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([2620:10d:c090:500::7:c71d]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-aee7de19c14sm8596571a12.20.2025.03.03.11.23.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 Mar 2025 11:23:51 -0800 (PST) Message-ID: Date: Mon, 3 Mar 2025 11:23:49 -0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/4 v2] cgroup: separate rstat locks for subsystems To: Shakeel Butt , Yosry Ahmed Cc: =?UTF-8?Q?Michal_Koutn=C3=BD?= , tj@kernel.org, mhocko@kernel.org, hannes@cmpxchg.org, akpm@linux-foundation.org, linux-mm@kvack.org, cgroups@vger.kernel.org, kernel-team@meta.com References: <20250227215543.49928-1-inwardvessel@gmail.com> <20250227215543.49928-4-inwardvessel@gmail.com> Content-Language: en-US From: JP Kobryn In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: kjpmkif5t5aw65fc1tzfax7d838yxbr6 X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: D4EEEC0004 X-Rspam-User: X-HE-Tag: 1741029832-290787 X-HE-Meta: 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 5reA4BGw 7ulPlYchvln3DYj1KNcepLM3eJODRYxBYt3vbxZ4dG0kOi4VdNXoSZQvjCooxS8pRv+RUu9AJWqJleEyvOFM/k/eLHzbHsViwLpyImOc9OYGupv9hTvK6GGdZTxQj0GzWvLwP24wsxy4eZSmbqrsgZCi/ISYRtj/kg6pVgCdNo2261PtTHlolgmWXX7dWohdjGV1FogZapIFtfZCAp/B4z3pLaNDjDYZGoEf/ctd2pHAm1ztoZh6vuI5h2+2Fbm1WOb29roJHazZvIDlCz/U9bijt50gs8TxcPGiNftDoxIxoCo7Ckk2qVjVrbyh/50QbeV5WnHgzCQdwqABiDa7VQ9SKIPpl+jSf4intkH5LxBq0vrcSdOWdhGzNfkhf9YKHQekcbswuBXwjd8iIx93QAuXVwPQ1iEQHD/FTcs/WGqZIu95Qpgy4h+XJy5OWJgj1fL2mBGX0MbkHces= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000014, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/3/25 10:40 AM, Shakeel Butt wrote: > On Mon, Mar 03, 2025 at 06:29:53PM +0000, Yosry Ahmed wrote: >> On Mon, Mar 03, 2025 at 04:22:42PM +0100, Michal Koutný wrote: >>> On Thu, Feb 27, 2025 at 01:55:42PM -0800, inwardvessel wrote: >>>> From: JP Kobryn >>> ... >>>> +static inline bool is_base_css(struct cgroup_subsys_state *css) >>>> +{ >>>> + return css->ss == NULL; >>>> +} >>> >>> Similar predicate is also used in cgroup.c (various cgroup vs subsys >>> lifecycle functions, e.g. css_free_rwork_fn()). I think it'd better >>> unified, i.e. open code the predicate here or use the helper in both >>> cases (css_is_cgroup() or similar). >>> >>>> void __init cgroup_rstat_boot(void) >>>> { >>>> - int cpu; >>>> + struct cgroup_subsys *ss; >>>> + int cpu, ssid; >>>> >>>> - for_each_possible_cpu(cpu) >>>> - raw_spin_lock_init(per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu)); >>>> + for_each_subsys(ss, ssid) { >>>> + spin_lock_init(&cgroup_rstat_subsys_lock[ssid]); >>>> + } >>> >>> Hm, with this loop I realize it may be worth putting this lock into >>> struct cgroup_subsys_state and initializing them in >>> cgroup_init_subsys() to keep all per-subsys data in one pack. >> >> I thought about this, but this would have unnecessary memory overhead as >> we only need one lock per-subsystem. So having a lock in every single >> css is wasteful. >> >> Maybe we can put the lock in struct cgroup_subsys? Then we can still >> initialize them in cgroup_init_subsys(). >> > > Actually one of things I was thinking about if we can just not have > per-subsystem lock at all. At the moment, it is protecting > rstat_flush_next field (today in cgroup and JP's series it is in css). > What if we make it a per-cpu then we don't need the per-subsystem lock > all? Let me know if I missed something which is being protected by this > lock. > > This is help the case where there are multiple same subsystem stat > flushers, possibly of differnt part of cgroup tree. Though they will > still compete on per-cpu lock but still would be better than a > sub-system level lock. Right, the trade-off would mean one subsystem flushing could contend for a cpu where a different subsystem is updating and vice versa.