linux-kernel - Re: [RFC 0/3] Implementation of cgroup isolation

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Date:	Tue, 29 Mar 2011 22:08:00 +0800
From:	Zhu Yanhai <zhu.yanhai@...il.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/3] Implementation of cgroup isolation

2011/3/29 Zhu Yanhai <zhu.yanhai@...il.com>:
> Hi,
>
> 2011/3/29 Michal Hocko <mhocko@...e.cz>:
>> Isn't this an overhead that would slow the whole thing down. Consider
>> that you would need to lookup page_cgroup for every page and touch
>> mem_cgroup to get the limit.
>
> Current almost has did such things, say the direct reclaim path:
> shrink_inactive_list()
>   ->isolate_pages_global()
>      ->isolate_lru_pages()
>         ->mem_cgroup_del_lru(for each page it wants to isolate)
>            and in mem_cgroup_del_lru() we have:
oops, the below code is from mem_cgroup_rotate_lru_list not
mem_cgroup_del_lru, the correct one should be:
[code]
pc = lookup_page_cgroup(page);
	/* can happen while we handle swapcache. */
	if (!TestClearPageCgroupAcctLRU(pc))
		return;
	VM_BUG_ON(!pc->mem_cgroup);
	/*
	 * We don't check PCG_USED bit. It's cleared when the "page" is finally
	 * removed from global LRU.
	 */
	mz = page_cgroup_zoneinfo(pc);
	MEM_CGROUP_ZSTAT(mz, lru) -= 1;
	if (mem_cgroup_is_root(pc->mem_cgroup))
		return;
[/code]
Anyway, the point still stands.

-zyh
> [code]
>        pc = lookup_page_cgroup(page);
>        /*
>         * Used bit is set without atomic ops but after smp_wmb().
>         * For making pc->mem_cgroup visible, insert smp_rmb() here.
>         */
>        smp_rmb();
>        /* unused or root page is not rotated. */
>        if (!PageCgroupUsed(pc) || mem_cgroup_is_root(pc->mem_cgroup))
>                return;
> [/code]
> By calling mem_cgroup_is_root(pc->mem_cgroup) we already brought the
> struct mem_cgroup into cache.
> So probably things won't get worse at least.
>
> Thanks,
> Zhu Yanhai
>
>> The point of the isolation is to not touch the global reclaim path at
>> all.
>>
>>> 3) shrink the cgroups who have set a reserve_limit, and leave them with only
>>> the reserve_limit bytes they need. if nr_reclaimed is meet, goto finish.
>>> 4) OOM
>>>
>>> Does it make sense?
>>
>> It sounds like a good thing - in that regard it is more generic than
>> a simple flag - but I am afraid that the implementation wouldn't be
>> that easy to preserve the performance and keep the balance between
>> groups. But maybe it can be done without too much cost.
>>
>> Thanks
>> --
>> Michal Hocko
>> SUSE Labs
>> SUSE LINUX s.r.o.
>> Lihovarska 1060/12
>> 190 00 Praha 9
>> Czech Republic
>>
>
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