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Message-ID: <82918a12-d83e-10c0-0e04-eec26657b699@shopee.com>
Date:   Tue, 21 Feb 2023 18:29:39 +0800
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Subject: Re: [PATCH] mm/memcg: Skip high limit check in root memcg
To:     Michal Hocko <mhocko@suse.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
References: <20230210094550.5125-1-haifeng.xu@shopee.com>
 <Y+uvRKo7OQ02yB4K@dhcp22.suse.cz>
From:   Haifeng Xu <haifeng.xu@shopee.com>
In-Reply-To: <Y+uvRKo7OQ02yB4K@dhcp22.suse.cz>
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On 2023/2/14 23:56, Michal Hocko wrote:
> 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
> 

test steps:
1. mkdir -p /sys/fs/cgroup/memory/test
2. echo $$ > /sys/fs/cgroup/memory/test/cgroup.procs
3. ./mmap_test

The test result show that with or without the patch, the time taken is almost the same.
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