Messages in this thread | | | From | Waiman Long <> | Subject | Re: [PATCH v3 2/5] mm/memcg: Introduce obj_cgroup_uncharge_mod_state() | Date | Thu, 15 Apr 2021 15:44:56 -0400 |
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On 4/15/21 3:40 PM, Johannes Weiner wrote: > On Thu, Apr 15, 2021 at 02:47:31PM -0400, Waiman Long wrote: >> On 4/15/21 2:10 PM, Johannes Weiner wrote: >>> On Thu, Apr 15, 2021 at 12:35:45PM -0400, Waiman Long wrote: >>>> 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. >>> Yeah it's more obvious after getting to patch 5. >>> >>> But with the irq disabling gone entirely, is there still an incentive >>> to combine the atomic section at all? Disabling preemption is pretty >>> cheap, so it wouldn't matter to just do it twice. >>> >>> I.e. couldn't the final sequence in slab code simply be >>> >>> objcg_uncharge() >>> mod_objcg_state() >>> >>> again and each function disables preemption (and in the rare case >>> irqs) as it sees fit? >>> >>> You lose the irqsoff batching in the cold path, but as you say, hit >>> rates are pretty good, and it doesn't seem worth complicating the code >>> for the cold path. >>> >> That does make sense, though a little bit of performance may be lost. I will >> try that out to see how it work out performance wise. > Thanks. > > Even if we still end up doing it, it's great to have that cost > isolated, so we know how much extra code complexity corresponds to how > much performance gain. It seems the task/irq split could otherwise be > a pretty localized change with no API implications. > I still want to move mod_objcg_state() function to memcontrol.c though as I don't want to put any obj_stock stuff in mm/slab.h.
Cheers, Longman
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