From: George Dunlap <george.dunlap@citrix.com>
To: "Jürgen Groß" <jgross@suse.com>, xen-devel@lists.xenproject.org
Cc: George Dunlap <george.dunlap@eu.citrix.com>,
Dario Faggioli <dfaggioli@suse.com>
Subject: Re: [Xen-devel] [PATCH] xen/sched: fix locking in sched_tick_[suspend|resume]()
Date: Fri, 4 Oct 2019 15:34:01 +0100 [thread overview]
Message-ID: <2965839e-6be2-720f-ad1a-8e725f0bffaa@citrix.com> (raw)
In-Reply-To: <e8364443-4f7f-62b7-f00d-4687ae07628b@suse.com>
On 10/4/19 3:24 PM, Jürgen Groß wrote:
> On 04.10.19 16:08, George Dunlap wrote:
>> On 10/4/19 7:40 AM, Juergen Gross wrote:
>>> sched_tick_suspend() and sched_tick_resume() should not call the
>>> scheduler specific timer handlers in case the cpu they are running on
>>> is just being moved to or from a cpupool.
>>>
>>> Use a new percpu lock for that purpose.
>>
>> Is there a reason we can't use the pcpu_schedule_lock() instead of
>> introducing a new one? Sorry if this is obvious, but it's been a while
>> since I poked around this code.
>
> Lock contention would be higher especially with credit2 which is using a
> per-core or even per-socket lock. We don't care about other scheduling
> activity here, all we need is a guard against our per-cpu scheduler
> data being changed beneath our feet.
Is this code really being called so often that we need to worry about
this level of contention?
We already have a *lot* of locks; and in this case you're adding a
second lock which interacts with the per-scheduler cpu lock. This just
seems like asking for trouble.
I won't Nack the patch, but I don't think I would ack it without clear
evidence that the extra lock has a performance improvement that's worth
the cost of the extra complexity.
-George
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next prev parent reply other threads:[~2019-10-04 14:34 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-04 6:40 [Xen-devel] [PATCH] xen/sched: fix locking in sched_tick_[suspend|resume]() Juergen Gross
2019-10-04 7:50 ` Andrew Cooper
2019-10-04 14:29 ` Dario Faggioli
2019-10-04 14:49 ` Andrew Cooper
2019-10-04 14:08 ` George Dunlap
2019-10-04 14:24 ` Jürgen Groß
2019-10-04 14:34 ` George Dunlap [this message]
2019-10-04 14:43 ` Jürgen Groß
2019-10-04 14:56 ` George Dunlap
2019-10-04 15:03 ` Jürgen Groß
2019-10-04 15:37 ` George Dunlap
2019-10-04 15:40 ` Jürgen Groß
2019-10-04 16:09 ` George Dunlap
2019-10-06 18:05 ` Jürgen Groß
2019-10-07 6:09 ` Jürgen Groß
2019-10-07 8:49 ` Dario Faggioli
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