Re: [PATCH 2/2] smp: introduce kick_active_cpus_sync()

[Yury Norov <ynorov@caviumnetworks.com>]

On Mon, Mar 26, 2018 at 02:57:35PM -0400, Steven Rostedt wrote:
> 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

I know that I had to copy the comment from kick_all_cpus_sync(), but I
don't like copy-pasting in general, and as Steven told, this smp_mb() is
already inside synchronization routine, so we may hope that users of
kick_*_cpus_sync() will explain better what for they need it...
 
> 
> > 
> > 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

I need v2 anyway, and I will add comments to address all questions in this
thread.

I also hope that we'll agree that for powerpc it's also safe to delay
synchronization, and if so, we will have no users of kick_all_cpus_sync(),
and can drop it.

(It looks like this, because nohz_full userspace CPU cannot have pending
IPIs, but I'd like to get confirmation from powerpc people.)

Would it make sense to rename kick_all_cpus_sync() to smp_mb_sync(), which
would stand for 'synchronous memory barrier on all online CPUs'?

Yury