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

[Jürgen Groß <jgross@xxxxxxxx>]

On 02.03.20 15:23, Igor Druzhinin wrote:
> On 02/03/2020 14:03, Jürgen Groß wrote:
> > 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?
> I apologies - my previous test result was from machine booted in core mode.
> I'm now testing it properly and the original patch seems to do the trick but
> I still don't understand how you can avoid the race with only 1 counter -
> it's always possible that CPU1 is still in cpu_count checking loop (even if
> cpu_count is currently 0) when cpu_count is reinitialized.
I guess this is very very unlikely.

> I'm looking at your current version now. Was the removal of get_cpu_maps()
> and recursion protection intentional? I suspect it would only work on the
> latest master so I need to keep those for 4.13 testing.
Oh, sorry, this seems to be an old version.

Here comes the correct one.


Juergen

Attachment: v3-0002-xen-rcu-don-t-use-stop_machine_run-for-rcu_barrie.patch
Description: Text Data

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