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

[Yu Zhao]

On Thu, Feb 23, 2023 at 10:43 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> 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.

Will fix.

> >   $ 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?

No. I have three benchmarks running at the moment:
1. Windows SQL server guest on x86 host,
2. Apache Spark guest on arm64 host, and
3. Memcached guest on ppc64 host.

If you are really interested in that, I can reprioritize -- I need to
stop 1) and use that machine to get the number for you.

> > @@ -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.

I'm not sure I fully understand. Let's present the problem on the MM
side: assuming KVM supports lockless walks, batching can still be
worse (very unlikely), because GFNs can exhibit no memory locality at
all. So this option allows userspace to disable batching.

I fully understand why you don't want MM to call into KVM directly. No
acceptable ways to set up a clear interface between MM and KVM other
than the MMU notifier?