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From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Dmitry Isaikin <isaikin-dmitry@yandex.ru>
Cc: Jan Beulich <jbeulich@suse.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Ian Jackson <iwj@xenproject.org>, Julien Grall <julien@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	<xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v1] domctl: hold domctl lock while domain is destroyed
Date: Thu, 16 Sep 2021 15:08:03 +0200	[thread overview]
Message-ID: <YUNBswZ3rAFeDa6I@MacBook-Air-de-Roger.local> (raw)
In-Reply-To: <22a4b3c7-7faa-607b-244a-0ab99d7ccfa9@suse.com>

On Thu, Sep 16, 2021 at 02:30:39PM +0200, Jan Beulich wrote:
> On 16.09.2021 13:10, Dmitry Isaikin wrote:
> > From: Dmitry Isaykin <isaikin-dmitry@yandex.ru>
> > 
> > This significantly speeds up concurrent destruction of multiple domains on x86.
> 
> This effectively is a simplistic revert of 228ab9992ffb ("domctl:
> improve locking during domain destruction"). There it was found to
> actually improve things; now you're claiming the opposite. It'll
> take more justification, clearly identifying that you actually
> revert an earlier change, and an explanation why then you don't
> revert that change altogether. You will want to specifically also
> consider the cleaning up of huge VMs, where use of the (global)
> domctl lock may hamper progress of other (parallel) operations on
> the system.
> 
> > I identify the place taking the most time:
> > 
> >     do_domctl(case XEN_DOMCTL_destroydomain)
> >       -> domain_kill()
> >            -> domain_relinquish_resources()
> >                 -> relinquish_memory(d, &d->page_list, PGT_l4_page_table)
> > 
> > My reference setup: Intel(R) Xeon(R) CPU E5-2680 v4 @ 2.40GHz, Xen 4.14.
> > 
> > I use this command for test:
> > 
> >     for i in $(seq 1 5) ; do xl destroy test-vm-${i} & done
> > 
> > Without holding the lock all calls of `relinquish_memory(d, &d->page_list, PGT_l4_page_table)`
> > took on my setup (for HVM with 2GB of memory) about 3 seconds for each destroying domain.
> > 
> > With holding the lock it took only 100 ms.
> 
> I'm further afraid I can't make the connection. Do you have an
> explanation for why there would be such a massive difference?
> What would prevent progress of relinquish_memory() with the
> domctl lock not held?

I would recommend to Dmitry to use lock profiling with and without
this change applied and try to spot which lock is causing the
contention as a starting point. That should be fairly easy and could
share some light.

Regards, Roger.


  reply	other threads:[~2021-09-16 13:14 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-16 11:10 Dmitry Isaikin
2021-09-16 12:30 ` Jan Beulich
2021-09-16 13:08   ` Roger Pau Monné [this message]
2021-09-16 17:52   ` Andrew Cooper
2021-09-17  6:17     ` Jan Beulich
2021-09-17  9:27       ` Julien Grall
2021-09-17  9:41         ` Andrew Cooper
2021-09-17  9:47           ` Jan Beulich
2021-09-17 16:01             ` Julien Grall
2021-09-20  8:19               ` Jan Beulich

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