linux-kernel - Re: [PATCH v8 1/5] mm: introduce a common interface for balloon pages mobility

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Date:	Thu, 23 Aug 2012 13:01:07 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rafael Aquini <aquini@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v8 1/5] mm: introduce a common interface for balloon
 pages mobility

On Wed, Aug 22, 2012 at 11:19:04PM -0300, Rafael Aquini wrote:
> On Wed, Aug 22, 2012 at 12:33:17PM +0300, Michael S. Tsirkin wrote:
> > Hmm, so this will busy wait which is unelegant.
> > We need some event IMO.
> 
> No, it does not busy wait. leak_balloon() is mutual exclusive with migration
> steps, so for the case we have one racing against the other, we really want
> leak_balloon() dropping the mutex temporarily to allow migration complete its
> work of refilling vb->pages list. Also, leak_balloon() calls tell_host(), which
> will potentially make it to schedule for each round of vb->pfns leak_balloon()
> will release.

tell_host might not even cause an exit to host. Even if it does
it does not involve guest scheduler.

> So, when remove_common() calls leak_balloon() looping on
> vb->num_pages, that won't become a tight loop. 
> The scheme was apparently working before this series, and it will remain working
> after it.

It seems that before we would always leak all requested memory
in one go. I can't tell why we have a while loop there at all.
Rusty, could you clarify please?

> 
> > Also, reading num_pages without a lock here
> > which seems wrong.
> 
> I'll protect it with vb->balloon_lock mutex. That will be consistent with the
> lock protection scheme this patch is introducing for struct virtio_balloon
> elements.
> 
> 
> > A similar concern applies to normal leaking
> > of the balloon: here we might leak less than
> > required, then wait for the next config change
> > event.
> 
> Just as before, same thing here. If you leaked less than required, balloon()
> will keep calling leak_balloon() until the balloon target is reached. This
> scheme was working before, and it will keep working after this patch.
>

IIUC we never hit this path before.
 
> > How about we signal config_change
> > event when pages are back to pages_list?
> 
> I really don't know what to tell you here, but, to me, it seems like an
> overcomplication that isn't directly entangled with this patch purposes.
> Besides, you cannot expect compation / migration happening and racing against
> leak_balloon() all the time to make them signal events to the later, so we might
> just be creating a wait-forever condition for leak_balloon(), IMHO.

So use wait_event or similar, check for existance of isolated pages.

> Cheers!
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