On 6/17/26 17:03, Shakeel Butt wrote: > On Wed, Jun 17, 2026 at 01:11:16PM +0200, David Hildenbrand (Arm) wrote: >> On 6/10/26 21:52, JP Kobryn 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_add_drain_all to capture callers of __lru_add_drain_all and >> > whether they set the force flag for all CPUs. The tracepoint resembles >> > the signature of the enclosing function, but is needed because of >> > potential inlining. >> > >> > Signed-off-by: JP Kobryn <[email protected]> >> > --- >> > include/trace/events/pagemap.h | 37 ++++++++++++++++++++++++++++++++++ >> > mm/swap.c | 7 ++++++- >> > 2 files changed, 43 insertions(+), 1 deletion(-) >> > >> > diff --git a/include/trace/events/pagemap.h >> > b/include/trace/events/pagemap.h >> > index 171524d3526d..ff3da07ccb40 100644 >> > --- a/include/trace/events/pagemap.h >> > +++ b/include/trace/events/pagemap.h >> > @@ -77,6 +77,43 @@ 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_add_drain_all, >> > + >> > + TP_PROTO(bool force_all_cpus), >> > + >> > + TP_ARGS(force_all_cpus), >> > + >> > + TP_STRUCT__entry( >> > + __field(bool, force_all_cpus ) >> > + ), >> > + >> > + TP_fast_assign( >> > + __entry->force_all_cpus = force_all_cpus; >> > + ), >> > + >> > + TP_printk("force_all_cpus=%s", >> > + __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..e14b7612f896 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. */ >> > @@ -869,6 +872,8 @@ static inline void __lru_add_drain_all(bool >> > force_all_cpus) >> > if (WARN_ON(!mm_percpu_wq)) >> > return; >> > >> > + trace_mm_lru_add_drain_all(force_all_cpus); >> > + >> > /* >> > * Guarantee folio_batch counter stores visible by this CPU >> > * are visible to other CPUs before loading the current drain >> >> Given that trace events can quickly become stable ABI [1], are we really >> sure we >> want to add this? > > Yes, I think so as this is useful to get insights into lru cache draining. > Trace events being stable or not is secondary IMHO. If in future we > rearchitect > the lru page handling where there is no cache draining anymore, we can make > these a noops.
Yeah and I don't recall ever that a change to a mm tracepoint would ever break someone who'd complain and we'd have to revert it. These are niche enough. So I think the risk is low. >> >> [1] https://lore.kernel.org/r/[email protected] >> >> -- >> Cheers, >> >> David
