From: Yury Norov <ynorov@caviumnetworks.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrea Parri <andrea.parri@amarulasolutions.com>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Chris Metcalf <cmetcalf@mellanox.com>,
Christopher Lameter <cl@linux.com>,
Russell King - ARM Linux <linux@armlinux.org.uk>,
Mark Rutland <mark.rutland@arm.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will.deacon@arm.com>,
Pekka Enberg <penberg@kernel.org>,
David Rientjes <rientjes@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Andrew Morton <akpm@linux-foundation.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
linux-arm-kernel@lists.infradead.org,
linuxppc-dev@lists.ozlabs.org, kvm-ppc@vger.kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] smp: introduce kick_active_cpus_sync()
Date: Wed, 28 Mar 2018 15:59:32 +0300 [thread overview]
Message-ID: <20180328125932.dhzwoxhext4h7hgh@yury-thinkpad> (raw)
In-Reply-To: <20180326145735.57ba306b@gandalf.local.home>
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
next prev parent reply other threads:[~2018-03-28 12:59 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-25 17:50 [PATCH 0/2] smp: don't kick CPUs running idle or nohz_full tasks Yury Norov
2018-03-25 17:50 ` [PATCH 1/2] rcu: declare rcu_eqs_special_set() in public header Yury Norov
2018-03-25 19:12 ` Paul E. McKenney
2018-03-25 19:18 ` Yury Norov
2018-03-25 17:50 ` [PATCH 2/2] smp: introduce kick_active_cpus_sync() Yury Norov
2018-03-25 19:23 ` Paul E. McKenney
2018-03-25 20:11 ` Yury Norov
2018-03-26 12:45 ` Paul E. McKenney
2018-03-28 13:36 ` Yury Norov
2018-03-28 13:56 ` Paul E. McKenney
2018-03-28 14:41 ` Yury Norov
2018-03-28 14:45 ` Paul E. McKenney
2018-03-26 8:53 ` Andrea Parri
2018-03-26 18:57 ` Steven Rostedt
2018-03-28 12:59 ` Yury Norov [this message]
2018-03-27 10:21 ` Will Deacon
2018-03-28 10:58 ` Yury Norov
2018-04-01 11:11 ` Yury Norov
2018-04-01 14:10 ` Paul E. McKenney
2018-04-03 13:48 ` Mark Rutland
2018-04-04 3:36 ` Yury Norov
2018-04-04 9:08 ` Mark Rutland
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