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

[Tejun Heo]

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.

Thanks.

-- 
tejun