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

Waiman Long
2021-04-15T16:35:55Z

On 4/15/21 12:30 PM, Johannes Weiner wrote:
> 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.
>
That will also work. However, the reason I did that was because of patch
5 in the series. I could put the get_obj_stock() and put_obj_stock()
code in slab.h and allowed them to be used directly in various places,
but hiding in one function is easier.

Anyway, I can change the patch if you think that is the right way.

Cheers,
Longman