Subject: Re: [PATCH mm-unstable v1 5/5] mm: multi-gen LRU: use mmu_notifier_test_clear_young()

From: Sean Christopherson <[email-masked]>
Date: Thu, 23 Feb 2023 09:43:31 -0800

On Thu, Feb 16, 2023, Yu Zhao wrote:
> An existing selftest can quickly demonstrate the effectiveness of this
> patch. On a generic workstation equipped with 128 CPUs and 256GB DRAM:

Not my area of maintenance, but a non-existent changelog (for all intents and
purposes) for a change of this size and complexity is not acceptable.

>   $ sudo max_guest_memory_test -c 64 -m 250 -s 250
> 
>   MGLRU      run2
>   ---------------
>   Before    ~600s
>   After      ~50s
>   Off       ~250s
> 
>   kswapd (MGLRU before)
>     100.00%  balance_pgdat
>       100.00%  shrink_node
>         100.00%  shrink_one
>           99.97%  try_to_shrink_lruvec
>             99.06%  evict_folios
>               97.41%  shrink_folio_list
>                 31.33%  folio_referenced
>                   31.06%  rmap_walk_file
>                     30.89%  folio_referenced_one
>                       20.83%  __mmu_notifier_clear_flush_young
>                         20.54%  kvm_mmu_notifier_clear_flush_young
>   =>                      19.34%  _raw_write_lock
> 
>   kswapd (MGLRU after)
>     100.00%  balance_pgdat
>       100.00%  shrink_node
>         100.00%  shrink_one
>           99.97%  try_to_shrink_lruvec
>             99.51%  evict_folios
>               71.70%  shrink_folio_list
>                 7.08%  folio_referenced
>                   6.78%  rmap_walk_file
>                     6.72%  folio_referenced_one
>                       5.60%  lru_gen_look_around
>   =>                    1.53%  __mmu_notifier_test_clear_young

Do you happen to know how much of the improvement is due to batching, and how
much is due to using a walkless walk?

> @@ -5699,6 +5797,9 @@ static ssize_t show_enabled(struct kobject *kobj, struct kobj_attribute *attr, c
>  	if (arch_has_hw_nonleaf_pmd_young() && get_cap(LRU_GEN_NONLEAF_YOUNG))
>  		caps |= BIT(LRU_GEN_NONLEAF_YOUNG);
>  
> +	if (kvm_arch_has_test_clear_young() && get_cap(LRU_GEN_SPTE_WALK))
> +		caps |= BIT(LRU_GEN_SPTE_WALK);

As alluded to in patch 1, unless batching the walks even if KVM does _not_ support
a lockless walk is somehow _worse_ than using the existing mmu_notifier_clear_flush_young(),
I think batching the calls should be conditional only on LRU_GEN_SPTE_WALK.  Or
if we want to avoid batching when there are no mmu_notifier listeners, probe
mmu_notifiers.  But don't call into KVM directly.