linux-kernel - Re: [PATCH] mm/memcg: Disable task obj_stock for PREEMPT

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Message-ID: <0e282deb-ead2-2ce6-26cf-f15adcab79ca@suse.cz>
Date:   Mon, 9 Aug 2021 11:28:41 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Waiman Long <longman@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Luis Goncalves <lgoncalv@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] mm/memcg: Disable task obj_stock for PREEMPT_RT

On 8/9/21 11:07 AM, Michal Hocko wrote:
> 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?

Yes, thanks.

> ---
> 
> From 5aa1c8ce0d88b8c6d59ba95c7e36ca07dc2b2161 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@...e.com>
> 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@...e.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  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);
>  
>