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Date: Mon, 25 Jun 2012 17:13:53 +0400
From: Glauber Costa <glommer@parallels.com>
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To: Michal Hocko <mhocko@suse.cz>
CC: <linux-mm@kvack.org>, Pekka Enberg <penberg@kernel.org>,
        Cristoph Lameter <cl@linux.com>, David Rientjes <rientjes@google.com>,
        <cgroups@vger.kernel.org>, <devel@openvz.org>,
        <kamezawa.hiroyu@jp.fujitsu.com>, <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Suleiman Souhlal <suleiman@google.com>
Subject: Re: [PATCH v4 07/25] memcg: Reclaim when more than one page needed.
References: <1340015298-14133-1-git-send-email-glommer@parallels.com> <1340015298-14133-8-git-send-email-glommer@parallels.com> <20120620134738.GG5541@tiehlicka.suse.cz> <4FE227F8.3000504@parallels.com> <20120621211923.GC31759@tiehlicka.suse.cz>
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>>>> +
>>>>   	ret = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
>>>>   	if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
>>>>   		return CHARGE_RETRY;
>>>> @@ -2234,8 +2235,10 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>>>>   	 * unlikely to succeed so close to the limit, and we fall back
>>>>   	 * to regular pages anyway in case of failure.
>>>>   	 */
>>>> -	if (nr_pages == 1 && ret)
>>>> +	if (nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER) && ret) {
>>>> +		cond_resched();
>>>>   		return CHARGE_RETRY;
>>>> +	}
>>>
>>> What prevents us from looping for unbounded amount of time here?
>>> Maybe you need to consider the number of reclaimed pages here.
>>
>> Why would we even loop here? It will just return CHARGE_RETRY, it is
>> up to the caller to decide whether or not it will retry.
>
> Yes, but the test was original to prevent oom when we managed to reclaim
> something. And something might be enough for a single page but now you
> have high order allocations so we can retry without any success.
>

So,

Most of the kmem allocations are likely to be quite small as well. For 
the slab, we're dealing with the order of 2-3 pages, and for other 
allocations that may happen, like stack, they will be in the order of 2 
pages as well.

So one thing I could do here, is define a threshold, say, 3, and only 
retry for that very low threshold, instead of following COSTLY_ORDER.
I don't expect two or three pages to be much less likely to be freed 
than a single page.

I am fine with ripping of the cond_resched as well.

Let me know if you would be okay with that.