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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Tue, 18 Aug 2020 15:00:35 -0400
Message-ID: <CAEXW_YQu9MAV-3ym0EFB0NmomWkLsBtZCT9sShnzo+vv=8sLgg@mail.gmail.com>
Subject: Re: [PATCH] rcu: shrink each possible cpu krcp
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Uladzislau Rezki <urezki@gmail.com>, qiang.zhang@windriver.com,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        rcu <rcu@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
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On Tue, Aug 18, 2020 at 1:18 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Mon, Aug 17, 2020 at 06:03:54PM -0400, Joel Fernandes wrote:
> > On Fri, Aug 14, 2020 at 2:51 PM Uladzislau Rezki <urezki@gmail.com> wrote:
> > >
> > > > From: Zqiang <qiang.zhang@windriver.com>
> > > >
> > > > Due to cpu hotplug. some cpu may be offline after call "kfree_call_rcu"
> > > > func, if the shrinker is triggered at this time, we should drain each
> > > > possible cpu "krcp".
> > > >
> > > > Signed-off-by: Zqiang <qiang.zhang@windriver.com>
> > > > ---
> > > >  kernel/rcu/tree.c | 6 +++---
> > > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index 8ce77d9ac716..619ccbb3fe4b 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -3443,7 +3443,7 @@ kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
> > > >       unsigned long count = 0;
> > > >
> > > >       /* Snapshot count of all CPUs */
> > > > -     for_each_online_cpu(cpu) {
> > > > +     for_each_possible_cpu(cpu) {
> > > >               struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> > > >
> > > >               count += READ_ONCE(krcp->count);
> > > > @@ -3458,7 +3458,7 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> > > >       int cpu, freed = 0;
> > > >       unsigned long flags;
> > > >
> > > > -     for_each_online_cpu(cpu) {
> > > > +     for_each_possible_cpu(cpu) {
> > > >               int count;
> > > >               struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> > > >
> > > > @@ -3491,7 +3491,7 @@ void __init kfree_rcu_scheduler_running(void)
> > > >       int cpu;
> > > >       unsigned long flags;
> > > >
> > > > -     for_each_online_cpu(cpu) {
> > > > +     for_each_possible_cpu(cpu) {
> > > >               struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> > > >
> > > >               raw_spin_lock_irqsave(&krcp->lock, flags);
> > > >
> > > I agree that it can happen.
> > >
> > > Joel, what is your view?
> >
> > Yes I also think it is possible. The patch LGTM. Another fix could be
> > to drain the caches in the CPU offline path and save the memory. But
> > then it will take hit during __get_free_page(). If CPU
> > offlining/online is not frequent, then it will save the lost memory.
> >
> > I wonder how other per-cpu caches in the kernel work in such scenarios.
> >
> > Thoughts?
>
> Do I count this as an ack or a review?  If not, what precisely would
> you like the submitter to do differently?

Hi Paul,
The patch is correct and is definitely an improvement. I was thinking
about whether we should always do what the patch is doing when
offlining CPUs to save memory but now I feel that may not be that much
of a win to justify more complexity.

You can take it with my ack:

Acked-by: Joel Fernandes <joel@joelfernandes.org>

 thanks,

 - Joel