linux-kernel - Re: [PATCH v6 6/9] mm: multigenerational lru: aging

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Date:   Thu, 6 Jan 2022 17:06:42 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        page-reclaim@...gle.com, x86@...nel.org,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>
Subject: Re: [PATCH v6 6/9] mm: multigenerational lru: aging

I am still reading through the series. It is a lot of code and quite
hard to wrap ones head around so these are mostly random things I have
run into. More will likely follow up.

On Tue 04-01-22 13:22:25, Yu Zhao wrote:
[...]
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index aba18cd101db..028afdb81c10 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1393,18 +1393,24 @@ mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
>  
>  static inline void lock_page_memcg(struct page *page)
>  {
> +	/* to match folio_memcg_rcu() */
> +	rcu_read_lock();
>  }
>  
>  static inline void unlock_page_memcg(struct page *page)
>  {
> +	rcu_read_unlock();
>  }
>  
>  static inline void folio_memcg_lock(struct folio *folio)
>  {
> +	/* to match folio_memcg_rcu() */
> +	rcu_read_lock();
>  }
>  
>  static inline void folio_memcg_unlock(struct folio *folio)
>  {
> +	rcu_read_unlock();
>  }

This should go into a separate patch and merge it independently. I
haven't really realized that !MEMCG configuration has a different
locking scopes.

[...]
> diff --git a/include/linux/oom.h b/include/linux/oom.h
> index 2db9a1432511..9c7a4fae0661 100644
> --- a/include/linux/oom.h
> +++ b/include/linux/oom.h
> @@ -57,6 +57,22 @@ struct oom_control {
>  extern struct mutex oom_lock;
>  extern struct mutex oom_adj_mutex;
>  
> +#ifdef CONFIG_MMU
> +extern struct task_struct *oom_reaper_list;
> +extern struct wait_queue_head oom_reaper_wait;
> +
> +static inline bool oom_reaping_in_progress(void)
> +{
> +	/* a racy check can be used to reduce the chance of overkilling */
> +	return READ_ONCE(oom_reaper_list) || !waitqueue_active(&oom_reaper_wait);
> +}
> +#else
> +static inline bool oom_reaping_in_progress(void)
> +{
> +	return false;
> +}
> +#endif

I do not like this. These are internal oom reaper's and no code should
really make any decisions based on that. oom_reaping_in_progress is not
telling much anyway. This is a global queue for oom reaper that can
contain oom victims from different oom scopes (e.g. global OOM, memcg
OOM or memory policy OOM).

Your lru_gen_age_node uses this to decide whether to trigger
out_of_memory and that is clearly wrong for the above reasons.
out_of_memory is designed to skip over any action if there is an oom
victim pending from the oom domain (have a look at oom_evaluate_task).

[...]

> +static bool age_lruvec(struct lruvec *lruvec, struct scan_control *sc,
> +		       unsigned long min_ttl)
> +{
> +	bool need_aging;
> +	long nr_to_scan;
> +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> +	int swappiness = get_swappiness(memcg);
> +	DEFINE_MAX_SEQ(lruvec);
> +	DEFINE_MIN_SEQ(lruvec);
> +
> +	if (mem_cgroup_below_min(memcg))
> +		return false;

mem_cgroup_below_min requires effective values to be calculated for the
reclaimed hierarchy. Have a look at mem_cgroup_calculate_protection
-- 
Michal Hocko
SUSE Labs