linux-block.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Stefan Hajnoczi <stefanha@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: virtualization@lists.linux-foundation.org,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jason Wang <jasowang@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>, Jens Axboe <axboe@kernel.dk>,
	slp@redhat.com, sgarzare@redhat.com,
	"Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll()
Date: Tue, 25 May 2021 14:19:22 +0100	[thread overview]
Message-ID: <YKz5WqaeU0R0jQMu@stefanha-x1.localdomain> (raw)
In-Reply-To: <20210524145928.GA3873@lst.de>

[-- Attachment #1: Type: text/plain, Size: 1946 bytes --]

On Mon, May 24, 2021 at 04:59:28PM +0200, Christoph Hellwig wrote:
> On Thu, May 20, 2021 at 03:13:05PM +0100, Stefan Hajnoczi wrote:
> > Possible drawbacks of this approach:
> > 
> > - Hardware virtio_blk implementations may find virtqueue_disable_cb()
> >   expensive since it requires DMA. If such devices become popular then
> >   the virtio_blk driver could use a similar approach to NVMe when
> >   VIRTIO_F_ACCESS_PLATFORM is detected in the future.
> > 
> > - If a blk_poll() thread is descheduled it not only hurts polling
> >   performance but also delays completion of non-REQ_HIPRI requests on
> >   that virtqueue since vq notifications are disabled.
> 
> Yes, I think this is a dangerous configuration.  What argument exists
> again just using dedicated poll queues?

Aside from what Paolo described (the lack of a hardware interface to
designate polling queues), the poll_queues=N parameter needs to be added
to the full guest and host software stack. Users also need to learn
about this so they can enable it in all the places. This is much more
hassle for users to configure. The dynamic polling mode approach
requires no configuration.

Paolo's suggestion is to notify the driver when irqs need to be
re-enabled if the polling thread is descheduled. I actually have a
prototype that achieves something similar here:
https://gitlab.com/stefanha/linux/-/commits/cpuidle-haltpoll-virtqueue

It's a different approach from the current patch series: the cpuidle
driver provides poll_start/stop() callbacks and virtio_blk participates
in cpuidle-haltpoll. That means all virtio-blk devices temporarily use
polling mode while the vCPU is doing cpuidle-haltpoll polling. The neat
thing about the cpuidle approach is:
1. Applications don't need to set the RWF_HIPRI flag!
2. Other drivers besides virtio_blk can participate in polling too.

Maybe we should go with cpuidle polling instead?

Stefan

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

  parent reply	other threads:[~2021-05-25 13:19 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-20 14:13 [PATCH 0/3] virtio_blk: blk-mq io_poll support Stefan Hajnoczi
2021-05-20 14:13 ` [PATCH 1/3] virtio: add virtioqueue_more_used() Stefan Hajnoczi
2021-05-25  2:23   ` Jason Wang
2021-05-25  8:48     ` Stefan Hajnoczi
2021-05-20 14:13 ` [PATCH 2/3] virtio_blk: avoid repeating vblk->vqs[qid] Stefan Hajnoczi
2021-05-25  2:25   ` Jason Wang
2021-05-20 14:13 ` [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll() Stefan Hajnoczi
2021-05-24 14:59   ` Christoph Hellwig
2021-05-25  7:22     ` Paolo Bonzini
2021-05-25  7:38       ` Ming Lei
2021-05-25  8:06         ` Paolo Bonzini
2021-05-25 13:20         ` Stefan Hajnoczi
2021-05-25 13:19     ` Stefan Hajnoczi [this message]
2021-05-25  3:21   ` Jason Wang
2021-05-25  8:59     ` Stefan Hajnoczi
2021-05-27  5:48       ` Jason Wang
2021-06-03 15:24         ` Stefan Hajnoczi
2021-05-27  2:44   ` Ming Lei
2021-06-03 15:12     ` Stefan Hajnoczi
2021-06-03 15:30 ` [PATCH 0/3] virtio_blk: blk-mq io_poll support Stefan Hajnoczi
2021-06-16  7:43   ` Christoph Hellwig

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=YKz5WqaeU0R0jQMu@stefanha-x1.localdomain \
    --to=stefanha@redhat.com \
    --cc=axboe@kernel.dk \
    --cc=hch@lst.de \
    --cc=jasowang@redhat.com \
    --cc=linux-block@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mst@redhat.com \
    --cc=pbonzini@redhat.com \
    --cc=sgarzare@redhat.com \
    --cc=slp@redhat.com \
    --cc=virtualization@lists.linux-foundation.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).