linux-kernel - Re: [PATCH 1/4] memcg: simplify consume

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Message-ID: <20250430151148.-SqLG7kP@linutronix.de>
Date: Wed, 30 Apr 2025 17:11:48 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>, bpf@...r.kernel.org
Subject: Re: [PATCH 1/4] memcg: simplify consume_stock

On 2025-04-29 21:37:26 [-0700], Shakeel Butt wrote:
> > > -	if (gfpflags_allow_spinning(gfp_mask))
> > > -		local_lock_irqsave(&memcg_stock.stock_lock, flags);
> > > -	else if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags))
> > > +	if (nr_pages > MEMCG_CHARGE_BATCH ||
> > > +	    !local_trylock_irqsave(&memcg_stock.stock_lock, flags))
> > 
> > I don't think it's a good idea.
> > spin_trylock() will fail often enough in PREEMPT_RT.
> > Even during normal boot I see preemption between tasks and they
> > contend on the same cpu for the same local_lock==spin_lock.
> > Making them take slow path is a significant behavior change
> > that needs to be carefully considered.
> 
> I didn't really think too much about PREEMPT_RT kernels as I assume
> performance is not top priority but I think I get your point. Let me

Not sure if this is performance nor simply failing to allocate memory.

> explain and correct me if I am wrong. On PREEMPT_RT kernel, the local
> lock is a spin lock which is actually a mutex but with priority
> inheritance. A task having the local lock can still get context switched
> (but will remain on same CPU run queue) and the newer task can try to
> acquire the memcg stock local lock. If we just do trylock, it will
> always go to the slow path but if we do local_lock() then it will sleeps
> and possibly gives its priority to the task owning the lock and possibly
> make that task to get the CPU. Later the task slept on memcg stock lock
> will wake up and go through fast path.

So far correct. On PREEMPT_RT a task with spinlock_t or local_lock_t can
get preempted while owning the lock. The local_lock_t is a per-CPU lock.

Sebastian