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 12:28, 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.

Igor

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