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a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1785316894; b=nOsnB+DMxHaQiTsnrEye8goLrKp4WYdn076xflSTD3+TX6fuvrEPbB6+p3aG62V7IHAwNd oRWex5Ip28PXHXPsWwSvcl2e0Phix+pNqYGCZ9rpm/hpMLmUwwYXV9mJYmkql02NC/nuf1 a833WbX2fivonqtrZpZMY3+djVM8lSo= ARC-Authentication-Results: i=1; imf17.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=kV2KAngc; spf=pass (imf17.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 2E9694189E; Wed, 29 Jul 2026 09:21:33 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 790081F000E9; Wed, 29 Jul 2026 09:21:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785316893; bh=1ck/H1VPUEgL0UJhdG6kDLw/19qHzYtBNITX6iLnKog=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=kV2KAngcdMNxcJho/xhAEZXwMDIP49qRzAfHR4lrnJTE9dXajTsyQ+/sZ9D/G1XKS H7Lcc5KvPOJuEoaJmFEJ95v3GW5s/4jqUKu+eL+wV15oxWQsBouINUehSHN5tF6M95 +VCbFq8eAOJS09HOB0ITH9R6dPsOu5XGwMx04zAlJZsdxWaG0n4nbL2C+GR7WSbkBX 41a/357wULvkt59XWNlPdF2czG22aRKLlKK6Mjir1Q7lBsJnTc/iCgzaZK+7W1rg31 VlZxYFjOj0KdkQ/kte/wCJfjAngUQlu/WLr8v/hxqsqwLBMfi9uev0RtpLgLjqx3dQ 98lkFNsgWbLrg== Message-ID: Date: Wed, 29 Jul 2026 11:21:29 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch() To: Breno Leitao , Andrew Morton , Zi Yan , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , paulmck@kernel.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260727-kcompact-v1-1-bdfefddd6874@debian.org> 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: <20260727-kcompact-v1-1-bdfefddd6874@debian.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 13D7240009 X-Stat-Signature: ek7uzz4cimg4moob8be1ueg3aywdef3c X-HE-Tag: 1785316893-52230 X-HE-Meta: U2FsdGVkX1+LhHj6rCQgiylzc5tI105j039Sb2IPnDkEIK6SzsqlcAkIc+Ar/KBGvuOqf6WtiG2RRPuo2o2/zov3XCAZAGnP2jpv3ko3V09Ra5MtNPxVvU9f1y4TplvApRhLkZeAswB8CKhX0cPiMoY5tks7apGfISyEDbUHiWw6+JIinxePH6oDFg6jKkln1+uqElTz4fXOnJhcvIrCi7BmhGSgmWqvgnIdPy7l9Px03grTq8P43MAZleAQ9Zy7zjy0AELyxtCWW+fQg71iWyCmt6Av2KUBfyRqi3lWC4Ff9Ph6mJ1tEp644N8i97d3DmIOCbRxNiYoxmKPx3JvD4ubmhDcqVRa55r3L+ln/7WuV6p4NJv/zF+r/UyUZNYfaeLKUiW8CeLsfbugg/Bmt6pnAm7pfKvDjPZe70GdM8vTHGUOpzcDdnBXbxbgG7GjpVfVE6+sGHam7dM1hgPTGY1o6+sDJXDYoXaQrLI5q7JM4T5+rC8v5yUw0xg71vgYXoFAPFfT5hFf4639LKbv/nghRMR6IdRX5zjp5s8VROJFj4tGg0womQlC/nS0sg/NUkpZ7LZY7xqqXeq/u3Bq915S11TF3t+UJIHeF9PdY5xHbcJ2kPkQ8JwYbXCAuQ9+cOXWU8mtBhS71ctDYDBrhPMwlKdXK6OrxNm42aM1qXrQ+MmXth9Lg1YvKo9UcxEe2ApOfA2LnE92NVcWJyAh6kpCOZfdZ7cKWaQN5vUG6R09NYcy2wzfgbiqHB2PUzvEaWAMlC6LGkMqeVEzcZhjfPyA4zd92yj5zpWOk9NcYLe0PxTLgiJMH5xyUW16SUVKrgUJdH4Ydy2IN7KTLifTOLDfFeVI18WK1fNMXj/Vx2NvsZlNCuUw5CRWjgpAtF06yOlGY2LPSI/Fakk2N+5VwPl5xT+iqYhM3MIi0gSy71R0SGWcj/AxKU2Ca2vKuIM7W+vDB2POYuh5qjlXr7w 3r8QYkbx VBoEklYJZQkrCB/S/A//EQWXj03ZjrMbEpgO/R6ZioW2Z+LpxFgyO6vBRN0r+vQ3xtES5zhWge8z1t1qfH/lwCPHDJKiq2gt9J15V/7D7DEZ0tkuOU2NVHCf0KMCjizSeayENRexlta3JbP/kzVW6pXNgaHE1Cx8oC4OnHuppK4xrIMdG4NDU7Rudu46ZWVPpjH1KzUwNc/ZOesqET3QwmbPlBKjV04UCvuaYd9XWGPe0L+YrEOAms3hQ3P54+LvH1ARxEFeEFS+e1af0yFPzjl0eTREeZTiuRDrYhPBVHpQ/WcpS1bkA602/NBUxHl/dxSXH9T2QfOUry45xNpAF5RAN38+9h0NRxuaB5aL93qa9vybOBntoyBeEXNAaQpMOjb8Sk9CvsuraivDMnFa7GY3UnNRSc7/X+jQ2TfkRyBXh4dJdc35aAbjkAgLN/YiJFCPcNA2lqsWZ1MI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/27/26 15:50, Breno Leitao wrote: > migrate_pages_batch() unmaps each folio before moving it, and every > unmap runs the mmu_notifier invalidate callbacks. On KVM hosts > try_to_migrate() ends up in kvm_mmu_notifier_invalidate_range_start() -> > tdp_mmu_zap_leafs(), which is expensive, so unmapping a large batch keeps > the CPU busy for a long time. > > The loop already calls cond_resched(), but on PREEMPTION kernels that is > a no-op, and involuntary preemption is not a Tasks-RCU quiescent state. > > A long batch therefore never reports a quiescent state, and the > migrating task (e.g. kcompactd) becomes a Tasks-RCU holdout, stalling the > Tasks-RCU grace period for minutes, which is common at Meta fleet: > > INFO: rcu_tasks detected stalls on tasks: > 0000000055349ecc: .. nvcsw: 1157401/1157401 holdout: 1 idle_cpu: -1/56 task:kcompactd0 state:R running task > Call Trace: > tdp_mmu_zap_leafs > tdp_mmu_next_root > gfn_to_pfn_cache_invalidate_start > kvm_mmu_notifier_invalidate_range_start > __mmu_notifier_invalidate_range_start > try_to_migrate_one > try_to_migrate > migrate_pages_batch > migrate_pages > compact_zone > compact_node > kcompactd > kthread > > Use cond_resched_tasks_rcu_qs() so a quiescent state is reported even > when cond_resched() does nothing. > > PS: This has also been discussed at [1] > > Link: https://lore.kernel.org/all/amdWVTs0WKOxguxP@gmail.com/ [1] > Signed-off-by: Breno Leitao > --- Thanks! Acked-by: David Hildenbrand (Arm) -- Cheers, David