Re: [PATCH] mm: memcg/slab: optimize objcg stock draining

[Roman Gushchin]

On Tue, Jan 05, 2021 at 10:05:20PM -0800, Shakeel Butt wrote:
> On Tue, Jan 5, 2021 at 8:22 PM Roman Gushchin <guro@xxxxxx> wrote:
> >
> > Imran Khan reported a regression in hackbench results caused by the
> > commit f2fe7b09a52b ("mm: memcg/slab: charge individual slab objects
> > instead of pages"). The regression is noticeable in the case of
> > a consequent allocation of several relatively large slab objects,
> > e.g. skb's. As soon as the amount of stocked bytes exceeds PAGE_SIZE,
> > drain_obj_stock() and __memcg_kmem_uncharge() are called, and it leads
> > to a number of atomic operations in page_counter_uncharge().
> >
> > The corresponding call graph is below (provided by Imran Khan):
> >   |__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
> >
> > Instead of directly uncharging the accounted kernel memory, it's
> > possible to refill the generic page-sized per-cpu stock instead.
> > It's a much faster operation, especially on a default hierarchy.
> > As a bonus, __memcg_kmem_uncharge_page() will also get faster,
> > so the freeing of page-sized kernel allocations (e.g. large kmallocs)
> > will become faster.
> >
> > A similar change has been done earlier for the socket memory by
> > the commit 475d0487a2ad ("mm: memcontrol: use per-cpu stocks for
> > socket memory uncharging").
> >
> > Signed-off-by: Roman Gushchin <guro@xxxxxx>
> > Reported-by: Imran Khan <imran.f.khan@xxxxxxxxxx>
> 
> I remember seeing this somewhere
> https://lore.kernel.org/linux-mm/20190423154405.259178-1-shakeelb@xxxxxxxxxx/

Yes, we've discussed it a couple of times, as I remember. Looks like now
we finally have a good reasoning/benchmark, thanks to Imran.

> 
> Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>

Thank you for the review!