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SubjectRe: [PATCH 2/2] smp: introduce kick_active_cpus_sync()
On Mon, 26 Mar 2018 10:53:13 +0200
Andrea Parri <andrea.parri@amarulasolutions.com> wrote:

> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -724,6 +724,30 @@ void kick_all_cpus_sync(void)
> > }
> > EXPORT_SYMBOL_GPL(kick_all_cpus_sync);
> >
> > +/**
> > + * kick_active_cpus_sync - Force CPUs that are not in extended
> > + * quiescent state (idle or nohz_full userspace) sync by sending
> > + * IPI. Extended quiescent state CPUs will sync at the exit of
> > + * that state.
> > + */
> > +void kick_active_cpus_sync(void)
> > +{
> > + int cpu;
> > + struct cpumask kernel_cpus;
> > +
> > + smp_mb();
>
> (A general remark only:)
>
> checkpatch.pl should have warned about the fact that this barrier is
> missing an accompanying comment (which accesses are being "ordered",
> what is the pairing barrier, etc.).

He could have simply copied the comment above the smp_mb() for
kick_all_cpus_sync():

/* Make sure the change is visible before we kick the cpus */

The kick itself is pretty much a synchronization primitive.

That is, you make some changes and then you need all CPUs to see it,
and you call: kick_active_cpus_synch(), which is the barrier to make
sure you previous changes are seen on all CPUS before you proceed
further. Note, the matching barrier is implicit in the IPI itself.

-- Steve


>
> Moreover if, as your reply above suggested, your patch is relying on
> "implicit barriers" (something I would not recommend) then even more
> so you should comment on these requirements.
>
> This could: (a) force you to reason about the memory ordering stuff,
> (b) easy the task of reviewing and adopting your patch, (c) easy the
> task of preserving those requirements (as implementations changes).
>
> Andrea
>
>
> > +
> > + cpumask_clear(&kernel_cpus);
> > + preempt_disable();
> > + for_each_online_cpu(cpu) {
> > + if (!rcu_eqs_special_set(cpu))
> > + cpumask_set_cpu(cpu, &kernel_cpus);
> > + }
> > + smp_call_function_many(&kernel_cpus, do_nothing, NULL, 1);
> > + preempt_enable();
> > +}
> > +EXPORT_SYMBOL_GPL(kick_active_cpus_sync);
> > +
> > /**
> > * wake_up_all_idle_cpus - break all cpus out of idle
> > * wake_up_all_idle_cpus try to break all cpus which is in idle state even
> > diff --git a/mm/slab.c b/mm/slab.c
> > index 324446621b3e..678d5dbd6f46 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -3856,7 +3856,7 @@ static int __do_tune_cpucache(struct kmem_cache *cachep, int limit,
> > * cpus, so skip the IPIs.
> > */
> > if (prev)
> > - kick_all_cpus_sync();
> > + kick_active_cpus_sync();
> >
> > check_irq_on();
> > cachep->batchcount = batchcount;
> > --
> > 2.14.1
> >

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