linux-kernel - Re: [PATCH v2 3/5] mm: memcg/percpu: per-memcg percpu memory statistics

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Date:   Sun, 21 Jun 2020 18:48:03 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v2 3/5] mm: memcg/percpu: per-memcg percpu memory
 statistics

On Mon, Jun 08, 2020 at 04:08:17PM -0700, Roman Gushchin wrote:
> Percpu memory can represent a noticeable chunk of the total
> memory consumption, especially on big machines with many CPUs.
> Let's track percpu memory usage for each memcg and display
> it in memory.stat.
> 
> A percpu allocation is usually scattered over multiple pages
> (and nodes), and can be significantly smaller than a page.
> So let's add a byte-sized counter on the memcg level:
> MEMCG_PERCPU_B. Byte-sized vmstat infra created for slabs
> can be perfectly reused for percpu case.
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> Acked-by: Dennis Zhou <dennis@...nel.org>
> ---
>  Documentation/admin-guide/cgroup-v2.rst |  4 ++++
>  include/linux/memcontrol.h              |  8 ++++++++
>  mm/memcontrol.c                         |  4 +++-
>  mm/percpu.c                             | 10 ++++++++++
>  4 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index ce3e05e41724..7c1e784239bf 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1274,6 +1274,10 @@ PAGE_SIZE multiple when read back.
>  		Amount of memory used for storing in-kernel data
>  		structures.
>  
> +	  percpu
> +		Amount of memory used for storing per-cpu kernel
> +		data structures.
> +
>  	  sock
>  		Amount of memory used in network transmission buffers
>  
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index eede46c43573..7ed3af71a6fb 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -32,11 +32,19 @@ struct kmem_cache;
>  enum memcg_stat_item {
>  	MEMCG_SWAP = NR_VM_NODE_STAT_ITEMS,
>  	MEMCG_SOCK,
> +	MEMCG_PERCPU_B,
>  	/* XXX: why are these zone and not node counters? */
>  	MEMCG_KERNEL_STACK_KB,
>  	MEMCG_NR_STAT,
>  };
>  
> +static __always_inline bool memcg_stat_item_in_bytes(enum memcg_stat_item item)
> +{
> +	if (item == MEMCG_PERCPU_B)
> +		return true;
> +	return vmstat_item_in_bytes(item);

This patch is now in -next and this line causes a warning from clang,
which shows up in every translation unit that includes this header,
which is a lot:

include/linux/memcontrol.h:45:30: warning: implicit conversion from
enumeration type 'enum memcg_stat_item' to different enumeration type
'enum node_stat_item' [-Wenum-conversion]
        return vmstat_item_in_bytes(item);
               ~~~~~~~~~~~~~~~~~~~~ ^~~~
1 warning generated.

I assume this conversion is intentional; if so, it seems like expecting
a specific enum is misleading. Perhaps this should be applied on top?

Cheers,
Nathan

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 2499f78cf32d..bddeb4ce7a4f 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -38,7 +38,7 @@ enum memcg_stat_item {
 	MEMCG_NR_STAT,
 };
 
-static __always_inline bool memcg_stat_item_in_bytes(enum memcg_stat_item item)
+static __always_inline bool memcg_stat_item_in_bytes(int item)
 {
 	if (item == MEMCG_PERCPU_B)
 		return true;
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 084ee1c17160..52d7961a24f0 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -211,7 +211,7 @@ enum node_stat_item {
  * measured in pages). This defines the API part, the internal representation
  * might be different.
  */
-static __always_inline bool vmstat_item_in_bytes(enum node_stat_item item)
+static __always_inline bool vmstat_item_in_bytes(int item)
 {
 	/*
 	 * Global and per-node slab counters track slab pages.