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SubjectRe: [PATCH v3 03/19] mm: memcg: convert vmstat slab counters to bytes
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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@fb.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?

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