Re: [PATCH v3 2/5] mm/memcg: Introduce obj_cgroup_uncharge_mod_state()

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Apr 13, 2021 at 09:20:24PM -0400, Waiman Long wrote:
> In memcg_slab_free_hook()/pcpu_memcg_free_hook(), obj_cgroup_uncharge()
> is followed by mod_objcg_state()/mod_memcg_state(). Each of these
> function call goes through a separate irq_save/irq_restore cycle. That
> is inefficient.  Introduce a new function obj_cgroup_uncharge_mod_state()
> that combines them with a single irq_save/irq_restore cycle.
>
> @@ -3292,6 +3296,25 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
>  	refill_obj_stock(objcg, size);
>  }
>  
> +void obj_cgroup_uncharge_mod_state(struct obj_cgroup *objcg, size_t size,
> +				   struct pglist_data *pgdat, int idx)

The optimization makes sense.

But please don't combine independent operations like this into a
single function. It makes for an unclear parameter list, it's a pain
in the behind to change the constituent operations later on, and it
has a habit of attracting more random bools over time. E.g. what if
the caller already has irqs disabled? What if it KNOWS that irqs are
enabled and it could use local_irq_disable() instead of save?

Just provide an __obj_cgroup_uncharge() that assumes irqs are
disabled, combine with the existing __mod_memcg_lruvec_state(), and
bubble the irq handling up to those callsites which know better.