Re: [Xen-devel] [PATCH 2/2] xen/rcu: don't use stop_machine_run() for rcu_barrier()

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

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

Igor

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