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Re: [Qemu-devel] [RFC PATCH 1/3] balloon: Allow nested inhibits
From: |
Peter Xu |
Subject: |
Re: [Qemu-devel] [RFC PATCH 1/3] balloon: Allow nested inhibits |
Date: |
Thu, 19 Jul 2018 12:45:17 +0800 |
User-agent: |
Mutt/1.10.0 (2018-05-17) |
On Wed, Jul 18, 2018 at 10:37:36AM -0600, Alex Williamson wrote:
> On Wed, 18 Jul 2018 14:40:15 +0800
> Peter Xu <address@hidden> wrote:
>
> > On Tue, Jul 17, 2018 at 04:47:37PM -0600, Alex Williamson wrote:
> > > A simple true/false internal state does not allow multiple users. Fix
> > > this within the existing interface by converting to a counter, so long
> > > as the counter is elevated, ballooning is inhibited.
> > >
> > > Signed-off-by: Alex Williamson <address@hidden>
> > > ---
> > > balloon.c | 7 ++++---
> > > 1 file changed, 4 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/balloon.c b/balloon.c
> > > index 6bf0a9681377..2a6a7e1a22a0 100644
> > > --- a/balloon.c
> > > +++ b/balloon.c
> > > @@ -37,16 +37,17 @@
> > > static QEMUBalloonEvent *balloon_event_fn;
> > > static QEMUBalloonStatus *balloon_stat_fn;
> > > static void *balloon_opaque;
> > > -static bool balloon_inhibited;
> > > +static int balloon_inhibited;
> > >
> > > bool qemu_balloon_is_inhibited(void)
> > > {
> > > - return balloon_inhibited;
> > > + return balloon_inhibited > 0;
> > > }
> > >
> > > void qemu_balloon_inhibit(bool state)
> > > {
> > > - balloon_inhibited = state;
> > > + balloon_inhibited += (state ? 1 : -1);
> > > + assert(balloon_inhibited >= 0);
> >
> > Better do it atomically?
>
> I'd assumed we're protected by the BQL anywhere this is called. Is
> that not the case? Generally when I try to add any sort of locking to
> QEMU it's shot down because the code paths are already serialized.
Ah I see. :-)
But I guess current code might call these without BQL. For example,
qemu_balloon_inhibit() has:
postcopy_ram_listen_thread
qemu_loadvm_state_main
loadvm_process_command
loadvm_postcopy_handle_listen
postcopy_ram_enable_notify
qemu_balloon_inhibit
While the whole stack is inside a standalone thread to load QEMU for
postcopy incoming migration rather than the main thread.
Thanks,
--
Peter Xu
Re: [Qemu-devel] [RFC PATCH 0/3] Balloon inhibit enhancements, Alex Williamson, 2018/07/18