On Thu, Apr 15, 2021 at 03:44:56PM -0400, Waiman Long wrote: > 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. No objection from me! That's actually a nice cleanup, IMO. Not sure why it was separated from the rest of the objcg interface implementation to begin with.
