Re: [PATCH v3] mm/lruvec: trace LRU add drains and drain-all requests

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

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 <jp.kobryn@linux.dev>
>> > ---
>> >  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/20260603130006.7d2c4a62@gandalf.local.home
>> 
>> -- 
>> Cheers,
>> 
>> David