Re: [PATCH 1/4] memcg: simplify consume_stock

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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