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 14:47:31 -0400 |
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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.
Cheers, Longman
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