linux-kernel - Re: [PATCH RFC 2/2] mm: kmem: add direct objcg pointer to task

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Date:   Tue, 20 Dec 2022 20:44:51 +0000
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH RFC 2/2] mm: kmem: add direct objcg pointer to task_struct

On Tue, Dec 20, 2022 at 10:27:45AM -0800, 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.
> 
> The old indirect way is still used for remote memcg charging.
> 
> Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>

This looks good too. Few comments below:

[...]
> +
> +#ifdef CONFIG_MEMCG_KMEM
> +static void mem_cgroup_kmem_attach(struct cgroup_taskset *tset)
> +{
> +	struct task_struct *task;
> +	struct cgroup_subsys_state *css;
> +
> +	cgroup_taskset_for_each(task, css, tset) {
> +		struct mem_cgroup *memcg;
> +
> +		if (task->objcg)
> +			obj_cgroup_put(task->objcg);
> +
> +		rcu_read_lock();
> +		memcg = container_of(css, struct mem_cgroup, css);
> +		task->objcg = __get_obj_cgroup_from_memcg(memcg);
> +		rcu_read_unlock();
> +	}
> +}
> +#else
> +static void mem_cgroup_kmem_attach(struct cgroup_taskset *tset) {}
> +#endif /* CONFIG_MEMCG_KMEM */
> +
> +#if defined(CONFIG_MEMCG_KMEM) || defined(CONFIG_MEMCG_KMEM)

I think you want CONFIG_LRU_GEN in the above check.

>  static void mem_cgroup_attach(struct cgroup_taskset *tset)
>  {
> +	mem_cgroup_lru_gen_attach(tset);
> +	mem_cgroup_kmem_attach(tset);
>  }
> -#endif /* CONFIG_LRU_GEN */
> +#endif
>  
>  static int seq_puts_memcg_tunable(struct seq_file *m, unsigned long value)
>  {
> @@ -6816,9 +6872,15 @@ struct cgroup_subsys memory_cgrp_subsys = {
>  	.css_reset = mem_cgroup_css_reset,
>  	.css_rstat_flush = mem_cgroup_css_rstat_flush,
>  	.can_attach = mem_cgroup_can_attach,
> +#if defined(CONFIG_MEMCG_KMEM) || defined(CONFIG_MEMCG_KMEM)

Same here.

>  	.attach = mem_cgroup_attach,
> +#endif
>  	.cancel_attach = mem_cgroup_cancel_attach,
>  	.post_attach = mem_cgroup_move_task,
> +#ifdef CONFIG_MEMCG_KMEM
> +	.fork = mem_cgroup_fork,
> +	.exit = mem_cgroup_exit,
> +#endif
>  	.dfl_cftypes = memory_files,
>  	.legacy_cftypes = mem_cgroup_legacy_files,
>  	.early_init = 0,
> -- 
> 2.39.0
>