From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 78F943C5530 for ; Thu, 26 Mar 2026 10:29:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774520996; cv=none; b=SVWF0aeMe63wfk6AK6Y2A4H5rHglIUTnVuo8w9gDh1ZmN2y9txRKTa5p4X15FUS0j2KaeihX/B3Z36Lap+bVywWP8mJDzs+GVEQ/mKTT13rP2Hfd6Or+iHUXegyDEjQR1PXIOLjx8HVoZgjZBjy5bNMB5j0pojxvx1P5Oht2HWc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774520996; c=relaxed/simple; bh=r+HgD2u/Tq9CkbDGZ3lEw+byiueM4J5FP2Q5P+aBkaM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=BB07I+eTjWBRzCytCbAH0bUiX24I3++2IMfqMQBGtXtqO0GqaTE/uhiw7UEkBH17Xde3OAR6Om3r9G2uuE6UGiKutmCdP45AYum93odFJK5R8+DiYgktRHatIOwkSfhysNM/m4jzaQ/4Yf4uE6zE4volM/gZXbKByuzfsdbyUzQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=eFad7qen; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="eFad7qen" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 129A4C116C6; Thu, 26 Mar 2026 10:29:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774520996; bh=r+HgD2u/Tq9CkbDGZ3lEw+byiueM4J5FP2Q5P+aBkaM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=eFad7qenAiwEVed1LXvS4QaXK2JMFhGBaPYAYgHB6A1moVO1UUUcPfEnRq4thbQzf xV5JT4zamYEBbTOKqunfvM+AVzTh/mnRLLD1rQPR+rYzVEBI/etuZYyEGlPNesXAPm uy2TPA5e5kW55GOGJIFWu+prPRS6lgc5T6/YKgnVO07ZFfMenrP7oFcg2OlrJqiVyG jb/NyXpOLuPBU3uoF3UmHO+KHKgMQ3HqOX1zD0utenGax9mnzRkeZ0UArTZDqVMSoq TTvl5A7QPtAgl4Vd2K/V1B8fDCXN4jtNGjl9KHEtDGPfKqMI352RrgvYhi3ZwoMYrc pAGz4MW4Mo6oQ== Message-ID: Date: Thu, 26 Mar 2026 11:29:49 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] sched/numa, mm: Skip page promotion if cpu pid is valid To: Donet Tom , Andrew Morton , Ingo Molnar , Peter Zijlstra Cc: Ritesh Harjani , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Baolin Wang , Ying Huang , Juri Lelli , Mel Gorman , Vincent Guittot , Dietmar Eggemann , Steven Rostedt References: <20260326071216.11883-1-donettom@linux.ibm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/26/26 08:12, Donet Tom wrote: > If memory tiering is disabled, cpupid of slow memory pages may > contain a valid CPU and PID. If tiering is enabled at runtime, > there is a chance that in should_numa_migrate_memory(), this > valid CPU/PID is treated as a last access timestamp, leading > to unnecessary promotion. Is that measurable? Should we at least have a Fixes: ? > > Prevent this by skipping promotion when cpupid is valid. > > Signed-off-by: Donet Tom > --- > kernel/sched/fair.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 4b43809a3fb1..f5830a5a94d5 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -2001,6 +2001,13 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio, > unsigned int latency, th, def_th; > long nr = folio_nr_pages(folio); > /* * When ... > + /* When tiering is enabled at runtime, last_cpupid may > + * hold a valid cpupid instead of an access timestamp. > + * If so, skip page promotion. > + */ > + if (cpupid_valid(folio_last_cpupid(folio))) > + return false; > + IIUC, as timestamp we use jiffies_to_msecs(). So, soon after bootup, we would no longer get false positives for cpupid_valid(). I suppose overflows are not a problem, correct? So what we're saying is that folio_use_access_time()==true does not imply that there is actually a valid time in there. In numa_migrate_check() we could still use the valid cpuid I guess and make that code a bit clearer? diff --git a/mm/memory.c b/mm/memory.c index 631205a384e1..ba68933a9e4a 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -6119,10 +6119,9 @@ int numa_migrate_check(struct folio *folio, struct vm_fault *vmf, * For memory tiering mode, cpupid of slow memory page is used * to record page access time. So use default value. */ - if (folio_use_access_time(folio)) + *last_cpupid = folio_last_cpupid(folio); + if (!cpupid_valid(*last_cpupid)) *last_cpupid = (-1 & LAST_CPUPID_MASK); - else - *last_cpupid = folio_last_cpupid(folio); /* Record the current PID accessing VMA */ vma_set_access_pid_bit(vma); The change itself here looks reasonable to me. Acked-by: David Hildenbrand (Arm) -- Cheers, David