Re: [PATCH v4 09/14] memcg: kmem accounting lifecycle management

[Kamezawa Hiroyuki]

(2012/10/12 17:41), Michal Hocko wrote:
> On Fri 12-10-12 11:47:17, Glauber Costa wrote:
> > On 10/11/2012 05:11 PM, Michal Hocko wrote:
> > > On Mon 08-10-12 14:06:15, Glauber Costa wrote:
> > > > Because kmem charges can outlive the cgroup, we need to make sure that
> > > > we won't free the memcg structure while charges are still in flight.
> > > > For reviewing simplicity, the charge functions will issue
> > > > mem_cgroup_get() at every charge, and mem_cgroup_put() at every
> > > > uncharge.
> > > >
> > > > This can get expensive, however, and we can do better. mem_cgroup_get()
> > > > only really needs to be issued once: when the first limit is set. In the
> > > > same spirit, we only need to issue mem_cgroup_put() when the last charge
> > > > is gone.
> > > >
> > > > We'll need an extra bit in kmem_accounted for that: KMEM_ACCOUNTED_DEAD.
> > > > it will be set when the cgroup dies, if there are charges in the group.
> > > > If there aren't, we can proceed right away.
> > > >
> > > > Our uncharge function will have to test that bit every time the charges
> > > > drop to 0. Because that is not the likely output of
> > > > res_counter_uncharge, this should not impose a big hit on us: it is
> > > > certainly much better than a reference count decrease at every
> > > > operation.
> > > >
> > > > [ v3: merged all lifecycle related patches in one ]
> > > >
> > > > Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
> > > > CC: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > > > CC: Christoph Lameter <cl@xxxxxxxxx>
> > > > CC: Pekka Enberg <penberg@xxxxxxxxxxxxxx>
> > > > CC: Michal Hocko <mhocko@xxxxxxx>
> > > > CC: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > > CC: Suleiman Souhlal <suleiman@xxxxxxxxxx>
> > >
> > > OK, I like the optimization. I have just one comment to the
> > > memcg_kmem_dead naming but other than that
> > >
> > > Acked-by: Michal Hocko <mhocko@xxxxxxx>
> > >
> > > [...]
> > > > +static bool memcg_kmem_dead(struct mem_cgroup *memcg)
> > >
> > > The name is tricky because it doesn't tell you that it clears the flag
> > > which made me scratch my head when reading comment in kmem_cgroup_destroy
> > memcg_kmem_finally_kill_that_bastard() ?
>
> memcg_kmem_test_and_clear_dead? I know long but at least clear that the
> flag is cleared. Or just open code it.

I agree. Ack by me with that naming.

Thanks,
-Kame



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