From: "Jürgen Groß" <jgross@suse.com>
To: Igor Druzhinin <igor.druzhinin@citrix.com>,
xen-devel@lists.xenproject.org
Cc: Stefano Stabellini <sstabellini@kernel.org>,
Julien Grall <julien@xen.org>, Wei Liu <wl@xen.org>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
George Dunlap <George.Dunlap@eu.citrix.com>,
Andrew Cooper <andrew.cooper3@citrix.com>,
Ian Jackson <ian.jackson@eu.citrix.com>,
Jan Beulich <jbeulich@suse.com>
Subject: Re: [Xen-devel] [PATCH 2/2] xen/rcu: don't use stop_machine_run() for rcu_barrier()
Date: Mon, 17 Feb 2020 16:07:16 +0100 [thread overview]
Message-ID: <84b1512b-817c-8c37-350f-0b1fdbd33b6f@suse.com> (raw)
In-Reply-To: <a22149d0-7858-864d-fcdc-590d165dd9c3@citrix.com>
On 17.02.20 15:23, Igor Druzhinin wrote:
> On 17/02/2020 12:30, Igor Druzhinin wrote:
>> On 17/02/2020 12:28, Jürgen Groß wrote:
>>> On 17.02.20 13:26, Igor Druzhinin wrote:
>>>> On 17/02/2020 07:20, Juergen Gross wrote:
>>>>> Today rcu_barrier() is calling stop_machine_run() to synchronize all
>>>>> physical cpus in order to ensure all pending rcu calls have finished
>>>>> when returning.
>>>>>
>>>>> As stop_machine_run() is using tasklets this requires scheduling of
>>>>> idle vcpus on all cpus imposing the need to call rcu_barrier() on idle
>>>>> cpus only in case of core scheduling being active, as otherwise a
>>>>> scheduling deadlock would occur.
>>>>>
>>>>> There is no need at all to do the syncing of the cpus in tasklets, as
>>>>> rcu activity is started in __do_softirq() called whenever softirq
>>>>> activity is allowed. So rcu_barrier() can easily be modified to use
>>>>> softirq for synchronization of the cpus no longer requiring any
>>>>> scheduling activity.
>>>>>
>>>>> As there already is a rcu softirq reuse that for the synchronization.
>>>>>
>>>>> Finally switch rcu_barrier() to return void as it now can never fail.
>>>>>
>>>>
>>>> Would this implementation guarantee progress as previous implementation
>>>> guaranteed?
>>>
>>> Yes.
>>
>> Thanks, I'll put it to test today to see if it solves our use case.
>
> Just manually tried it - gives infinite (up to stack size) trace like:
>
> (XEN) [ 1.496520] [<ffff82d08022e435>] F softirq.c#__do_softirq+0x85/0x90
> (XEN) [ 1.496561] [<ffff82d08022e475>] F process_pending_softirqs+0x35/0x37
> (XEN) [ 1.496600] [<ffff82d080221101>] F rcupdate.c#rcu_process_callbacks+0x1df/0x1f6
> (XEN) [ 1.496643] [<ffff82d08022e435>] F softirq.c#__do_softirq+0x85/0x90
> (XEN) [ 1.496685] [<ffff82d08022e475>] F process_pending_softirqs+0x35/0x37
> (XEN) [ 1.496726] [<ffff82d080221101>] F rcupdate.c#rcu_process_callbacks+0x1df/0x1f6
> (XEN) [ 1.496766] [<ffff82d08022e435>] F softirq.c#__do_softirq+0x85/0x90
> (XEN) [ 1.496806] [<ffff82d08022e475>] F process_pending_softirqs+0x35/0x37
> (XEN) [ 1.496847] [<ffff82d080221101>] F rcupdate.c#rcu_process_callbacks+0x1df/0x1f6
> (XEN) [ 1.496887] [<ffff82d08022e435>] F softirq.c#__do_softirq+0x85/0x90
> (XEN) [ 1.496927] [<ffff82d08022e475>] F process_pending_softirqs+0x35/0x37
Interesting I didn't run into this problem. Obviously I managed to
forget handling the case of recursion.
Juergen
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prev parent reply other threads:[~2020-02-17 15:07 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-17 7:20 [Xen-devel] [PATCH 0/2] xen/rcu: let rcu work better with core scheduling Juergen Gross
2020-02-17 7:20 ` [Xen-devel] [PATCH 1/2] xen/rcu: use rcu softirq for forcing quiescent state Juergen Gross
2020-02-17 7:20 ` [Xen-devel] [PATCH 2/2] xen/rcu: don't use stop_machine_run() for rcu_barrier() Juergen Gross
2020-02-17 11:49 ` Julien Grall
2020-02-17 12:11 ` Jürgen Groß
2020-02-17 12:17 ` Roger Pau Monné
2020-02-17 12:32 ` Jürgen Groß
2020-02-17 12:49 ` Roger Pau Monné
2020-02-17 13:17 ` Jürgen Groß
2020-02-17 13:47 ` Roger Pau Monné
2020-02-17 13:56 ` Jürgen Groß
2020-02-17 12:26 ` Igor Druzhinin
2020-02-17 12:28 ` Jürgen Groß
2020-02-17 12:30 ` Igor Druzhinin
2020-02-17 14:23 ` Igor Druzhinin
2020-02-17 15:07 ` Jürgen Groß [this message]
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