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

From: Michal Hocko <mhocko@suse.com>
To: Yu Zhao <yuzhao@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andi Kleen <ak@linux.intel.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
	Jesse Barnes <jsbarnes@google.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Matthew Wilcox <willy@infradead.org>,
	Mel Gorman <mgorman@suse.de>,
	Michael Larabel <Michael@michaellarabel.com>,
	Rik van Riel <riel@surriel.com>, Vlastimil Babka <vbabka@suse.cz>,
	Will Deacon <will@kernel.org>, Ying Huang <ying.huang@intel.com>,
	linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	page-reclaim@google.com, x86@kernel.org,
	Konstantin Kharlamov <Hi-Angel@yandex.ru>
Subject: Re: [PATCH v6 6/9] mm: multigenerational lru: aging
Date: Thu, 6 Jan 2022 17:06:42 +0100	[thread overview]
Message-ID: <YdcTkrl84Xzg2dSz@dhcp22.suse.cz> (raw)
In-Reply-To: <20220104202227.2903605-7-yuzhao@google.com>

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