[Devel] Re: [PATCH v3 04/13] kmem accounting basic infrastructure

[Glauber Costa]

On 09/30/2012 12:23 PM, Tejun Heo wrote:
> Hello, Glauber.
> 
> On Thu, Sep 27, 2012 at 10:30:36PM +0400, Glauber Costa wrote:
>>> But that happens only when pages enter and leave slab and if it still
>>> is significant, we can try to further optimize charging.  Given that
>>> this is only for cases where memcg is already in use and we provide a
>>> switch to disable it globally, I really don't think this warrants
>>> implementing fully hierarchy configuration.
>>
>> Not totally true. We still have to match every allocation to the right
>> cache, and that is actually our heaviest hit, responsible for the 2, 3 %
>> we're seeing when this is enabled. It is the kind of path so hot that
>> people frown upon branches being added, so I don't think we'll ever get
>> this close to being free.
> 
> Sure, depening on workload, any addition to alloc/free could be
> noticeable.  I don't know.  I'll write more about it when replying to
> Michal's message.  BTW, __memcg_kmem_get_cache() does seem a bit
> heavy.  I wonder whether indexing from cache side would make it
> cheaper?  e.g. something like the following.
> 
> 	kmem_cache *__memcg_kmem_get_cache(cachep, gfp)
> 	{
> 		struct kmem_cache *c;
> 
> 		c = cachep->memcg_params->caches[percpu_read(kmemcg_slab_idx)];
> 		if (likely(c))
> 			return c;
> 		/* try to create and then fall back to cachep */
> 	}
> 
> where kmemcg_slab_idx is updated from sched notifier (or maybe add and
> use current->kmemcg_slab_idx?).  You would still need __GFP_* and
> in_interrupt() tests but current->mm and PF_KTHREAD tests can be
> rolled into index selection.
> 

How big would this array be? there can be a lot more kmem_caches than
there are memcgs. That is why it is done from memcg side.