Re: [Xen-devel] [PATCH v2 0/4] xen/rcu: let rcu work better with core scheduling

[Jürgen Groß <jgross@xxxxxxxx>]

On 02.03.20 14:25, Igor Druzhinin wrote:
> On 28/02/2020 07:10, Jürgen Groß wrote:
> > I think you are just narrowing the window of the race:
> >
> > It is still possible to have two cpus entering rcu_barrier() and to
> > make it into the if ( !initial ) clause.
> >
> > Instead of introducing another atomic I believe the following patch
> > instead of yours should do it:
> >
> > diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c
> > index e6add0b120..0d5469a326 100644
> > --- a/xen/common/rcupdate.c
> > +++ b/xen/common/rcupdate.c
> > @@ -180,23 +180,17 @@ static void rcu_barrier_action(void)
> >
> >   void rcu_barrier(void)
> >   {
> > -    int initial = atomic_read(&cpu_count);
> > -
> >       while ( !get_cpu_maps() )
> >       {
> >           process_pending_softirqs();
> > -        if ( initial && !atomic_read(&cpu_count) )
> > +        if ( !atomic_read(&cpu_count) )
> >               return;
> >
> >           cpu_relax();
> > -        initial = atomic_read(&cpu_count);
> >       }
> >
> > -    if ( !initial )
> > -    {
> > -        atomic_set(&cpu_count, num_online_cpus());
> > +    if ( atomic_cmpxchg(&cpu_count, 0, num_online_cpus()) == 0 )
> >           cpumask_raise_softirq(&cpu_online_map, RCU_SOFTIRQ);
> > -    }
> >
> >       while ( atomic_read(&cpu_count) )
> >       {
> >
> > Could you give that a try, please?
> With this patch I cannot disable SMT at all.
>
> The problem that my diff solved was a race between 2 consecutive
> rcu_barrier operations on CPU0 (the pattern specific to SMT-on/off
> operation) where some CPUs didn't exit the cpu_count checking loop
> completely but cpu_count is already reinitialized on CPU0 - this
> results in some CPUs being stuck in the loop.
Ah, okay, then I believe a combination of the two patches is needed.

Something like the attached version?


Juergen

Attachment: 0002-xen-rcu-don-t-use-stop_machine_run-for-rcu_barrier.patch
Description: Text Data

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