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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Tue, 18 Aug 2020 23:55:11 +0200
To:     "Paul E. McKenney" <paulmck@kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        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>
Subject: Re: [PATCH] rcu: shrink each possible cpu krcp
Message-ID: <20200818215511.GA2538@pc636>
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> On Tue, Aug 18, 2020 at 03:00:35PM -0400, Joel Fernandes wrote:
> > 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>
> 
> Thank you all!  I wordsmithed a bit as shown below, so please let
> me know if I messed anything up.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit fe5d89cc025b3efe682cac122bc4d39f4722821e
> Author: Zqiang <qiang.zhang@windriver.com>
> Date:   Fri Aug 14 14:45:57 2020 +0800
> 
>     rcu: Shrink each possible cpu krcp
>     
>     CPUs can go offline shortly after kfree_call_rcu() has been invoked,
>     which can leave memory stranded until those CPUs come back online.
>     This commit therefore drains the kcrp of each CPU, not just the
>     ones that happen to be online.
>     
>     Acked-by: Joel Fernandes <joel@joelfernandes.org>
>     Signed-off-by: Zqiang <qiang.zhang@windriver.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 02ca8e5..d9f90f6 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3500,7 +3500,7 @@ 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);
> @@ -3515,7 +3515,7 @@ 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);
>  
> @@ -3548,7 +3548,7 @@ 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);
>

Should we just clean a krc of a CPU when it goes offline?

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b8ccd7b5af82..6decb9ad2421 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2336,10 +2336,15 @@ int rcutree_dead_cpu(unsigned int cpu)
 {
        struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
        struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
+       struct kfree_rcu_cpu *krcp;
 
        if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
                return 0;
 
+       /* Drain the kcrp of this CPU. IRQs should be disabled? */
+       krcp = this_cpu_ptr(&krc)
+       schedule_delayed_work(&krcp->monitor_work, 0);
+

A cpu can be offlined and its krp will be stuck until a shrinker is involved.
Maybe be never.

--
Vlad Rezki