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

["David Hildenbrand (Arm)" <david@kernel.org>]

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?

[1] https://lore.kernel.org/r/20260603130006.7d2c4a62@gandalf.local.home

-- 
Cheers,

David