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 <[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? > > 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.
