Re: [PATCH] mm/memcg: Disable task obj_stock for PREEMPT_RT
From: Vlastimil Babka <hidden>
Date: 2021-08-09 09:28:47
Also in:
linux-mm, lkml
On 8/9/21 11:07 AM, Michal Hocko wrote:
On Wed 04-08-21 10:33:41, Michal Hocko wrote:quoted
On Wed 04-08-21 09:39:23, Vlastimil Babka wrote:quoted
On 8/4/21 1:21 AM, Thomas Gleixner wrote:quoted
/* * 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?
Yes, thanks.
--- From 5aa1c8ce0d88b8c6d59ba95c7e36ca07dc2b2161 Mon Sep 17 00:00:00 2001 From: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org> 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-IBi9RG/b67k@public.gmane.org>
Acked-by: Vlastimil Babka <redacted>
quoted hunk ↗ jump to hunk
--- 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);