On Tue 04-02-14 10:55:08, Johannes Weiner wrote: > On Tue, Feb 04, 2014 at 02:28:55PM +0100, Michal Hocko wrote: > > Johannes Weiner has pointed out that __mem_cgroup_try_charge duplicates > > try_get_mem_cgroup_from_mm for charges which came without a memcg. The > > only reason seems to be a tiny optimization when css_tryget is not > > called if the charge can be consumed from the stock. Nevertheless > > css_tryget is very cheap since it has been reworked to use per-cpu > > counting so this optimization doesn't give us anything these days. > > > > So let's drop the code duplication so that the code is more readable. > > While we are at it also remove a very confusing comment in > > try_get_mem_cgroup_from_mm. > > > > Signed-off-by: Michal Hocko <mhocko@xxxxxxx> > > --- > > mm/memcontrol.c | 49 ++++++++----------------------------------------- > > 1 file changed, 8 insertions(+), 41 deletions(-) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index 53385cd4e6f0..042e4ff36c05 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -1081,11 +1081,7 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm) > > > > if (!mm) > > return NULL; > > While you're at it, this check also seems unnecessary. Yes, it will be removed in a later patch. I wanted to have it in a separate patch for a better bisectability just in case I have really missed mm-might-by-NULL case. > > - /* > > - * Because we have no locks, mm->owner's may be being moved to other > > - * cgroup. We use css_tryget() here even if this looks > > - * pessimistic (rather than adding locks here). > > - */ > > + > > rcu_read_lock(); > > do { > > memcg = mem_cgroup_from_task(rcu_dereference(mm->owner)); > > @@ -2759,45 +2755,15 @@ again: > > goto done; > > css_get(&memcg->css); > > } else { > > - struct task_struct *p; > > - > > - rcu_read_lock(); > > - p = rcu_dereference(mm->owner); > > - /* > > - * Because we don't have task_lock(), "p" can exit. > > - * In that case, "memcg" can point to root or p can be NULL with > > - * race with swapoff. Then, we have small risk of mis-accouning. > > - * But such kind of mis-account by race always happens because > > - * we don't have cgroup_mutex(). It's overkill and we allo that > > - * small race, here. > > - * (*) swapoff at el will charge against mm-struct not against > > - * task-struct. So, mm->owner can be NULL. > > - */ > > - memcg = mem_cgroup_from_task(p); > > - if (!memcg) > > + memcg = try_get_mem_cgroup_from_mm(mm); > > + if (!memcg) { > > memcg = root_mem_cgroup; > > - if (mem_cgroup_is_root(memcg)) { > > - rcu_read_unlock(); > > - goto done; > > - } > > - if (consume_stock(memcg, nr_pages)) { > > - /* > > - * It seems dagerous to access memcg without css_get(). > > - * But considering how consume_stok works, it's not > > - * necessary. If consume_stock success, some charges > > - * from this memcg are cached on this cpu. So, we > > - * don't need to call css_get()/css_tryget() before > > - * calling consume_stock(). > > - */ > > - rcu_read_unlock(); > > goto done; > > } > > - /* after here, we may be blocked. we need to get refcnt */ > > - if (!css_tryget(&memcg->css)) { > > - rcu_read_unlock(); > > - goto again; > > - } > > - rcu_read_unlock(); > > + if (mem_cgroup_is_root(memcg)) > > + goto done_put; > > + if (consume_stock(memcg, nr_pages)) > > + goto done_put; > > These two are actually the same in the if (*ptr) branch. True, I just wanted to have the patch minimalistic and do just a single thing here. Duplicity will vanish in the next patch. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>