Re: [PATCH v3] mm/lruvec: trace LRU add drains and drain-all requests

[Barry Song]

On Thu, Jun 11, 2026 at 3:53 AM JP Kobryn <jp.kobryn@xxxxxxxxx> wrote:
>
> LRU add batches can be drained before they reach capacity. This can be a
> source of LRU lock contention, but it is not currently possible to
> attribute these drains to callers with existing tracepoints.
>
> Add mm_lru_add_drain to report the CPU and lru_add batch count when an
> lru_add batch is drained. This allows tracing to distinguish full drains
> from partial drains and attribute them to the calling stack.
>
> Add mm_lru_add_drain_all to capture callers of __lru_add_drain_all and
> whether they set the force flag for all CPUs. The tracepoint resembles
> the signature of the enclosing function, but is needed because of
> potential inlining.
>
> Signed-off-by: JP Kobryn <jp.kobryn@xxxxxxxxx>

Reviewed-by: Barry Song <baohua@xxxxxxxxxx>

Some minor nits:

[...]

> +       unsigned int nr_folios_add = folio_batch_count(fbatch);
>
> -       if (folio_batch_count(fbatch))
> +       if (nr_folios_add) {
>                 folio_batch_move_lru(fbatch, lru_add);
> +               trace_mm_lru_add_drain(cpu, nr_folios_add);
> +       }

Would "nr_folios" work here, given the surrounding lru_add context?
Alternatively, nr_folios_added might make the meaning a little clearer.

Best Regards
Barry