On Wed 04-08-21 10:33:41, Michal Hocko wrote:
> On Wed 04-08-21 09:39:23, Vlastimil Babka wrote:
> > On 8/4/21 1:21 AM, Thomas Gleixner wrote:
> > > /*
> > > * The only protection from memory hotplug vs. drain_stock races is
> > > * that we always operate on local CPU stock here with IRQ disabled
> > > */
> > > - local_irq_save(flags);
> > > + local_lock_irqsave(memcg_stock_lock, flags);
> > > ...
> > > if (use_task_obj_stock())
> > > drain_obj_stock(&stock->task_obj);
> > >
> > > which is incomprehensible garbage.
> > >
> > > The comment above the existing local_irq_save() is garbage w/o any local
> > > lock conversion already today (and even before the commit which
> > > introduced stock::task_obj) simply because that comment does not explain
> > > the why.
> >
> > Michal, this seems to be your comment from commit 72f0184c8a00 ("mm, memcg:
> > remove hotplug locking from try_charge"). Was "memory hotplug" a mistake,
> > because the rest of the commit is about cpu hotplug, and I don't really see a
> > memory hotplug connection there?
>
> This part of the changelog tried to explain that part IIRC
> "
> We can get rid of {get,put}_online_cpus, fortunately. We do not have to
> be worried about races with memory hotplug because drain_local_stock,
> which is called from both the WQ draining and the memory hotplug
> contexts, is always operating on the local cpu stock with IRQs disabled.
> "
>
> Now I have to admit I do not remember all the details and from a quick
> look the memory hotplug doesn't seem to be draining memcg pcp stock.
> Maybe this has been removed since then. The only stock draining outside
> of the memcg code seems to be memcg_hotplug_cpu_dead callback. That
> would indicate that I really meant the cpu hotplug here indeed.
Does this look better?
---
>From 5aa1c8ce0d88b8c6d59ba95c7e36ca07dc2b2161 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@xxxxxxxx>
Date: Mon, 9 Aug 2021 10:59:04 +0200
Subject: [PATCH] memcg: fix up drain_local_stock comment
Thomas and Vlastimil have noticed that the comment in drain_local_stock
doesn't quite make sense. It talks about a synchronization with the
memory hotplug but there is no actual memory hotplug involvement here.
I meant to talk about cpu hotplug here. Fix that up and hopefuly make
the comment more helpful by referencing the cpu hotplug callback as
well.
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
mm/memcontrol.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index eb8e87c4833f..f7be7b01395e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2205,8 +2205,9 @@ static void drain_local_stock(struct work_struct *dummy)
unsigned long flags;
/*
- * The only protection from memory hotplug vs. drain_stock races is
- * that we always operate on local CPU stock here with IRQ disabled
+ * The only protection from cpu hotplug (memcg_hotplug_cpu_dead) vs.
+ * drain_stock races is that we always operate on local CPU stock
+ * here with IRQ disabled
*/
local_irq_save(flags);
--
2.30.1
--
Michal Hocko
SUSE Labs
