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 <mho...@suse.cz>
> ---
> 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.
> - /*
> - * 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.
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