Re: [PATCH] virtio pmem: fix async flush ordering

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Nov 21, 2019 at 9:37 PM Pankaj Gupta <pagupta@redhat.com> wrote:
>
>
> > > > > > >
> > > > > > > I added that and was about to push this out, but what about the
> > > > > > > fact
> > > > > > > that now the guest will synchronously wait for flushing to occur.
> > > > > > > The
> > > > > > > goal of the child bio was to allow that to be an I/O wait with
> > > > > > > overlapping I/O, or at least not blocking the submission thread.
> > > > > > > Does
> > > > > > > the block layer synchronously wait for PREFLUSH requests? If not I
> > > > > > > think a synchronous wait is going to be a significant performance
> > > > > > > regression. Are there any numbers to accompany this change?
> > > > > >
> > > > > > Why not just swap the parent child relationship in the PREFLUSH case?
> > > > >
> > > > > I we are already inside parent bio "make_request" function and we
> > > > > create
> > > > > child
> > > > > bio. How we exactly will swap the parent/child relationship for
> > > > > PREFLUSH
> > > > > case?
> > > > >
> > > > > Child bio is queued after parent bio completes.
> > > >
> > > > Sorry, I didn't quite mean with bio_split, but issuing another request
> > > > in front of the real bio. See md_flush_request() for inspiration.
> > >
> > > o.k. Thank you. Will try to post patch today to be considered for 5.4.
> > >
> >
> > I think it is too late for v5.4-final, but we can get it in the
> > -stable queue. Let's take the time to do it right and get some testing
> > on it.
>
> Sure.
>
> Just sharing probable patch for early feedback, if I am doing it correctly?
> I will test it thoroughly.
>
> Thanks,
> Pankaj
>
> ========
>
> diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
> index 10351d5b49fa..c683e0e2515c 100644
> --- a/drivers/nvdimm/nd_virtio.c
> +++ b/drivers/nvdimm/nd_virtio.c
> @@ -112,6 +112,12 @@ int async_pmem_flush(struct nd_region *nd_region, struct bio *bio)
>                 bio_copy_dev(child, bio);
>                 child->bi_opf = REQ_PREFLUSH;
>                 child->bi_iter.bi_sector = -1;
> +
> +               if (unlikely(bio->bi_opf & REQ_PREFLUSH)) {
> +                       struct request_queue *q = bio->bi_disk->queue;
> +                       q->make_request_fn(q, child);
> +                       return 0;
> +               }
>                 bio_chain(child, bio);
>                 submit_bio(child);

In the md case there is a lower level device to submit to. In this
case I expect you would

- create a flush workqueue
- queue the bio that workqueue and wait for any previous flush request
to complete (md_flush_request does this)
- run virtio_pmem_flush
- complete the original bio

Is there a way to make virtio_pmem_flush() get an interrupt when the
flush is complete rather than synchronously waiting. That way if you
get a storm of flush requests you can coalesce them like
md_flush_request() does.
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