Re: [PATCH] rcu: shrink each possible cpu krcp

Uladzislau Rezki
2020-08-18T21:55:20Z

> 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