linux-kernel - Re: [RFC PATCH v3 06/12] xen-blkfront: add callbacks for PM suspend and hibernation

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Date:   Mon, 9 Mar 2020 10:54:07 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Anchal Agarwal <anchalag@...zon.com>
CC:     <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
        <hpa@...or.com>, <x86@...nel.org>, <boris.ostrovsky@...cle.com>,
        <jgross@...e.com>, <linux-pm@...r.kernel.org>,
        <linux-mm@...ck.org>, <kamatam@...zon.com>,
        <sstabellini@...nel.org>, <konrad.wilk@...cle.com>,
        <axboe@...nel.dk>, <davem@...emloft.net>, <rjw@...ysocki.net>,
        <len.brown@...el.com>, <pavel@....cz>, <peterz@...radead.org>,
        <eduval@...zon.com>, <sblbir@...zon.com>,
        <xen-devel@...ts.xenproject.org>, <vkuznets@...hat.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <dwmw@...zon.co.uk>, <fllinden@...ozn.com>,
        <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH v3 06/12] xen-blkfront: add callbacks for PM suspend
 and hibernation

On Fri, Mar 06, 2020 at 06:40:33PM +0000, Anchal Agarwal wrote:
> On Fri, Feb 21, 2020 at 03:24:45PM +0100, Roger Pau Monné wrote:
> > On Fri, Feb 14, 2020 at 11:25:34PM +0000, Anchal Agarwal wrote:
> > >  	blkfront_gather_backend_features(info);
> > >  	/* Reset limits changed by blk_mq_update_nr_hw_queues(). */
> > >  	blkif_set_queue_limits(info);
> > > @@ -2046,6 +2063,9 @@ static int blkif_recover(struct blkfront_info *info)
> > >  		kick_pending_request_queues(rinfo);
> > >  	}
> > >  
> > > +	if (frozen)
> > > +		return 0;
> > 
> > I have to admit my memory is fuzzy here, but don't you need to
> > re-queue requests in case the backend has different limits of indirect
> > descriptors per request for example?
> > 
> > Or do we expect that the frontend is always going to be resumed on the
> > same backend, and thus features won't change?
> > 
> So to understand your question better here, AFAIU the  maximum number of indirect 
> grefs is fixed by the backend, but the frontend can issue requests with any 
> number of indirect segments as long as it's less than the number provided by 
> the backend. So by your question you mean this max number of MAX_INDIRECT_SEGMENTS 
> 256 on backend can change ?

Yes, number of indirect descriptors supported by the backend can
change, because you moved to a different backend, or because the
maximum supported by the backend has changed. It's also possible to
resume on a backend that has no indirect descriptors support at all.

> > > @@ -2625,6 +2671,62 @@ static void blkif_release(struct gendisk *disk, fmode_t mode)
> > >  	mutex_unlock(&blkfront_mutex);
> > >  }
> > >  
> > > +static int blkfront_freeze(struct xenbus_device *dev)
> > > +{
> > > +	unsigned int i;
> > > +	struct blkfront_info *info = dev_get_drvdata(&dev->dev);
> > > +	struct blkfront_ring_info *rinfo;
> > > +	/* This would be reasonable timeout as used in xenbus_dev_shutdown() */
> > > +	unsigned int timeout = 5 * HZ;
> > > +	int err = 0;
> > > +
> > > +	info->connected = BLKIF_STATE_FREEZING;
> > > +
> > > +	blk_mq_freeze_queue(info->rq);
> > > +	blk_mq_quiesce_queue(info->rq);
> > 
> > Don't you need to also drain the queue and make sure it's empty?
> > 
> blk_mq_freeze_queue and blk_mq_quiesce_queue should take care of running HW queues synchronously
> and making sure all the ongoing dispatches have finished. Did I understand your question right?

Can you please add some check to that end? (ie: that there are no
pending requests on any queue?)

Thanks, Roger.