On 6/9/26 6:21 PM, Shakeel Butt wrote: > 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 <[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. > > 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.
Right, but couldn't that already be done with fentry or kprobe? If we only need the calling stack and the argument value of force_all_cpus I don't see a strong need for a dedicated tracepoint.
