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From: Shakeel Butt To: JP Kobryn Cc: Barry Song , linux-mm@kvack.org, willy@infradead.org, usama.arif@linux.dev, akpm@linux-foundation.org, vbabka@kernel.org, mhocko@suse.com, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, kasong@tencent.com, qi.zheng@linux.dev, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, chrisl@kernel.org, shikemeng@huaweicloud.com, nphamcs@gmail.com, baoquan.he@linux.dev, youngjun.park@lge.com, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH] mm/lruvec: trace LRU add drains and drain-all queuing Message-ID: References: <20260609041156.31127-1-jp.kobryn@linux.dev> <3e55e520-b979-4b1c-874c-3b4e5ca629e2@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT 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 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 > >>> --- > >>> 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.