linux-kernel - Re: [PATCH v2 06/11] memcg: kmem controller infrastructure

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Date:	Thu, 23 Aug 2012 11:51:31 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Greg Thelen <gthelen@...gle.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <devel@...nvz.org>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<kamezawa.hiroyu@...fujitsu.com>, Christoph Lameter <cl@...ux.com>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...nel.org>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH v2 06/11] memcg: kmem controller infrastructure


>>> Perhaps we're just trying to take a conservative initial implementation
>>> which is consistent with user visible pages.
>>>
>>
>> The way I see it, is not about being conservative, but rather about my
>> physical safety. It is quite easy and natural to assume that "all
>> modifications to page cgroup are done under lock". So someone modifying
>> this later will likely find out about this exception in a rather
>> unpleasant way. They know where I live, and guns for hire are everywhere.
>>
>> Note that it is not unreasonable to believe that we can modify this
>> later. This can be a way out, for example, for the memcg lifecycle problem.
>>
>> I agree with your analysis and we can ultimately remove it, but if we
>> cannot pinpoint any performance problems to here, maybe consistency
>> wins. Also, the locking operation itself is a bit expensive, but the
>> biggest price is the actual contention. If we'll have nobody contending
>> for the same page_cgroup, the problem - if exists - shouldn't be that
>> bad. And if we ever have, the lock is needed.
> 
> Sounds reasonable. Another reason we might have to eventually revisit
> this lock is the fact that lock_page_cgroup() is not generally irq_safe.
> I assume that slab pages may be freed in softirq and would thus (in an
> upcoming patch series) call __memcg_kmem_free_page.  There are a few
> factors that might make it safe to grab this lock here (and below in
> __memcg_kmem_free_page) from hard/softirq context:
> * the pc lock is a per page bit spinlock.  So we only need to worry
>   about interrupting a task which holds the same page's lock to avoid
>   deadlock.
> * for accounted kernel pages, I am not aware of other code beyond
>   __memcg_kmem_charge_page and __memcg_kmem_free_page which grab pc
>   lock.  So we shouldn't find __memcg_kmem_free_page() called from a
>   context which interrupted a holder of the page's pc lock.
> 

All very right.


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