Re: [PATCH -v2 1/6] memcg: do not replicate try_get_mem_cgroup_from_mm in __mem_cgroup_try_charge

[Michal Hocko <mhocko@suse.cz>]

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@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.

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

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