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b=AKrz7BsVVtgchEJ5poESGGoOf+5w+YoDmJiFZdfkzwcVyuGrSfpS34jmPqTXUbdsP rrQaziZTzHRme5MaFUI7WpziGeuA2L8OcX2eMOi+zDrK2ljlRtAgr/C+uJn44rv1H+ m8sWmAapRcDZAyBJXa0M8j+v9RG4n5DiB04iFsvqwVYqKOYIjropUlljZ3SezzYE3N LOjjeKz9bnI20Nu5H84DlpXk3x0cqCXfcuLODJNjmSLhiTnaBy4fWmLm600XTfI5Qr NF7lRfR5azYVrL1aQAYyNklSJrDR6nse8WDe9OsyiXR9wBsj33KZwMIFjLBKzmDYGd NBGvb6rxjG5bw== Message-ID: <7dd35e65-e0c2-423d-a82d-c9e488e85e33@kernel.org> Date: Tue, 31 Mar 2026 12:04:10 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] sched/numa, mm: Skip page promotion if cpu pid is valid To: "Huang, Ying" , Donet Tom Cc: 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 References: <20260326071216.11883-1-donettom@linux.ibm.com> <2b8f30a6-a8d1-4ea5-8078-5eec399c8609@linux.ibm.com> <87cy0kpfdx.fsf@DESKTOP-5N7EMDA> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 43DF5160011 X-Stat-Signature: e81fatr6fxoaaqiudr81xkbmeucj1rpq X-Rspamd-Server: rspam06 X-HE-Tag: 1774951457-230472 X-HE-Meta: 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 hnUyRP+v BwV+BLZ9ZKjYK7gGEmdgidiQWqoMBFRebsMvptBxdvDDgMNx8Vciue7Wp4k7e18KpToDVbVKt8T/Qy9OyHewnZuZYkEv2zTmB9t2rN3KyU3jF/ToHsoECaB40OpDnBX8pstjV6d3TS/+psm2jmYsOY5imnRfWFiI1EaQXJrk2umBHCNS/4O4SjncX3CFrJYCekwV/ahM/C/7hEVhIzPW0yXis0dZOa4u3w/ZfU4GBjZRnShYUNU+cOpiwBqaRUbW/s09XQEjiKKPHDvBqV3MQrlmp5MWGS7PMFEQYNvQJRMpaCtkkcHf7hLOhBvQHP2VGjKfC Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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? Would make it all easier to get ... -- Cheers, David