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Received: from DESKTOP-5N7EMDA(mailfrom:ying.huang@linux.alibaba.com fp:SMTPD_---0X0AXFuR_1775036903 cluster:ay36) by smtp.aliyun-inc.com; Wed, 01 Apr 2026 17:48:33 +0800 From: "Huang, Ying" To: "David Hildenbrand (Arm)" Cc: Donet Tom , Andrew Morton , Ingo Molnar , Peter Zijlstra , 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 Subject: Re: [PATCH] sched/numa, mm: Skip page promotion if cpu pid is valid In-Reply-To: <7dd35e65-e0c2-423d-a82d-c9e488e85e33@kernel.org> (David Hildenbrand's message of "Tue, 31 Mar 2026 12:04:10 +0200") References: <20260326071216.11883-1-donettom@linux.ibm.com> <2b8f30a6-a8d1-4ea5-8078-5eec399c8609@linux.ibm.com> <87cy0kpfdx.fsf@DESKTOP-5N7EMDA> <7dd35e65-e0c2-423d-a82d-c9e488e85e33@kernel.org> Date: Wed, 01 Apr 2026 17:48:23 +0800 Message-ID: <874ilvui3c.fsf@DESKTOP-5N7EMDA> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspamd-Queue-Id: 6E30E8000B X-Stat-Signature: azobn9mfwgmmjxticeg9qsap93he7ynw X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1775036918-629941 X-HE-Meta: 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 +gNZYmDs BF5K/+I0d5t8EDP5oUMYyWsLZBjFA4y/pRtCMn7TZQk4Qr/u5jVQc5YpQ7Avzr5os2HxSQcWW8ltxspoiw6KGYFpHHLGYvdYOvDFIxmYOhqZBGv0n0Mbdcwb5B91k5OJe2UM0LDROOfopGhraSxYowKTtLq2UkV596tmeiETYkxVQiAg3Ui6C1/eswoNvoX73WGrJPUYk8SChPwmLPSArknEocE5bVO4/PJTAK1XS7tw3g6dRi0MdfeMaM0YqLXEkWb32vh3+BCizuZWwHYVkmvkkUKWyOW2QGpDBN61wgb9wUuty/0UHJrdbXLDoegBwjeWtVcDpoOJPlcTwf1ujlDGe18Hf9h0BXNl1 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: "David Hildenbrand (Arm)" writes: > On 3/31/26 10:33, Huang, Ying wrote: >> Hi, Donet, >> >> Donet Tom writes: >> >>> On 3/26/26 3:59 PM, David Hildenbrand (Arm) wrote: >>>> Is that measurable? Should we at least have a Fixes: ? >>>> >>>> /* >>>> * When ... >>>> >>>> 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? >>> >>> Thank you, David, for guiding me in the right direction. >>> >>> I initially thought that overflows would not occur, and therefore >>> cpupid_valid() would not produce false positives. However, >>> after looking into it further, it appears that overflow can >>> happen when storing the access time. >>> >>> The last_cpupid field is used to store the last access time. >>> From the code, it appears that 21 bits are used for this >>> (#define LAST_CPUPID_SHIFT (LAST__PID_SHIFT + LAST__CPU_SHIFT)). >>> >>> With 21 bits, the maximum value that can be stored is >> >> It can be less than 21 bits, if CONFIG_NR_CPUS is small. >> >> DEFINE(NR_CPUS_BITS, order_base_2(CONFIG_NR_CPUS)); >> >>> 2097151ms (35Hrs) . If the access time exceeds this >>> range, it can overflow, which may lead to cpupid_valid() >>> returning false positives. >>> >>> I think we need a reliable way to determine cpupid_valid() that >>> does not produce false positives. >> >> Yes. IMHO, false positives is unavoidable. So, the patch fixes a >> temporal performance issue at the cost of a longstanding performance >> issue. Right? > > > Could we set aside a bit to indicate "cpuid vs. time" ? We'd lose one > bit for time, to we care? Do we need one more bit for time and cpupid? However, page flags are precious resources. > Would make it all easier to get ... --- Best Regards, Huang, Ying