of *starting* stopped queues from insi= de > > that function. >=20 > Yes, it is run queues, as you can see in my comment. >=20 > The reality is that we can't run queue without clearing the STOPPED state= . Hello Ming, I think it's completely wrong to make blk_mq_unquiesce_queue() to clear the STOPPED state. Stopping a queue is typically used to tell the block layer t= o stop dispatching requests and not to resume dispatching requests until the STOPPED flag is cleared. If stopping and quiescing a queue happen simultaneously then dispatching should not occur before both blk_mq_unquiesce_queue() and blk_mq_start_hw_queue() have been called. Your patch would make dispatching start too early. Bart.=