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Re: [PATCH for-4.16] Revert "domctl: improve locking during domain destruction"
- To: Ian Jackson <iwj@xxxxxxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Tue, 9 Nov 2021 16:15:24 +0100
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- Cc: <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Hongyan Xia <hongyxia@xxxxxxxxxx>, Dmitry Isaikin <isaikin-dmitry@xxxxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue, Nov 09, 2021 at 03:04:56PM +0000, Ian Jackson wrote:
> Roger Pau Monne writes ("[PATCH for-4.16] Revert "domctl: improve locking
> during domain destruction""):
> > This reverts commit 228ab9992ffb1d8f9d2475f2581e68b2913acb88.
> >
> > Performance analysis has shown that dropping the domctl lock during
> > domain destruction greatly increases the contention in the heap_lock,
> > thus making parallel destruction of domains slower.
> ...
> > Given the current point in the release, revert the commit and
> > reinstate holding the domctl lock during domain destruction. Further
> > work should be done in order to re-add more fine grained locking to
> > the domain destruction path once a proper solution to avoid the
> > heap_lock contention is found.
> > ---
> ...
> > Since this is a revert and not new code I think the risk is lower.
> > There's however some risk, as the original commit was from 2017, and
> > hence the surrounding code has changed a bit. It's also a possibility
> > that some other parts of the domain destruction code now rely on this
> > more fine grained locking. Local tests however haven't shown issues.
>
> From a release management point of view I don't regard this as the
> kind of "revert" that ought to get any kind of special consideration.
> The tree has been like this since 2017 and Xen 4.11 and many changes
> have been happened since.
>
> So I am going to treat this as an effectively new change.
>
> AIUI it is a proposal to improve performance, not a bugfix. Was this
> change posted (or, proposed on-list) before the Xen 4.16 Last Posting
> Date (24th of September) ? Even if it was, it would need a freeze
> exception.
It was posted here:
https://lore.kernel.org/xen-devel/de46590ad566d9be55b26eaca0bc4dc7fbbada59.1585063311.git.hongyxia@xxxxxxxxxx/
Which was missing a spin_barrier, and in a different form here:
https://lore.kernel.org/xen-devel/2e7044de3cd8a6768a20250e61fe262f3a018724.1631790362.git.isaikin-dmitry@xxxxxxxxx/
Thanks, Roger.
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