linux-kernel - Re: [PATCH] mm: optimize stat output for 11% sys time reduce

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Message-Id: <20260108111027.172f19a9a86667e8e0142042@linux-foundation.org>
Date: Thu, 8 Jan 2026 11:10:27 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Jianyue Wu <wujianyue000@...il.com>
Cc: jianyuew@...dia.com, hannes@...xchg.org, mhocko@...nel.org,
 roman.gushchin@...ux.dev, shakeel.butt@...ux.dev, muchun.song@...ux.dev,
 linux-mm@...ck.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: optimize stat output for 11% sys time reduce

On Thu,  8 Jan 2026 17:37:29 +0800 Jianyue Wu <wujianyue000@...il.com> wrote:

> From: Jianyue Wu <wujianyue000@...il.com>
> 
> Replace seq_printf/seq_buf_printf with lightweight helpers to avoid
> printf parsing in memcg stats output.
> 
> Key changes:
> - Add memcg_seq_put_name_val() for seq_file "name value\n" formatting
> - Add memcg_seq_buf_put_name_val() for seq_buf "name value\n" formatting
> - Update __memory_events_show(), swap_events_show(),
>   memory_stat_format(), memory_numa_stat_show(), and related helpers
> 
> Performance:
> - 1M reads of memory.stat+memory.numa_stat
> - Before: real 0m9.663s, user 0m4.840s, sys 0m4.823s
> - After:  real 0m9.051s, user 0m4.775s, sys 0m4.275s (~11.4% sys drop)
> 
> Tests:
> - Script:
>   for ((i=1; i<=1000000; i++)); do
>       : > /dev/null < /sys/fs/cgroup/memory.stat
>       : > /dev/null < /sys/fs/cgroup/memory.numa_stat
>   done
> 

I suspect there are workloads which read these files frequently.

I'd be interested in learning "how frequently".  Perhaps
ascii-through-sysfs simply isn't an appropriate API for this data?

> @@ -1795,25 +1795,33 @@ static int memcg_numa_stat_show(struct seq_file *m, void *v)
>  	mem_cgroup_flush_stats(memcg);
>  
>  	for (stat = stats; stat < stats + ARRAY_SIZE(stats); stat++) {
> -		seq_printf(m, "%s=%lu", stat->name,
> -			   mem_cgroup_nr_lru_pages(memcg, stat->lru_mask,
> -						   false));
> -		for_each_node_state(nid, N_MEMORY)
> -			seq_printf(m, " N%d=%lu", nid,
> -				   mem_cgroup_node_nr_lru_pages(memcg, nid,
> -							stat->lru_mask, false));
> +		seq_puts(m, stat->name);
> +		seq_put_decimal_ull(m, "=",
> +				    (u64)mem_cgroup_nr_lru_pages(memcg, stat->lru_mask,
> +								  false));
> +		for_each_node_state(nid, N_MEMORY) {
> +			seq_put_decimal_ull(m, " N", nid);
> +		seq_put_decimal_ull(m, "=",
> +				    (u64)mem_cgroup_node_nr_lru_pages(memcg, nid,
> +								       stat->lru_mask, false));

The indenting went wrong here.

The patch does do a lot of ugly tricks to constrain the number of
columns used.  Perhaps introduce some new local variables to clean this
up?