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

[JP Kobryn <jp.kobryn@linux.dev>]

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 <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.

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.