blk-mq: improvement CPU hotplug (simplified version) v2 [LWN.net]

From:		Christoph Hellwig <hch-AT-lst.de>
To:		unlisted-recipients:; (no To-header on input)
Subject:		blk-mq: improvement CPU hotplug (simplified version) v2
Date:		Mon, 18 May 2020 08:39:28 +0200
Message-ID:		<20200518063937.757218-1-hch@lst.de>
Cc:		linux-block-AT-vger.kernel.org, John Garry <john.garry-AT-huawei.com>, Bart Van Assche <bvanassche-AT-acm.org>, Hannes Reinecke <hare-AT-suse.com>, Thomas Gleixner <tglx-AT-linutronix.de>
Archive-link:		Article

Hi all,

this series ensures I/O is quiesced before a cpu and thus the managed
interrupt handler is shut down.

This patchset tries to address the issue by the following approach:

 - before the last cpu in hctx->cpumask is going to offline, mark this
   hctx as inactive

 - disable preempt during allocating tag for request, and after tag is
   allocated, check if this hctx is inactive. If yes, give up the
   allocation and try remote allocation from online CPUs

 - before hctx becomes inactive, drain all allocated requests on this
   hctx

The guts of the changes are from Ming Lei, I just did a bunch of prep
cleanups so that they can fit in more nicely.  The series also depends
on my "avoid a few q_usage_counter roundtrips v3" series.

Thanks John Garry for running lots of tests on arm64 with this previous
version patches and co-working on investigating all kinds of issues.

A git tree is available here:

    git://git.infradead.org/users/hch/block.git blk-mq-hotplug

Gitweb:

    http://git.infradead.org/users/hch/block.git/shortlog/ref...