Re: [PATCH V4 0/5] blk-mq: improvement on handling IO during CPU hotplug

From: Ming Lei <ming.lei@redhat.com>
To: John Garry <john.garry@huawei.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Bart Van Assche <bvanassche@acm.org>,
	Hannes Reinecke <hare@suse.com>, Christoph Hellwig <hch@lst.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Keith Busch <keith.busch@intel.com>
Subject: Re: [PATCH V4 0/5] blk-mq: improvement on handling IO during CPU hotplug
Date: Tue, 22 Oct 2019 21:45:39 +0800	[thread overview]
Message-ID: <20191022134537.GA13393@ming.t460p> (raw)
In-Reply-To: <4a42a062-8bca-9e03-c158-1c149986d383@huawei.com>

On Tue, Oct 22, 2019 at 12:19:17PM +0100, John Garry wrote:
> On 22/10/2019 01:16, Ming Lei wrote:
> > On Mon, Oct 21, 2019 at 03:02:56PM +0100, John Garry wrote:
> > > On 21/10/2019 13:53, Ming Lei wrote:
> > > > On Mon, Oct 21, 2019 at 12:49:53PM +0100, John Garry wrote:
> > > > > > > > 
> > > > > > > 
> > > > > > > Yes, we share tags among all queues, but we generate the tag - known as IPTT
> > > > > > > - in the LLDD now, as we can no longer use the request tag (as it is not
> > > > > > > unique per all queues):
> > > > > > > 
> > > > > > > https://github.com/hisilicon/kernel-dev/commit/087b95af374be6965583c1673032fb33bc8127e8#diff-f5d8fff19bc539a7387af5230d4e5771R188
> > > > > > > 
> > > > > > > As I said, the branch is messy and I did have to fix 087b95af374.
> > > > > > 
> > > > > > Firstly this way may waste lots of memory, especially the queue depth is
> > > > > > big, such as, hisilicon V3's queue depth is 4096.
> > > > > > 
> > > > > > Secondly, you have to deal with queue busy efficiently and correctly,
> > > > > > for example, your real hw tags(IPTT) can be used up easily, and how
> > > > > > will you handle these dispatched request?
> > > > > 
> > > > > I have not seen scenario of exhausted IPTT. And IPTT count is same as SCSI
> > > > > host.can_queue, so SCSI midlayer should ensure that this does not occur.
> > > > 
> > > 
> > > Hi Ming,
> 
> Hi Ming,
> 
> > > 
> > > > That check isn't correct, and each hw queue should have allowed
> > > > .can_queue in-flight requests.
> > > 
> > > There always seems to be some confusion or disagreement on this topic.
> > > 
> > > I work according to the comment in scsi_host.h:
> > > 
> > > "Note: it is assumed that each hardware queue has a queue depth of
> > >   can_queue. In other words, the total queue depth per host
> > >   is nr_hw_queues * can_queue."
> > > 
> > > So I set Scsi_host.can_queue = HISI_SAS_MAX_COMMANDS (=4096)
> > 
> > I believe all current drivers set .can_queue as single hw queue's depth.
> > If you set .can_queue as HISI_SAS_MAX_COMMANDS which is HBA's queue
> > depth, the hisilicon sas driver will HISI_SAS_MAX_COMMANDS * nr_hw_queues
> > in-flight requests.
> 
> Yeah, but the SCSI host should still limit max IOs over all queues to
> .can_queue:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/scsi/scsi_mid_low_api.txt#n1083
> 

That limit is actually from legacy single-queue era, you should see that
I am removing it:

https://lore.kernel.org/linux-scsi/20191009093241.21481-2-ming.lei@redhat.com/

With this change, IOPS can be improved much on some fast SCSI storage.

Thanks,
Ming