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

Yu Zhao
2022-02-08T08:42:39Z

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@dhcp22.suse.cz/

<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@linux.ibm.com/

<snipped>