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Fri, 20 Jun 2025 14:29:03 +0800 From: "Huang, Ying" To: Li Zhijian Cc: linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, y-goto@fujitsu.com, Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider Subject: Re: [PATCH RFC] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE accounting In-Reply-To: <20250619075245.3272384-1-lizhijian@fujitsu.com> (Li Zhijian's message of "Thu, 19 Jun 2025 15:52:45 +0800") References: <20250619075245.3272384-1-lizhijian@fujitsu.com> Date: Fri, 20 Jun 2025 14:28:51 +0800 Message-ID: <87ldpn2afw.fsf@DESKTOP-5N7EMDA> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspam-User: X-Stat-Signature: kut6xjm3ow13hwnk6yae8wiiuy88gsff X-Rspamd-Queue-Id: F0BA8C0002 X-Rspamd-Server: rspam02 X-HE-Tag: 1750400947-402254 X-HE-Meta: U2FsdGVkX195yYIMQOic8UIoJQmgxXCt4QKlTxa+HqVO3t8WRKrjyb2EOprD+LLLTOnjGBA4NQo3JE5lGBRlyNAr0EqVljOz0P8xgxWEhGOqS216I/UShNyN2dfOAfxQ9XRTeczT5ap9ZEEg24jCl1Mlhpw1UhFiCQPcvCKtl97vYuYjlVMlqJJem69tjwY6neV3Rggzser34ocpz11klY4An/IT6DCBltO8sHXXtJ6PPk05+2HI8RLK7UYrlXBHoVKxBp733p9fhLk8lLOANK5JIXZZ8he7xyFI6UH1KwknmZApuXCYzthsoxmUgiQC1VFq9NamRjLqO+B0DcIAzmAx8tzwaxZZ+6HKX54IZaWJisCQDKllGpufGh/XI30hJjOM+mQWZj+ukHMYEn5THwsqDWuf0oM5uzt6mOV+CUyJ6O6IWXrEUrplSQhQCvi7Pq28RNpqvvMki7OHseA2dH9oV2/7js1JSqR7pTQiQ+IPxrZT+oWX1euotHj+rY9ZbMA3BLitDx16T7ZNc/Z2C/hTIjRoydpEKvYA+2jsxqwP2QxVIOk79+64tfFRJbRyFIgcnGPx65YLKY1r52AFrvMM++3/+cAxIwct0e05vFbGhm3x6usNPtFOExOAuDOvslTFKXmtjiBEL19TUmcq9fV/HhmWo9CTUra5mp78iUO3f8N/hDHz1bAFJdsytckP1wer+x4+LfMcw/H18h81hHg3LXRVC3Q6MhZ9Bo5FAFDLfJZU9+XNCAMLf27q4eFAeQxHP/KtM6odsrpl7ZYAkEasjA+n/ivx8sSNA3rYZnq3csBi4IVIZ2cR6qDt/vkUm+ELMo6yDTJ0zJ1zhEyVgPVrTEKGiKPOGFo/YgEKXvV1WTZHxsDcTyGLmTQlZL77mlEbaUNPMoykxA1vD2qAPp/auTzVu0I5n+hEnvcvHioviv3pGbPQPqV8xIN+PiSmxthdPx/ecYihOdPcV2W G08R8vM9 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Li Zhijian writes: > Goto-san reported confusing pgpromote statistics where > the pgpromote_success count significantly exceeded pgpromote_candidate. > The issue manifests under specific memory pressure conditions: > when top-tier memory (DRAM) is exhausted by memhog and allocation begins > in lower-tier memory (CXL). After terminating memhog, the stats show: The above description is confusing. The page promotion occurs when the size of the top-tier free space is large enough (after killing the memhog above). The accessed lower-tier memory will be promoted upon accessing to take full advantage of the more expensive top-tier memory. > $ grep -e pgpromote /proc/vmstat > pgpromote_success 2579 > pgpromote_candidate 1 > > This update increments PGPROMOTE_CANDIDATE within the free space branch > when a promotion decision is made, which may alter the mechanism of the > rate limit. Consequently, it becomes easier to reach the rate limit than > it was previously. > > For example: > Rate Limit = 100 pages/sec > Scenario: > T0: 90 free-space migrations > T0+100ms: 20-page migration request > > Before: > Rate limit is *not* reached: 0 + 20 = 20 < 100 > PGPROMOTE_CANDIDATE: 20 > After: > Rate limit is reached: 90 + 20 = 110 > 100 > PGPROMOTE_CANDIDATE: 110 Yes. The rate limit will be influenced by the change. So, more tests may be needed to verify it will not incurs regressions. > > Reported-by: Yasunori Gotou (Fujitsu) > Signed-off-by: Li Zhijian > --- > > This is markes as RFC because I am uncertain whether we originally > intended for this or if it was overlooked. > > However, the current situation where pgpromote_candidate < pgpromote_success > is indeed confusing when interpreted literally. > > Cc: Huang Ying > Cc: Ingo Molnar > Cc: Peter Zijlstra > Cc: Juri Lelli > Cc: Vincent Guittot > Cc: Dietmar Eggemann > Cc: Steven Rostedt > Cc: Ben Segall > Cc: Mel Gorman > Cc: Valentin Schneider > --- > kernel/sched/fair.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 7a14da5396fb..4715cd4fa248 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1940,11 +1940,13 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio, > struct pglist_data *pgdat; > unsigned long rate_limit; > unsigned int latency, th, def_th; > + long nr = folio_nr_pages(folio) > > pgdat = NODE_DATA(dst_nid); > if (pgdat_free_space_enough(pgdat)) { > /* workload changed, reset hot threshold */ > pgdat->nbp_threshold = 0; > + mod_node_page_state(pgdat, PGPROMOTE_CANDIDATE, nr); > return true; > } > > @@ -1958,8 +1960,7 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio, > if (latency >= th) > return false; > > - return !numa_promotion_rate_limit(pgdat, rate_limit, > - folio_nr_pages(folio)); > + return !numa_promotion_rate_limit(pgdat, rate_limit, nr); > } > > this_cpupid = cpu_pid_to_cpupid(dst_cpu, current->pid); --- Best Regards, Huang, Ying