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

[Johannes Weiner <hannes@cmpxchg.org>]

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.