Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Waiman Long <longman@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
	linux-mm@kvack.org, Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT
Date: Fri, 10 Dec 2021 14:01:32 +0100	[thread overview]
Message-ID: <YbNPrGEjtKjzEjQa@linutronix.de> (raw)
In-Reply-To: <20211210025228.158196-1-longman@redhat.com>

On 2021-12-09 21:52:28 [-0500], Waiman Long wrote:
…
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
…
> @@ -2210,7 +2211,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>  	struct memcg_stock_pcp *stock;
>  	unsigned long flags;
>  
> -	local_irq_save(flags);
> +	local_lock_irqsave(&memcg_stock.lock, flags);

Why is this one using the lock? It isn't accessing irq_obj, right?

>  	stock = this_cpu_ptr(&memcg_stock);
>  	if (stock->cached != memcg) { /* reset if necessary */
> @@ -2779,29 +2780,28 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
>   * which is cheap in non-preempt kernel. The interrupt context object stock
>   * can only be accessed after disabling interrupt. User context code can
>   * access interrupt object stock, but not vice versa.
> + *
> + * This task and interrupt context optimization is disabled for PREEMPT_RT
> + * as there is no performance gain in this case.
>   */
>  static inline struct obj_stock *get_obj_stock(unsigned long *pflags)
>  {
> -	struct memcg_stock_pcp *stock;
> -
> -	if (likely(in_task())) {
> +	if (likely(in_task()) && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
>  		*pflags = 0UL;
>  		preempt_disable();
> -		stock = this_cpu_ptr(&memcg_stock);
> -		return &stock->task_obj;
> +		return this_cpu_ptr(&memcg_stock.task_obj);
>  	}

We usually add the local_lock_t to the object it protects, struct
obj_stock it this case.
That would give you two different locks (instead of one) so you wouldn't
have to use preempt_disable() to avoid lockdep's complains. Also it
would warn you if you happen to use that obj_stock in !in_task() which
is isn't possible now.
The only downside would be that drain_local_stock() needs to acquire two
locks.

>  
> -	local_irq_save(*pflags);
> -	stock = this_cpu_ptr(&memcg_stock);
> -	return &stock->irq_obj;
> +	local_lock_irqsave(&memcg_stock.lock, *pflags);
> +	return this_cpu_ptr(&memcg_stock.irq_obj);
>  }
>  
>  static inline void put_obj_stock(unsigned long flags)
>  {
> -	if (likely(in_task()))
> +	if (likely(in_task()) && !IS_ENABLED(CONFIG_PREEMPT_RT))
>  		preempt_enable();
>  	else
> -		local_irq_restore(flags);
> +		local_unlock_irqrestore(&memcg_stock.lock, flags);
>  }
>  
>  /*

Sebastian