On Tue, Jun 9, 2026 at 12:12 PM JP Kobryn <[email protected]> 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 <[email protected]> > --- > 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? Do you also need this_gen and lru_drain_gen to be traced? By the way, I'm not sure drain_all_queue is the best name here. Why not simply use add_drain_all()? It would match the existing function name better. Best Regards Barry
