Re: [PATCH v2] mm: add per-order mTHP alloc_success and alloc_fail counters

[Yu Zhao <yuzhao@google.com>]

On Thu, Mar 28, 2024 at 5:51 AM Barry Song <21cnbao@gmail.com> wrote:
>
> From: Barry Song <v-songbaohua@oppo.com>
>
> Profiling a system blindly with mTHP has become challenging due
> to the lack of visibility into its operations. Presenting the
> success rate of mTHP allocations appears to be pressing need.
>
> Recently, I've been experiencing significant difficulty debugging
> performance improvements and regressions without these figures.
> It's crucial for us to understand the true effectiveness of
> mTHP in real-world scenarios, especially in systems with
> fragmented memory.
>
> This patch sets up the framework for per-order mTHP counters,
> starting with the introduction of alloc_success and alloc_fail
> counters.  Incorporating additional counters should now be
> straightforward as well.
>
> The initial two unsigned longs for each event are unused, given
> that order-0 and order-1 are not mTHP. Nonetheless, this refinement
> improves code clarity.
>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> ---
>  -v2:
>  * move to sysfs and provide per-order counters; David, Ryan, Willy
>  -v1:
>  https://lore.kernel.org/linux-mm/20240326030103.50678-1-21cnbao@gmail.com/
>
>  include/linux/huge_mm.h | 17 +++++++++++++
>  mm/huge_memory.c        | 54 +++++++++++++++++++++++++++++++++++++++++
>  mm/memory.c             |  3 +++
>  3 files changed, 74 insertions(+)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index e896ca4760f6..27fa26a22a8f 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -264,6 +264,23 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
>                                           enforce_sysfs, orders);
>  }
>
> +enum thp_event_item {
> +       THP_ALLOC_SUCCESS,
> +       THP_ALLOC_FAIL,
> +       NR_THP_EVENT_ITEMS
> +};
> +
> +struct thp_event_state {
> +       unsigned long event[PMD_ORDER + 1][NR_THP_EVENT_ITEMS];
> +};
> +
> +DECLARE_PER_CPU(struct thp_event_state, thp_event_states);

Do we have existing per-CPU counters that cover all possible THP
orders? I.e., foo_counter[PMD_ORDER + 1][BAR_ITEMS]. I don't think we
do but I want to double check.

This might be fine if BAR_ITEMS is global, not per memcg. Otherwise on
larger systems, e.g., 512 CPUs which is not uncommon, we'd have high
per-CPU memory overhead. For Google's datacenters, per-CPU memory
overhead has been a problem.

I'm not against this patch since NR_THP_EVENT_ITEMS is not per memcg.
Alternatively, we could make the per-CPU counters to track only one
order and flush the local counter to a global atomic counter if the
new order doesn't match the existing order stored in the local
counter. WDYT?