Re: [RFC PATCH 2/7] blk-mq: add blk-mq flag of BLK_MQ_F_NO_MANAGED_IRQ

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,
	"James E . J . Bottomley" <jejb@linux.ibm.com>,
	"Martin K . Petersen" <martin.petersen@oracle.com>,
	linux-scsi@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: [RFC PATCH 2/7] blk-mq: add blk-mq flag of BLK_MQ_F_NO_MANAGED_IRQ
Date: Tue, 16 Jul 2019 14:08:30 +0800	[thread overview]
Message-ID: <20190716060828.GA1094@ming.t460p> (raw)
In-Reply-To: <ce954a60-5a7a-a13d-b999-6f973a440d22@huawei.com>

On Tue, Jul 16, 2019 at 10:53:00AM +0800, John Garry wrote:
> 在 12/07/2019 10:47, Ming Lei 写道:
> > We will stop hw queue and wait for completion of in-flight requests
> > when one hctx is becoming dead in the following patch. This way may
> > cause dead-lock for some stacking blk-mq drivers, such as dm-rq and
> > loop.
> > 
> > Add blk-mq flag of BLK_MQ_F_NO_MANAGED_IRQ and mark it for dm-rq and
> > loop, so we needn't to wait for completion of in-flight requests of
> > dm-rq & loop, then the potential dead-lock can be avoided.
> 
> Wouldn't it make more sense to have the flag name be like
> BLK_MQ_F_DONT_DRAIN_STOPPED_HCTX?
> 
> I did not think that blk-mq is specifically concerned with managed
> interrupts, but only their indirect effect.

I am fine with either one, however it is easier for drivers to
recognize if this flag should be set, given BLK_MQ_F_NO_MANAGED_IRQ
is self-explained.

Also on the other side, this patchset serves a generic blk-mq fix
for managed IRQ issue, so it is reasonable for all drivers which
don't use managed IRQ to set the flag.

> 
> > 
> > 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>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >   block/blk-mq-debugfs.c | 1 +
> >   drivers/block/loop.c   | 2 +-
> >   drivers/md/dm-rq.c     | 2 +-
> >   include/linux/blk-mq.h | 1 +
> >   4 files changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
> > index af40a02c46ee..24fff8c90942 100644
> > --- a/block/blk-mq-debugfs.c
> > +++ b/block/blk-mq-debugfs.c
> > @@ -240,6 +240,7 @@ static const char *const hctx_flag_name[] = {
> >   	HCTX_FLAG_NAME(TAG_SHARED),
> >   	HCTX_FLAG_NAME(BLOCKING),
> >   	HCTX_FLAG_NAME(NO_SCHED),
> > +	HCTX_FLAG_NAME(NO_MANAGED_IRQ),
> >   };
> >   #undef HCTX_FLAG_NAME
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index 44c9985f352a..199d76e8bf46 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -1986,7 +1986,7 @@ static int loop_add(struct loop_device **l, int i)
> >   	lo->tag_set.queue_depth = 128;
> >   	lo->tag_set.numa_node = NUMA_NO_NODE;
> >   	lo->tag_set.cmd_size = sizeof(struct loop_cmd);
> > -	lo->tag_set.flags = BLK_MQ_F_SHOULD_MERGE;
> > +	lo->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_NO_MANAGED_IRQ;
> 
> nit: at this point in the series you're setting a flag which is never
> checked.

Yeah, I see, and this way is a bit easier for review purpose.

thanks,
Ming