linux-kernel - Re: [PATCH v3 03/19] mm: memcg: convert vmstat slab counters to bytes

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Date:   Wed, 20 May 2020 14:25:22 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 03/19] mm: memcg: convert vmstat slab counters to bytes

On 4/22/20 10:46 PM, Roman Gushchin wrote:
> In order to prepare for per-object slab memory accounting, convert
> NR_SLAB_RECLAIMABLE and NR_SLAB_UNRECLAIMABLE vmstat items to bytes.
> 
> To make it obvious, rename them to NR_SLAB_RECLAIMABLE_B and
> NR_SLAB_UNRECLAIMABLE_B (similar to NR_KERNEL_STACK_KB).
> 
> Internally global and per-node counters are stored in pages,
> however memcg and lruvec counters are stored in bytes.
> This scheme may look weird, but only for now. As soon as slab
> pages will be shared between multiple cgroups, global and
> node counters will reflect the total number of slab pages.
> However memcg and lruvec counters will be used for per-memcg
> slab memory tracking, which will take separate kernel objects
> in the account. Keeping global and node counters in pages helps
> to avoid additional overhead.
> 
> The size of slab memory shouldn't exceed 4Gb on 32-bit machines,
> so it will fit into atomic_long_t we use for vmstats.
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> ---
>  drivers/base/node.c     |  4 ++--
>  fs/proc/meminfo.c       |  4 ++--
>  include/linux/mmzone.h  | 16 +++++++++++++---
>  kernel/power/snapshot.c |  2 +-
>  mm/memcontrol.c         | 11 ++++-------
>  mm/oom_kill.c           |  2 +-
>  mm/page_alloc.c         |  8 ++++----
>  mm/slab.h               | 15 ++++++++-------
>  mm/slab_common.c        |  4 ++--
>  mm/slob.c               | 12 ++++++------
>  mm/slub.c               |  8 ++++----
>  mm/vmscan.c             |  3 ++-
>  mm/workingset.c         |  6 ++++--
>  13 files changed, 53 insertions(+), 42 deletions(-)


> @@ -206,7 +206,17 @@ enum node_stat_item {
>  
>  static __always_inline bool vmstat_item_in_bytes(enum node_stat_item item)
>  {
> -	return false;
> +	/*
> +	 * Global and per-node slab counters track slab pages.
> +	 * It's expected that changes are multiples of PAGE_SIZE.
> +	 * Internally values are stored in pages.
> +	 *
> +	 * Per-memcg and per-lruvec counters track memory, consumed
> +	 * by individual slab objects. These counters are actually
> +	 * byte-precise.
> +	 */
> +	return (item == NR_SLAB_RECLAIMABLE_B ||
> +		item == NR_SLAB_UNRECLAIMABLE_B);
>  }

Ok, so this is no longer a no-op, but __always_inline here and inline in
global_node_page_state() should hopefully mean that for all users of
global_node_page_state(<constant>) the compiler will eliminate the branch for
non-slab counters. But there are also functions such as si_mem_available() that
use non-constant item. Maybe compiler is smart enough anyway, but perhaps it's
better to use global_node_page_state_pages() in such callers?

However __mod_node_page_state() and mode_node_state() will now branch always. I
wonder if the "API clean" goal is worth it...

> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1409,9 +1409,8 @@ static char *memory_stat_format(struct mem_cgroup *memcg)
>  		       (u64)memcg_page_state(memcg, MEMCG_KERNEL_STACK_KB) *
>  		       1024);
>  	seq_buf_printf(&s, "slab %llu\n",
> -		       (u64)(memcg_page_state(memcg, NR_SLAB_RECLAIMABLE) +
> -			     memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE)) *
> -		       PAGE_SIZE);
> +		       (u64)(memcg_page_state(memcg, NR_SLAB_RECLAIMABLE_B) +
> +			     memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE_B)));
>  	seq_buf_printf(&s, "sock %llu\n",
>  		       (u64)memcg_page_state(memcg, MEMCG_SOCK) *
>  		       PAGE_SIZE);
> @@ -1445,11 +1444,9 @@ static char *memory_stat_format(struct mem_cgroup *memcg)
>  			       PAGE_SIZE);
>  
>  	seq_buf_printf(&s, "slab_reclaimable %llu\n",
> -		       (u64)memcg_page_state(memcg, NR_SLAB_RECLAIMABLE) *
> -		       PAGE_SIZE);
> +		       (u64)memcg_page_state(memcg, NR_SLAB_RECLAIMABLE_B));
>  	seq_buf_printf(&s, "slab_unreclaimable %llu\n",
> -		       (u64)memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE) *
> -		       PAGE_SIZE);
> +		       (u64)memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE_B));

So here we are now printing in bytes instead of pages, right? That's fine for
OOM report, but in sysfs aren't we breaking existing users?