* Re: [PATCH V2 0/5] blk-mq: improvement on handling IO during CPU hotplug
2019-08-12 16:21 ` [PATCH V2 0/5] blk-mq: improvement on handling IO during CPU hotplug John Garry
@ 2019-08-12 22:45 ` Ming Lei
0 siblings, 0 replies; 2+ messages in thread
From: Ming Lei @ 2019-08-12 22:45 UTC (permalink / raw)
To: John Garry
Cc: Jens Axboe, linux-block, Minwoo Im, Bart Van Assche,
Hannes Reinecke, Christoph Hellwig, Thomas Gleixner, Keith Busch,
chenxiang, linux-scsi@vger.kernel.org
On Mon, Aug 12, 2019 at 05:21:44PM +0100, John Garry wrote:
> On 12/08/2019 14:46, Ming Lei wrote:
> > Hi John,
> >
> > On Mon, Aug 12, 2019 at 09:43:07PM +0800, Ming Lei wrote:
> > > Hi,
> > >
> > > Thomas mentioned:
> > > "
> > > That was the constraint of managed interrupts from the very beginning:
> > >
> > > The driver/subsystem has to quiesce the interrupt line and the associated
> > > queue _before_ it gets shutdown in CPU unplug and not fiddle with it
> > > until it's restarted by the core when the CPU is plugged in again.
> > > "
> > >
> > > But no drivers or blk-mq do that before one hctx becomes dead(all
> > > CPUs for one hctx are offline), and even it is worse, blk-mq stills tries
> > > to run hw queue after hctx is dead, see blk_mq_hctx_notify_dead().
> > >
> > > This patchset tries to address the issue by two stages:
> > >
> > > 1) add one new cpuhp state of CPUHP_AP_BLK_MQ_ONLINE
> > >
> > > - mark the hctx as internal stopped, and drain all in-flight requests
> > > if the hctx is going to be dead.
> > >
> > > 2) re-submit IO in the state of CPUHP_BLK_MQ_DEAD after the hctx becomes dead
> > >
> > > - steal bios from the request, and resubmit them via generic_make_request(),
> > > then these IO will be mapped to other live hctx for dispatch
> > >
> > > Please comment & review, thanks!
> > >
> > > V2:
> > > - patch4 & patch 5 in V1 have been merged to block tree, so remove
> > > them
> > > - address comments from John Garry and Minwoo
> > >
> > >
> > > Ming Lei (5):
> > > blk-mq: add new state of BLK_MQ_S_INTERNAL_STOPPED
> > > blk-mq: add blk-mq flag of BLK_MQ_F_NO_MANAGED_IRQ
> > > blk-mq: stop to handle IO before hctx's all CPUs become offline
> > > blk-mq: re-submit IO in case that hctx is dead
> > > blk-mq: handle requests dispatched from IO scheduler in case that hctx
> > > is dead
> > >
> > > block/blk-mq-debugfs.c | 2 +
> > > block/blk-mq-tag.c | 2 +-
> > > block/blk-mq-tag.h | 2 +
> > > block/blk-mq.c | 143 +++++++++++++++++++++++++++++++++----
> > > block/blk-mq.h | 3 +-
> > > drivers/block/loop.c | 2 +-
> > > drivers/md/dm-rq.c | 2 +-
> > > include/linux/blk-mq.h | 5 ++
> > > include/linux/cpuhotplug.h | 1 +
> > > 9 files changed, 146 insertions(+), 16 deletions(-)
> > >
> > > Cc: Bart Van Assche <bvanassche@acm.org>
> > > Cc: Hannes Reinecke <hare@suse.com>
> > > Cc: Christoph Hellwig <hch@lst.de>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Keith Busch <keith.busch@intel.com>
> > > --
> > > 2.20.1
> > >
> >
> > Sorry for forgetting to Cc you.
>
> Already subscribed :)
>
> I don't mean to hijack this thread, but JFYI we're getting around to test
> https://github.com/ming1/linux/commits/v5.2-rc-host-tags-V2 - unfortunately
> we're still seeing a performance regression. I can't see where it's coming
> from. We're double-checking the test though.
host-tag patchset is only for several particular drivers which use
private reply queue as completion queue.
This patchset is for handling generic blk-mq CPU hotplug issue, and
the several particular scsi drivers(hisi_sas_v3, hpsa, megaraid_sas and
mp3sas) won't be covered so far.
I'd suggest to move on for generic blk-mq devices first given now blk-mq
is the only request IO path now.
There are at least two choices for us to handle drivers/devices with
private completion queue:
1) host-tags
- performance issue shouldn't be hard to solve, given it is same with
with single tags in theory, and just corner cases is there.
What I am not glad with this approach is that blk-mq-tag code becomes mess.
2) private callback
- we could define private callback to drain each completion queue in
driver simply.
- problem is that the four drivers have to duplicate the same job
Thanks,
Ming
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