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From: David Stevens <stevensd@chromium.org>
Date: Mon, 18 Dec 2023 10:16:24 -0500
Message-ID: <CAD=HUj7h-cF929AbSL0bmZE7J=G1aQjZuYSgChPBkSGJ+wfoyQ@mail.gmail.com>
Subject: Re: [PATCH] virtio_balloon: stay awake while adjusting balloon
To: David Hildenbrand <david@redhat.com>
Cc: "Michael S . Tsirkin" <mst@redhat.com>, virtualization@lists.linux-foundation.org, 
	linux-kernel@vger.kernel.org
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On Mon, Dec 18, 2023 at 6:37=E2=80=AFAM David Hildenbrand <david@redhat.com=
> wrote:
>
> On 14.12.23 05:13, David Stevens wrote:
> > On Wed, Dec 13, 2023 at 5:44=E2=80=AFPM David Hildenbrand <david@redhat=
.com> wrote:
> >>
> >> On 11.12.23 12:43, David Stevens wrote:
> >>> From: David Stevens <stevensd@chromium.org>
> >>>
> >>
> >> Hi David,
> >>
> >>> Add a wakeup event for when the balloon is inflating or deflating.
> >>> Userspace can enable this wakeup event to prevent the system from
> >>> suspending while the balloon is being adjusted. This allows
> >>> /sys/power/wakeup_count to be used without breaking virtio_balloon's
> >>> cooperative memory management.
> >>
> >> Can you add/share some more details
> >
> > I'm working on enabling support for Linux s2Idle in our Android
> > virtual machine, to restrict apps from running in the background
> > without holding an Android partial wakelock. With the patch I recently
> > sent out [1], since crosvm advertises native PCI power management for
> > virtio devices, the Android guest can properly enter s2idle, and it
> > can be woken up by incoming IO. However, one of the remaining problems
> > is that when the host needs to reclaim memory from the guest via the
> > virtio-balloon, there is nothing preventing the guest from entering
> > s2idle before the balloon driver finishes returning memory to the
> > host.
>
> Thanks for the information. So you also want to wakeup the VM when
> wanting to get more memory from the VM?
>
> Using which mechanism would that wakeup happen? Likely not the device
> itself?

The wakeup would happen via the parent device's interrupt. I've sent a
new version of this patch that uses the parent device's wakeup event
instead of adding a new one.

> >
> > One alternative to this approach would be to add a virtballoon_suspend
> > callback to abort suspend if the balloon is inflating/adjusting.
> > However, it seems cleaner to just prevent suspend in the first place.
>
> Also, the PM notifier could also be used with very high priority, so the
> device would respond early to PM_SUSPEND_PREPARE.

One drawback of blocking suspend via a PM notifier is that the
behavior isn't configurable by userspace, whereas wakeup events can be
enabled/disabled by userspace.

-David