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

[Jürgen Groß <jgross@xxxxxxxx>]

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.


Juergen

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