Re: [PATCH v7 09/12] mm: multigenerational LRU: runtime switch

[Yu Zhao]

On Tue, Feb 08, 2022 at 01:18:59AM -0700, Yu Zhao wrote:
> Add /sys/kernel/mm/lru_gen/enabled as a runtime switch. Features that
> can be enabled or disabled include:
>   0x0001: the multigenerational LRU
>   0x0002: the page table walks, when arch_has_hw_pte_young() returns
>           true
>   0x0004: the use of the accessed bit in non-leaf PMD entries, when
>           CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG=y
>   [yYnN]: apply to all the features above
> E.g.,
>   echo y >/sys/kernel/mm/lru_gen/enabled
>   cat /sys/kernel/mm/lru_gen/enabled
>   0x0007
>   echo 5 >/sys/kernel/mm/lru_gen/enabled
>   cat /sys/kernel/mm/lru_gen/enabled
>   0x0005
> 
> NB: the page table walks happen on the scale of seconds under heavy
> memory pressure. Under such a condition, the mmap_lock contention is a
> lesser concern, compared with the LRU lock contention and the I/O
> congestion. So far the only well-known case of the mmap_lock
> contention is Android, due to Scudo [1] which allocates several
> thousand VMAs for merely a few hundred MBs. The SPF and the Maple Tree
> also have provided their own assessments [2][3]. However, if the page
> table walks do worsen the mmap_lock contention, the runtime switch can
> be used to disable this feature. In this case the multigenerational
> LRU will suffer a minor performance degradation, as shown previously.

Clarified the potential impact from the mmap_lock contention as
requested here:
https://lore.kernel.org/linux-mm/YdwQcl6D5Mbp9Z4h@xxxxxxxxxxxxxx/

<snipped>

> +static void lru_gen_change_state(bool enable)
> +{
> +	static DEFINE_MUTEX(state_mutex);
> +
> +	struct mem_cgroup *memcg;
> +
> +	cgroup_lock();
> +	cpus_read_lock();
> +	get_online_mems();
> +	mutex_lock(&state_mutex);
> +
> +	if (enable == lru_gen_enabled())
> +		goto unlock;
> +
> +	if (enable)
> +		static_branch_enable_cpuslocked(&lru_gen_caps[LRU_GEN_CORE]);
> +	else
> +		static_branch_disable_cpuslocked(&lru_gen_caps[LRU_GEN_CORE]);

Fixed the lockdep warning for memory hotplug:
https://lore.kernel.org/linux-mm/87a6g0nczg.fsf@xxxxxxxxxxxxx/

<snipped>