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Date: Wed, 25 Aug 2021 22:01:59 +0200
From: Pankaj Gupta <pankaj.gupta.linux@...il.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Linux NVDIMM <nvdimm@...ts.linux.dev>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
jmoyer <jmoyer@...hat.com>, David Hildenbrand <david@...hat.com>,
"Michael S. Tsirkin" <mst@...hat.com>,
Cornelia Huck <cohuck@...hat.com>,
Vishal L Verma <vishal.l.verma@...el.com>,
Dave Jiang <dave.jiang@...el.com>,
"Weiny, Ira" <ira.weiny@...el.com>,
Pankaj Gupta <pankaj.gupta@...os.com>
Subject: Re: [RFC v2 1/2] virtio-pmem: Async virtio-pmem flush
Hi Dan,
Thank you for the review. Please see my reply inline.
> > Implement asynchronous flush for virtio pmem using work queue
> > to solve the preflush ordering issue. Also, coalesce the flush
> > requests when a flush is already in process.
> >
> > Signed-off-by: Pankaj Gupta <pankaj.gupta@...os.com>
> > ---
> > drivers/nvdimm/nd_virtio.c | 72 ++++++++++++++++++++++++++++--------
> > drivers/nvdimm/virtio_pmem.c | 10 ++++-
> > drivers/nvdimm/virtio_pmem.h | 14 +++++++
> > 3 files changed, 79 insertions(+), 17 deletions(-)
> >
> > diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
> > index 10351d5b49fa..61b655b583be 100644
> > --- a/drivers/nvdimm/nd_virtio.c
> > +++ b/drivers/nvdimm/nd_virtio.c
> > @@ -97,29 +97,69 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
> > return err;
> > };
> >
> > +static void submit_async_flush(struct work_struct *ws);
> > +
> > /* The asynchronous flush callback function */
> > int async_pmem_flush(struct nd_region *nd_region, struct bio *bio)
> > {
> > - /*
> > - * Create child bio for asynchronous flush and chain with
> > - * parent bio. Otherwise directly call nd_region flush.
> > + /* queue asynchronous flush and coalesce the flush requests */
> > + struct virtio_device *vdev = nd_region->provider_data;
> > + struct virtio_pmem *vpmem = vdev->priv;
> > + ktime_t req_start = ktime_get_boottime();
> > +
> > + spin_lock_irq(&vpmem->lock);
> > + /* flush requests wait until ongoing flush completes,
> > + * hence coalescing all the pending requests.
> > */
> > - if (bio && bio->bi_iter.bi_sector != -1) {
> > - struct bio *child = bio_alloc(GFP_ATOMIC, 0);
> > -
> > - if (!child)
> > - return -ENOMEM;
> > - bio_copy_dev(child, bio);
> > - child->bi_opf = REQ_PREFLUSH;
> > - child->bi_iter.bi_sector = -1;
> > - bio_chain(child, bio);
> > - submit_bio(child);
> > - return 0;
> > + wait_event_lock_irq(vpmem->sb_wait,
> > + !vpmem->flush_bio ||
> > + ktime_before(req_start, vpmem->prev_flush_start),
> > + vpmem->lock);
> > + /* new request after previous flush is completed */
> > + if (ktime_after(req_start, vpmem->prev_flush_start)) {
> > + WARN_ON(vpmem->flush_bio);
> > + vpmem->flush_bio = bio;
> > + bio = NULL;
> > + }
>
> Why the dance with ->prev_flush_start vs just calling queue_work()
> again. queue_work() is naturally coalescing in that if the last work
> request has not started execution another queue attempt will be
> dropped.
How parent flush request will know when corresponding flush is completed?
>
> > + spin_unlock_irq(&vpmem->lock);
> > +
> > + if (!bio) {
> > + INIT_WORK(&vpmem->flush_work, submit_async_flush);
>
> I expect this only needs to be initialized once at driver init time.
yes, will fix this.
>
> > + queue_work(vpmem->pmem_wq, &vpmem->flush_work);
> > + return 1;
> > + }
> > +
> > + /* flush completed in other context while we waited */
> > + if (bio && (bio->bi_opf & REQ_PREFLUSH)) {
> > + bio->bi_opf &= ~REQ_PREFLUSH;
> > + submit_bio(bio);
> > + } else if (bio && (bio->bi_opf & REQ_FUA)) {
> > + bio->bi_opf &= ~REQ_FUA;
> > + bio_endio(bio);
>
> It's not clear to me how this happens, shouldn't all flush completions
> be driven from the work completion?
Requests should progress after notified by ongoing flush completion
event.
>
> > }
> > - if (virtio_pmem_flush(nd_region))
> > - return -EIO;
> >
> > return 0;
> > };
> > EXPORT_SYMBOL_GPL(async_pmem_flush);
> > +
> > +static void submit_async_flush(struct work_struct *ws)
> > +{
> > + struct virtio_pmem *vpmem = container_of(ws, struct virtio_pmem, flush_work);
> > + struct bio *bio = vpmem->flush_bio;
> > +
> > + vpmem->start_flush = ktime_get_boottime();
> > + bio->bi_status = errno_to_blk_status(virtio_pmem_flush(vpmem->nd_region));
> > + vpmem->prev_flush_start = vpmem->start_flush;
> > + vpmem->flush_bio = NULL;
> > + wake_up(&vpmem->sb_wait);
> > +
> > + /* Submit parent bio only for PREFLUSH */
> > + if (bio && (bio->bi_opf & REQ_PREFLUSH)) {
> > + bio->bi_opf &= ~REQ_PREFLUSH;
> > + submit_bio(bio);
> > + } else if (bio && (bio->bi_opf & REQ_FUA)) {
> > + bio->bi_opf &= ~REQ_FUA;
> > + bio_endio(bio);
> > + }
>
> Shouldn't the wait_event_lock_irq() be here rather than in
> async_pmem_flush()? That will cause the workqueue to back up and flush
> requests to coalesce.
but this is coalesced flush request?
> > +}
> > MODULE_LICENSE("GPL");
> > diff --git a/drivers/nvdimm/virtio_pmem.c b/drivers/nvdimm/virtio_pmem.c
> > index 726c7354d465..56780a6140c7 100644
> > --- a/drivers/nvdimm/virtio_pmem.c
> > +++ b/drivers/nvdimm/virtio_pmem.c
> > @@ -24,6 +24,7 @@ static int init_vq(struct virtio_pmem *vpmem)
> > return PTR_ERR(vpmem->req_vq);
> >
> > spin_lock_init(&vpmem->pmem_lock);
> > + spin_lock_init(&vpmem->lock);
>
> Why 2 locks?
One lock is for work queue and other for virtio flush completion.
Thanks,
Pankaj
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