Date:   Tue, 14 Feb 2023 16:56:52 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Haifeng Xu <haifeng.xu@shopee.com>
Cc:     hannes@cmpxchg.org, shakeelb@google.com, muchun.song@linux.dev,
        akpm@linux-foundation.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/memcg: Skip high limit check in root memcg
Message-ID: <Y+uvRKo7OQ02yB4K@dhcp22.suse.cz>
References: <20230210094550.5125-1-haifeng.xu@shopee.com>
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On Fri 10-02-23 09:45:50, Haifeng Xu wrote:
> The high limit checks the memory usage from given memcg to root memcg.
> However, there is no limit in root memcg. So this check makes no sense
> and we can ignore it.

Is this check actually addining any benefit? Have you measured aby
performance gains by this change?

> Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
> ---
>  mm/memcontrol.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 73afff8062f9..a31a56598f29 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2780,6 +2780,10 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  	do {
>  		bool mem_high, swap_high;
>  
> +		/* There is no need for root memcg to check high limit */
> +		if (mem_cgroup_is_root(memcg))
> +			break;
> +
>  		mem_high = page_counter_read(&memcg->memory) >
>  			READ_ONCE(memcg->memory.high);
>  		swap_high = page_counter_read(&memcg->swap) >
> -- 
> 2.25.1

-- 
Michal Hocko
SUSE Labs