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

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Date:   Wed, 4 Aug 2021 09:39:23 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Waiman Long <longman@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>
Cc:     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 8/4/21 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?