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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Tue, Jun 09, 2026 at 05:16:15PM -0700, JP Kobryn wrote:
> On 6/9/26 5:07 PM, JP Kobryn wrote:
> > On 6/9/26 12:44 AM, Barry Song wrote:
> >> On Tue, Jun 9, 2026 at 12:12 PM JP Kobryn <jp.kobryn@linux.dev> 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_drain_all_queue to report when lru_add_drain_all() queues
> >>> per-CPU drain work. This captures the requester stack and target CPU for
> >>> remote drain work. The event is named as a drain-all queue event because
> >>> the queued work can be needed for batches other than lru_add.
> >>>
> >>> Signed-off-by: JP Kobryn <jp.kobryn@linux.dev>
> >>> ---
> >>>  include/trace/events/pagemap.h | 40 ++++++++++++++++++++++++++++++++++
> >>>  mm/swap.c                      |  6 ++++-
> >>>  2 files changed, 45 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/include/trace/events/pagemap.h b/include/trace/events/pagemap.h
> >>> index 171524d3526d..ea8fc46bedb0 100644
> >>> --- a/include/trace/events/pagemap.h
> >>> +++ b/include/trace/events/pagemap.h
> >>> @@ -77,6 +77,46 @@ TRACE_EVENT(mm_lru_activate,
> >>>         TP_printk("folio=%p pfn=0x%lx", __entry->folio, __entry->pfn)
> >>>  );
> >>>
> >>> +TRACE_EVENT(mm_lru_add_drain,
> >>> +
> >>> +       TP_PROTO(int cpu, unsigned int nr),
> >>> +
> >>> +       TP_ARGS(cpu, nr),
> >>> +
> >>> +       TP_STRUCT__entry(
> >>> +               __field(int,            cpu     )
> >>> +               __field(unsigned int,   nr      )
> >>> +       ),
> >>> +
> >>> +       TP_fast_assign(
> >>> +               __entry->cpu    = cpu;
> >>> +               __entry->nr     = nr;
> >>> +       ),
> >>> +
> >>> +       TP_printk("cpu=%d nr=%u", __entry->cpu, __entry->nr)
> >>> +);
> >>> +
> >>> +TRACE_EVENT(mm_lru_drain_all_queue,
> >>> +
> >>> +       TP_PROTO(int target_cpu, bool force_all_cpus),
> >>> +
> >>> +       TP_ARGS(target_cpu, force_all_cpus),
> >>> +
> >>> +       TP_STRUCT__entry(
> >>> +               __field(int,    target_cpu      )
> >>> +               __field(bool,   force_all_cpus  )
> >>> +       ),
> >>> +
> >>> +       TP_fast_assign(
> >>> +               __entry->target_cpu     = target_cpu;
> >>> +               __entry->force_all_cpus = force_all_cpus;
> >>> +       ),
> >>> +
> >>> +       TP_printk("target_cpu=%d force_all_cpus=%s",
> >>> +               __entry->target_cpu,
> >>> +               __entry->force_all_cpus ? "true" : "false")
> >>> +);
> >>> +
> >>>  #endif /* _TRACE_PAGEMAP_H */
> >>>
> >>>  /* This part must be outside protection */
> >>> diff --git a/mm/swap.c b/mm/swap.c
> >>> index 588f50d8f1a8..c385b93582eb 100644
> >>> --- a/mm/swap.c
> >>> +++ b/mm/swap.c
> >>> @@ -694,9 +694,12 @@ void lru_add_drain_cpu(int cpu)
> >>>  {
> >>>         struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu);
> >>>         struct folio_batch *fbatch = &fbatches->lru_add;
> >>> +       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);
> >>> +       }
> >>>
> >>>         fbatch = &fbatches->lru_move_tail;
> >>>         /* Disabling interrupts below acts as a compiler barrier. */
> >>> @@ -928,6 +931,7 @@ static inline void __lru_add_drain_all(bool force_all_cpus)
> >>>                 if (cpu_needs_drain(cpu)) {
> >>>                         INIT_WORK(work, lru_add_drain_per_cpu);
> >>>                         queue_work_on(cpu, mm_percpu_wq, work);
> >>> +                       trace_mm_lru_drain_all_queue(cpu, force_all_cpus);
> >>
> >> Do you need tracing on each CPU individually, or is tracing the
> >> entire __lru_add_drain_all() invocation sufficient?
> > 
> > I think the latter would be fine. The remote work will invoke the
> > mm_lru_add_drain tracepoint, which will show up as kworker stacks. Since
> > the event already has the CPU, we could see where queued drains actually
> > ran.
> 
> Actually if it's just a single invocation and the only event data is the
> force flag, a tracepoint may not even be needed. Other probes can be
> installed on function invocation and read the single argument. I can
> drop this from v2 and keep the single mm_lru_add_drain tracepoint.

No we do want to trace the callers requesting to drain from all the CPUs. If you
trace just lru_add_drain_cpu() then you will only see that the drain is
requested for a given CPU but no information on the requester. 

Also as Barry said, I think single trace for whole __lru_add_drain_all() is good
enough.