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 69D7B3E8C78 for ; Tue, 31 Mar 2026 10:04:16 +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=1774951456; cv=none; b=gOUGhpaHoFdLEl8R7o9rZuw/zU5HT/2tdAXSP00dsy7t4raELRCaD46WQLjtqexNnk6v0GzbIUsF89m2Oc2GO4Cnz1ze26xtqNh0M6t652akRMYnruqtpzOZQ4LCg6YOrOszKUzCr2zAaB43ivEVSkofFYulAocIiPaBNgLQWYU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774951456; c=relaxed/simple; bh=49PG3nZl+98TuRwZIqz1zYmsoSpDs17xr568qzu1Eqg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GzB+co+05YXr1SB0YO42YOQdImBYrtx3OECsQ6kHUKtunugDW1OzNclPycKh8hC06SQFYsQQg/HidvumC/aqsYrstzMVXIiNGdcFFgCS1ujLVkB1XfufQZXzSf/RSKWpKXI9c8Hd8jKlYjcxG1TYwS1q2Bc25yYTcD83dIDA8+Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AKrz7BsV; 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="AKrz7BsV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 88CF1C2BCB0; Tue, 31 Mar 2026 10:04:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774951456; bh=49PG3nZl+98TuRwZIqz1zYmsoSpDs17xr568qzu1Eqg=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; 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 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: "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; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <87cy0kpfdx.fsf@DESKTOP-5N7EMDA> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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