Re: [patch 1/3] mm: memcontrol: do not kill uncharge batching in free_pages_and_swap_cache

Andrew Morton
2014-09-24T21:15:44Z

On Wed, 24 Sep 2014 17:03:22 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:

> > Obviously it's not very important - presumably the common case is that
> > the LRU contains lengthy sequences of pages from the same zone. Maybe.
>
> Even then, the end result is more concise and busts the lock where
> it's actually taken, making the whole thing a bit more obvious:

Yes, that did come out better.

> From: Michal Hocko <mhocko@suse.cz>
> Date: Fri, 5 Sep 2014 11:16:17 +0200
> Subject: [patch] mm: memcontrol: do not kill uncharge batching in
> free_pages_and_swap_cache
>
> free_pages_and_swap_cache limits release_pages to PAGEVEC_SIZE chunks.
> This is not a big deal for the normal release path but it completely
> kills memcg uncharge batching which reduces res_counter spin_lock
> contention. Dave has noticed this with his page fault scalability test
> case on a large machine when the lock was basically dominating on all
> CPUs:
> 80.18% 80.18% [kernel] [k] _raw_spin_lock
> |
> --- _raw_spin_lock
> |
> |--66.59%-- res_counter_uncharge_until
> | res_counter_uncharge
> | uncharge_batch
> | uncharge_list
> | mem_cgroup_uncharge_list
> | release_pages
> | free_pages_and_swap_cache
> | tlb_flush_mmu_free
> | |
> | |--90.12%-- unmap_single_vma
> | | unmap_vmas
> | | unmap_region
> | | do_munmap
> | | vm_munmap
> | | sys_munmap
> | | system_call_fastpath
> | | __GI___munmap
> | |
> | --9.88%-- tlb_flush_mmu
> | tlb_finish_mmu
> | unmap_region
> | do_munmap
> | vm_munmap
> | sys_munmap
> | system_call_fastpath
> | __GI___munmap
>
> In his case the load was running in the root memcg and that part
> has been handled by reverting 05b843012335 ("mm: memcontrol: use
> root_mem_cgroup res_counter") because this is a clear regression,
> but the problem remains inside dedicated memcgs.
>
> There is no reason to limit release_pages to PAGEVEC_SIZE batches other
> than lru_lock held times. This logic, however, can be moved inside the
> function. mem_cgroup_uncharge_list and free_hot_cold_page_list do not
> hold any lock for the whole pages_to_free list so it is safe to call
> them in a single run.
>
> In release_pages, break the lock at least every SWAP_CLUSTER_MAX (32)
> pages, then remove the batching from free_pages_and_swap_cache.

I beefed this paragraph up a bit:

: The release_pages() code was previously breaking the lru_lock each
: PAGEVEC_SIZE pages (ie, 14 pages). However this code has no usage of
: pagevecs so switch to breaking the lock at least every SWAP_CLUSTER_MAX
: (32) pages. This means that the lock acquisition frequency is
: approximately halved and the max hold times are approximately doubled.
:
: The now unneeded batching is removed from free_pages_and_swap_cache().

I doubt if the increased irq-off time will hurt anyone, but who knows...