Re: [PATCH V2 RESEND 4/5] blk-mq: re-submit IO in case that hctx is dead

[Ming Lei <ming.lei@redhat.com>]

On Mon, Oct 07, 2019 at 08:27:38AM +0200, Hannes Reinecke wrote:
> On 10/6/19 4:45 AM, Ming Lei wrote:
> > When all CPUs in one hctx are offline, we shouldn't run this hw queue
> > for completing request any more.
> > 
> > So steal bios from the request, and resubmit them, and finally free
> > the request in blk_mq_hctx_notify_dead().
> > 
> > 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.c | 48 +++++++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 41 insertions(+), 7 deletions(-)
> > 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index d991c122abf2..0b35fdbd1f17 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -2280,10 +2280,30 @@ static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
> >  	return 0;
> >  }
> >  
> > +static void blk_mq_resubmit_io(struct request *rq)
> > +{
> > +	struct bio_list list;
> > +	struct bio *bio;
> > +
> > +	bio_list_init(&list);
> > +	blk_steal_bios(&list, rq);
> > +
> > +	while (true) {
> > +		bio = bio_list_pop(&list);
> > +		if (!bio)
> > +			break;
> > +
> > +		generic_make_request(bio);
> > +	}
> > +
> > +	blk_mq_cleanup_rq(rq);
> > +	blk_mq_end_request(rq, 0);
> > +}
> > +
> Hmm. Not sure if this is a good idea.
> Shouldn't we call 'blk_mq_end_request()' before calling
> generic_make_request()?
> otherwise the cloned request might be completed before original one,
> which looks a bit dodgy to me; and might lead to quite a recursion if we
> have several dead cpus to content with ...

Good catch, we should have freed the old empty request before calling
generic_make_request(), will fix it in V3.


Thanks,
Ming