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Date:   Fri, 20 Oct 2023 07:33:33 +0200
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Subject: Re: [PATCH v5 2/6] mm: kmem: add direct objcg pointer to task_struct
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To:     Roman Gushchin <roman.gushchin@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        Dennis Zhou <dennis@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Naresh Kamboju <naresh.kamboju@linaro.org>
References: <20231019225346.1822282-1-roman.gushchin@linux.dev>
 <20231019225346.1822282-3-roman.gushchin@linux.dev>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <20231019225346.1822282-3-roman.gushchin@linux.dev>
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Precedence: bulk

On 10/20/23 00:53, Roman Gushchin wrote:
> To charge a freshly allocated kernel object to a memory cgroup, the
> kernel needs to obtain an objcg pointer. Currently it does it
> indirectly by obtaining the memcg pointer first and then calling to
> __get_obj_cgroup_from_memcg().
> 
> Usually tasks spend their entire life belonging to the same object
> cgroup. So it makes sense to save the objcg pointer on task_struct
> directly, so it can be obtained faster. It requires some work on fork,
> exit and cgroup migrate paths, but these paths are way colder.
> 
> To avoid any costly synchronization the following rules are applied:
> 1) A task sets it's objcg pointer itself.
> 
> 2) If a task is being migrated to another cgroup, the least
>    significant bit of the objcg pointer is set atomically.
> 
> 3) On the allocation path the objcg pointer is obtained locklessly
>    using the READ_ONCE() macro and the least significant bit is
>    checked. If it's set, the following procedure is used to update
>    it locklessly:
>        - task->objcg is zeroed using cmpxcg
>        - new objcg pointer is obtained
>        - task->objcg is updated using try_cmpxchg
>        - operation is repeated if try_cmpxcg fails
>    It guarantees that no updates will be lost if task migration
>    is racing against objcg pointer update. It also allows to keep
>    both read and write paths fully lockless.
> 
> Because the task is keeping a reference to the objcg, it can't go away
> while the task is alive.
> 
> This commit doesn't change the way the remote memcg charging works.
> 
> Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
> Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Shakeel Butt <shakeelb@google.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>