From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id ACE6BE571 for ; Sun, 15 Mar 2026 23:36:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773617796; cv=none; b=DtH2c2DiMvRsv8v5o/UJtJ9SAHAUoNorLdoAysTY7BbYOWfwZvVFkvthtfE7ng2jfq0jlJWBojRb7ABWLkkbddAmsQso8IYAgIem69Lt2LVlNyS/Hmuin452NEhtiruizT4AuhAbrp0bvihGShE+muZq6vJOGBVJrtgSBXyDsxY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773617796; c=relaxed/simple; bh=00iwuk2aKSEUk0a8QaUxn33sDZgOKJJlTJ4imzAyc48=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=d3AR83zxprC+/Snf4uYlceuR0IyE9TGzE/pBKZHE9n0YKuwpfSG+G5YnK3HJeGrMoAEYtcJ7M5BJ9Z9RfGMX4MXHx/Xw2aCSYEGTo0xVnxVG9E2Oun1bi45Jocn/SwBy1zfYq8VQyeoWBTOxgjXqQ0hHtdRK3cpfgvaoRnZgLAs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5E0091477; Sun, 15 Mar 2026 16:36:20 -0700 (PDT) Received: from [192.168.178.6] (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7B2A53F778; Sun, 15 Mar 2026 16:36:24 -0700 (PDT) Message-ID: <5b1228b7-5949-4a45-9f62-e8ce936de694@arm.com> Date: Mon, 16 Mar 2026 00:36:22 +0100 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 v4 7/9] sched/fair: Remove superfluous rcu_read_lock() in the wakeup path To: K Prateek Nayak , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Valentin Schneider , linux-kernel@vger.kernel.org Cc: Steven Rostedt , Ben Segall , Mel Gorman , Chen Yu , Shrikanth Hegde , Li Chen , "Gautham R. Shenoy" References: <20260312044434.1974-1-kprateek.nayak@amd.com> <20260312044434.1974-8-kprateek.nayak@amd.com> Content-Language: en-GB From: Dietmar Eggemann In-Reply-To: <20260312044434.1974-8-kprateek.nayak@amd.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 12.03.26 05:44, K Prateek Nayak wrote: > select_task_rq_fair() is always called with p->pi_lock held and IRQs > disabled which makes it equivalent of an RCU read-side. > > Since commit 71fedc41c23b ("sched/fair: Switch to > rcu_dereference_all()") switched to using rcu_dereference_all() in the > wakeup path, drop the explicit rcu_read_{lock,unlock}() in the fair > task's wakeup path. > > Future plans to reuse select_task_rq_fair() / > find_energy_efficient_cpu() in the fair class' balance callback will do > so with IRQs disabled and will comply with the requirements of > rcu_dereference_all() which makes this safe keeping in mind future > development plans too. > > Signed-off-by: K Prateek Nayak > --- > Changelog v3..v4: > > o No changes. > --- > kernel/sched/fair.c | 33 ++++++++++++--------------------- > 1 file changed, 12 insertions(+), 21 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index d57c02e82f3a..28853c0abb83 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8570,10 +8570,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) > struct perf_domain *pd; > struct energy_env eenv; > > - rcu_read_lock(); > pd = rcu_dereference_all(rd->pd); Got an RCU related warning when running EAS: [ 3.795872] EM: rcu read lock needed [ 3.795903] WARNING: ./include/linux/energy_model.h:251 at compute_energy+0x3 a4/0x3bc, CPU#1: swapper/1/0 [ 3.813755] Modules linked in: [ 3.816844] CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted 7.0.0-rc2-00295- g93189edc73c8-dirty #30 PREEMPT [ 3.826807] Hardware name: ARM Juno development board (r0) (DT) [ 3.832752] pstate: 600000c5 (nZCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--) [ 3.839750] pc : compute_energy+0x3a4/0x3bc [ 3.843970] lr : compute_energy+0x3a4/0x3bc [ 3.848185] sp : ffff8000838f39e0 [ 3.851518] x29: ffff8000838f3a00 x28: ffff0008008fe560 x27: 00000000000000f9 [ 3.858717] x26: ffff800082702250 x25: ffff000803211e00 x24: ffff8000838f3af8 [ 3.865913] x23: ffff0008009a3580 x22: 00000000000000f9 x21: 00000000ffffffff [ 3.873108] x20: ffff0008030377c0 x19: 00000000000000b7 x18: 0000000000000028 [ 3.880303] x17: ffff8008fc91f000 x16: ffff8000838f0000 x15: 0000000000000007 [ 3.887497] x14: fffffffffffc73af x13: 0a64656465656e20 x12: 0000000000008000 [ 3.894690] x11: ffff80008272ca18 x10: ffff8000825f7000 x9 : 0000000000000050 [ 3.901884] x8 : 0000000000000008 x7 : ffff800080190100 x6 : 0000000000000001 [ 3.909076] x5 : 0000000000000161 x4 : 0000000000000000 x3 : 0000000000000001 [ 3.916268] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff0008002e1ac0 [ 3.923461] Call trace: [ 3.925924] compute_energy+0x3a4/0x3bc (P) [ 3.930147] select_task_rq_fair+0x590/0x1990 [ 3.934541] try_to_wake_up+0x1f8/0xac4 [ 3.938420] wake_up_process+0x18/0x24 [ 3.942208] process_timeout+0x14/0x20 [ 3.945995] call_timer_fn+0xb8/0x470 We need to adapt em_cpu_energy() in include/linux/energy_model.h as well. ---8<--- >From 74f9067751b02f4bd0934ba6d47f2a204c763abe Mon Sep 17 00:00:00 2001 From: Dietmar Eggemann Date: Sun, 15 Mar 2026 23:45:39 +0100 Subject: [PATCH] PM: EM: Switch to rcu_dereference_all() in wakeup path em_cpu_energy() is part of the EAS (Fair) task wakeup path. Now that rcu_read_{,un}lock() have been removed from find_energy_efficient_cpu() switch to rcu_dereference_all() and check for rcu_read_lock_any_held() in em_cpu_energy() as well. In EAS (Fair) task wakeup path is a preempt/IRQ disabled region, so rcu_read_{,un}lock() can be removed. Signed-off-by: Dietmar Eggemann --- include/linux/energy_model.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h index e7497f804644..c909a8ba22e8 100644 --- a/include/linux/energy_model.h +++ b/include/linux/energy_model.h @@ -248,7 +248,7 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd, struct em_perf_state *ps; int i; - WARN_ONCE(!rcu_read_lock_held(), "EM: rcu read lock needed\n"); + lockdep_assert(rcu_read_lock_any_held()); if (!sum_util) return 0; @@ -267,7 +267,7 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd, * Find the lowest performance state of the Energy Model above the * requested performance. */ - em_table = rcu_dereference(pd->em_table); + em_table = rcu_dereference_all(pd->em_table); i = em_pd_get_efficient_state(em_table->state, pd, max_util); ps = &em_table->state[i]; -- 2.43.0