From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 0A4D63ACEE0 for ; Wed, 29 Jul 2026 09:21:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785316896; cv=none; b=BLnEEY8G/W53hvdpGI/DYtKTKy5BPR0CvfRGyBpuBD2B0DcNANxEEwgUGP+iIkbYWuTWhD+P4TaqopLmc1wA41BZ0c5MIMWSfUugXHrrD2BpyeZuX/1cAGwX/2RYsuoY9BXuWFuEDdOLon/rQdjnvdygSPTlS/CrMiLgzp2/22s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785316896; c=relaxed/simple; bh=yTN6dmZMZsMqQ/BlKmGTwoKN1GolLKbhV24y691DVHE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=r0uRRXmnDYi2r6ps9TkmB2P8i8zwz0sozcLSPL++4Eqc83tm+uSVEqMUh4EYXdvG1Vsi/Mq+HdnGHrWxy2NKFfE1nVwhipKa1+1s9ZZlAev1htFYZQYNhv6wBsuS+Q8RRBlPO5XmNhk0OqvyJsO7gjO+zexqwQTS+r41VXB30PI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kV2KAngc; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kV2KAngc" 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 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] 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; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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