Re: [PATCH 0/4] Memory controller soft limit patches (v3)

[Balbir Singh]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2009-03-02 14:32:50]:

> On Mon, 2 Mar 2009 10:10:43 +0530
> Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2009-03-02 09:24:04]:
> > 
> > > On Sun, 01 Mar 2009 11:59:59 +0530
> > > Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> > > 
> > > > 
> > > > From: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> 
> > > 
> > > At first, it's said "When cgroup people adds something, the kernel gets slow".
> > > This is my start point of reviewing. Below is comments to this version of patch.
> > > 
> > >  1. I think it's bad to add more hooks to res_counter. It's enough slow to give up
> > >     adding more fancy things..
> > 
> > res_counters was desgined to be extensible, why is adding anything to
> > it going to make it slow, unless we turn on soft_limits?
> > 
> You inserted new "if" logic in the core loop.
> (What I want to say here is not that this is definitely bad but that "isn't there
>  any alternatives which is less overhead.)
> 
> 
> > > 
> > >  2. please avoid to add hooks to hot-path. In your patch, especially a hook to
> > >     mem_cgroup_uncharge_common() is annoying me.
> > 
> > If soft limits are not enabled, the function does a small check and
> > leaves. 
> > 
> &soft_fail_res is passed always even if memory.soft_limit==ULONG_MAX
> res_counter_soft_limit_excess() adds one more function call and spinlock, and irq-off.
>

OK, I see that overhead.. I'll figure out a way to work around it.
 
> > > 
> > >  3. please avoid to use global spinlock more. 
> > >     no lock is best. mutex is better, maybe.
> > > 
> > 
> > No lock to update a tree which is update concurrently?
> > 
> Using tree/sort itself is nonsense, I believe.
> 

I tried using prio trees in the past, but they are not easy to update
either. I won't mind asking for suggestions for a data structure that
can scaled well, allow quick insert/delete and search.

> 
> > >  4. RB-tree seems broken. Following is example. (please note you do all ops
> > >     in lazy manner (once in HZ/4.)
> > > 
> > >    i). while running, the tree is constructed as following
> > > 
> > >              R           R=exceed=300M
> > >             / \ 
> > >            A   B      A=exceed=200M  B=exceed=400M
> > >    ii) A process B exits, but and usage goes down.
> > 
> > That is why we have the hook in uncharge. Even if we update and the
> > usage goes down, the tree is ordered by usage_in_excess which is
> > updated only when the tree is updated. So what you show below does not
> > occur. I think I should document the design better.
> > 
> 
> time_check==true. So, update-tree at uncharge() only happens once in HZ/4


No.. you are missing the point

==
        if (updated_tree) {
                spin_lock_irqsave(&memcg_soft_limit_tree_lock, flags);
                mem->last_tree_update = jiffies;
                mem->usage_in_excess = new_usage_in_excess;
                spin_unlock_irqrestore(&memcg_soft_limit_tree_lock,
flags);
        }
==

mem->usage_in_excess is the key for the RB-Tree and is updated only
when the tree is updated.

> ==
> @@ -1422,6 +1520,7 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
>  	mz = page_cgroup_zoneinfo(pc);
>  	unlock_page_cgroup(pc);
> 
> +	mem_cgroup_check_and_update_tree(mem, true);
>  	/* at swapout, this memcg will be accessed to record to swap */
>  	if (ctype != MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
>  		css_put(&mem->css);
> ==
> Then, not-sorted RB-tree can be there.
> 
> BTW,
>    time_after(jiffies, 0)
> is buggy (see definition). If you want make this true always,
>    time_after(jiffies, jiffies +1)
>

HZ/4 is 250/4 jiffies in the worst case (62). We have
time_after(jiffies, next_update_interval) and next_update_interval is
set to last_tree_update + 62. Not sure if I got what you are pointing
to.

-- 
	Balbir
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