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t=1781720342; bh=65QRkQSPu45itKPZo5VCpczhZ2orHwojKEIsve8RGCg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=TQOFWsvacBh/H+QKfWlTUHQpbcJy5i705DeADqJ8QDA0XtY3ARIdOD4G6U5MHBQf9 vG/WIIoc5axb9rFBefsHIpYzWZ+N0hbwQEQNBL5u5417hBCwUg/PmGGbvX1WoZnpy5 6IaE+eGmciTr4YBs51A4NNr0nND7U3r7mN2OIi6MjOy6ICb35H2mQUe4ktcbJAiMKv zLWuYTbAOrPcb+eylcCF+g2mQ0ixJTmucUj09gNZTiDWpYWS/ZriQuLhB7nXR9ae0U s2pzK5PRzngFjmUbwJlVh/A16KcVmlmdYYgcWC34c2cfWEjXxAlrxS/qJanH7u9y29 xjgguslc/s0Jw== Message-ID: <1136baf3-3967-4202-9eaa-5fd667c235cf@kernel.org> Date: Wed, 17 Jun 2026 20:18:57 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/lruvec: trace LRU add drains and drain-all requests Content-Language: en-US To: Shakeel Butt , "David Hildenbrand (Arm)" Cc: JP Kobryn , linux-mm@kvack.org, willy@infradead.org, usama.arif@linux.dev, akpm@linux-foundation.org, mhocko@suse.com, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, kasong@tencent.com, qi.zheng@linux.dev, baohua@kernel.org, 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 References: <20260610195220.12403-1-jp.kobryn@linux.dev> <06122cae-e28b-4ded-a9dd-d380d31c5230@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: BBED212000A X-Rspam-User: X-Stat-Signature: 13ig7oyn59f4ytybxy6rmeyyup1gp5t3 X-HE-Tag: 1781720343-232056 X-HE-Meta: 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 RhTUfrYG Lbl+yd+aR7xhdXqP/Mlg6PhLdIb9CwG5RVph/DtJSH+vf1Cnqp3cLHRMFd66piBBVZiOD6tQyoNhFmyVBn22yWbUApLxh+qoP5+kDY8GPzbo7np1HncTJUEvwrSU6zs9QgBpl47p3GGFN08WXoTTbeEf68qar3slg3lpmkc4l9Eqa5ADCqIH1m2VG70+2C+nDIUmvu+q4UIU+95+G2PRLZ8lbIkzOrOjW4COSw1eSGjmavllV9USWrZ5YlBrbqgVyy55J7FkDGebXVTGTyfLGzMQWUcnFiAkZsmgf Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 >> > --- >> > 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