Re: [RFC PATCH] mm/memcontrol: Increase threshold for draining per-cpu stocked bytes.

[Roman Gushchin <guro@fb.com>]

On Tue, Jan 05, 2021 at 10:23:52AM -0800, Roman Gushchin wrote:
> On Tue, Jan 05, 2021 at 04:07:42PM +0000, Imran Khan wrote:
> > While allocating objects whose size is multiple of PAGE_SIZE,
> > say kmalloc-4K, we charge one page for extra bytes corresponding
> > to the obj_cgroup membership pointer and remainder of the charged
> > page gets added to per-cpu stocked bytes. If this allocation is
> > followed by another allocation of the same size, the stocked bytes
> > will not suffice and thus we endup charging an extra page
> > again for membership pointer and remainder of this page gets added
> > to per-cpu stocked bytes. This second addition will cause amount of
> > stocked bytes to go beyond PAGE_SIZE and hence will result in
> > invocation of drain_obj_stock.
> > 
> > So if we are in a scenario where we are consecutively allocating,
> > several PAGE_SIZE multiple sized objects, the stocked bytes will
> > never be enough to suffice a request and every second request will
> > trigger draining of stocked bytes.
> > 
> > For example invoking __alloc_skb multiple times with
> > 2K < packet size < 4K will give a call graph like:
> > 
> > __alloc_skb
> >     |
> >     |__kmalloc_reserve.isra.61
> >     |    |
> >     |    |__kmalloc_node_track_caller
> >     |    |    |
> >     |    |    |slab_pre_alloc_hook.constprop.88
> >     |    |     obj_cgroup_charge
> >     |    |    |    |
> >     |    |    |    |__memcg_kmem_charge
> >     |    |    |    |    |
> >     |    |    |    |    |page_counter_try_charge
> >     |    |    |    |
> >     |    |    |    |refill_obj_stock
> >     |    |    |    |    |
> >     |    |    |    |    |drain_obj_stock.isra.68
> >     |    |    |    |    |    |
> >     |    |    |    |    |    |__memcg_kmem_uncharge
> >     |    |    |    |    |    |    |
> >     |    |    |    |    |    |    |page_counter_uncharge
> >     |    |    |    |    |    |    |    |
> >     |    |    |    |    |    |    |    |page_counter_cancel
> >     |    |    |
> >     |    |    |
> >     |    |    |__slab_alloc
> >     |    |    |    |
> >     |    |    |    |___slab_alloc
> >     |    |    |    |
> >     |    |    |slab_post_alloc_hook
> > 
> > This frequent draining of stock bytes and resultant charging of pages
> > increases the CPU load and hence deteriorates the scheduler latency.
> > 
> > The above mentioned scenario and it's impact can be seen by running
> > hackbench with large packet size on v5.8 and subsequent kernels. The
> > deterioration in hackbench number starts appearing from v5.9 kernel,
> > 'commit f2fe7b09a52b ("mm: memcg/slab: charge individual slab objects
> > instead of pages")'.
> > 
> > Increasing the draining limit to twice of KMALLOC_MAX_CACHE_SIZE
> > (a safe upper limit for size of slab cache objects), will avoid draining
> > of stock, every second allocation request, for the above mentioned
> > scenario and hence will reduce the CPU load for such cases. For
> > allocation of smaller objects or other allocation patterns the behaviour
> > will be same as before.
> > 
> > This change increases the draining threshold for per-cpu stocked bytes
> > from PAGE_SIZE to KMALLOC_MAX_CACHE_SIZE * 2.
> 
> Hello, Imran!
> 
> Yes, it makes total sense to me.
> 
> Btw, in earlier versions of the new slab controller there was a separate stock
> for byte-sized charging and it was 32 pages large. Later Johannes suggested
> the current layered design and he thought that because of the layering a single
> page is enough for the upper layer.
> 
> > 
> > Below are the hackbench numbers with and without this change on
> > v5.10.0-rc7.
> > 
> > Without this change:
> >     # hackbench process 10 1000 -s 100000
> >     Running in process mode with 10 groups using 40 file descriptors
> >     each (== 400 tasks)
> >     Each sender will pass 100 messages of 100000 bytes
> >     Time: 4.401
> > 
> >     # hackbench process 10 1000 -s 100000
> >     Running in process mode with 10 groups using 40 file descriptors
> >     each (== 400 tasks)
> >     Each sender will pass 100 messages of 100000 bytes
> >     Time: 4.470
> > 
> > With this change:
> >     # hackbench process 10 1000 -s 100000
> >     Running in process mode with 10 groups using 40 file descriptors
> >     each (== 400 tasks)
> >     Each sender will pass 100 messages of 100000 bytes
> >     Time: 3.782
> > 
> >     # hackbench process 10 1000 -s 100000
> >     Running in process mode with 10 groups using 40 file descriptors
> >     each (== 400 tasks)
> >     Each sender will pass 100 messages of 100000 bytes
> >     Time: 3.827
> > 
> > As can be seen the change gives an improvement of about 15% in hackbench
> > numbers.
> > Also numbers obtained with the change are inline with those obtained
> > from v5.8 kernel.
> 
> The difference is quite impressive!
> 
> I wonder if you tried smaller values than KMALLOC_MAX_CACHE_SIZE * 2?
> Let's say 16 and 32?
> 
> KMALLOC_MAX_CACHE_SIZE * 2 makes sense to me, but then the whole construction
> with two layer caching is very questionable. Anyway, it's not a reason to not
> merge your patch, just something I wanna look at later.

Hm, can you, please, benchmark the following change (without your change)?

@@ -3204,7 +3204,7 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
 
 		if (nr_pages) {
 			rcu_read_lock();
-			__memcg_kmem_uncharge(obj_cgroup_memcg(old), nr_pages);
+			refill_stock(obj_cgroup_memcg(old), nr_pages);
 			rcu_read_unlock();
 		}