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client-ip=165.204.84.17; helo=satlexmb08.amd.com; pr=C Received: from satlexmb08.amd.com (165.204.84.17) by SJ5PEPF000001E8.mail.protection.outlook.com (10.167.242.196) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.270.5 via Frontend Transport; Wed, 29 Jul 2026 06:19:45 +0000 Received: from satlexmb10.amd.com (10.181.42.219) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.41; Wed, 29 Jul 2026 01:19:44 -0500 Received: from satlexmb07.amd.com (10.181.42.216) by satlexmb10.amd.com (10.181.42.219) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.41; Wed, 29 Jul 2026 01:19:44 -0500 Received: from [10.136.36.1] (10.180.168.240) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server id 15.2.2562.41 via Frontend Transport; Wed, 29 Jul 2026 01:19:40 -0500 Message-ID: <39ff2c76-d533-4b6c-9449-de19ed18e44a@amd.com> Date: Wed, 29 Jul 2026 11:49:39 +0530 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/core: Skip rq->avg_idle update without a valid idle_stamp To: "Shubhang Kaushik (Ampere)" , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , "Christopher Lameter" , Shubhang Kaushik , John Stultz CC: References: <20260728-master-v1-1-f95d9b0147d2@gentwo.org> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <20260728-master-v1-1-f95d9b0147d2@gentwo.org> Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: hcRsjwPgs+qPEcQJbqoRnn0ijA0z3eyEULZa7BhNApv1pbd26gXVyB+z1cQnky1Vven0tpjGfQ0JhBzA4z9opRmGIUhY4mbSf2IKcAyyaTmHZEl0PHn7lyWEk+02c/eXiOyIImFwzzsyVrbtbqiXCIP9wYypULCjSjI/oyvlyxzE29kEsr4p39MF4lyjtVuk/IJ24zxtRLzz57//PFkcnrhr+WGxnHEHP1gAm0Kwux4L8fJaKx/77dhQDCRHGGqz4R2F6OmFku+bywwOfLpZ0GZRClG0M3dqQT0hHSx/jg1lworF8JmeEOzbPzNW36wJtpt3FBuBNjT2hw4ozJvOER8NM0ZbqyJEc3KrAlByLJOiCmuSOciu/ZfmP8mlxJx+FeNrvFg3Lrpgy6xVPsWGjjcc7lif4Olm3hakFVA6mn4/Co9TV+Ceqs/U3ZgtfeYW X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 29 Jul 2026 06:19:45.0529 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: ef539105-9210-4d39-6da6-08deed396255 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb08.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SJ5PEPF000001E8.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA1PR12MB8697 (+John for awareness) Hello Shubhang, On 7/29/2026 4:54 AM, Shubhang Kaushik (Ampere) wrote: > Commit 4b603f1551a73 ("sched: Update rq->avg_idle when a task is moved > to an idle CPU") moved rq->avg_idle accounting out of the wakeup path and > into put_prev_task_idle(), so that the idle interval is consumed whenever > the idle task is switched out. > > The wakeup-side accounting that it replaced only updated rq->avg_idle > when rq->idle_stamp was non-zero. The new helper lost that validity > check and unconditionally computes: > > rq_clock(rq) - rq->idle_stamp > > If rq->idle_stamp is zero, this uses rq_clock(rq) as the sample. That is > not a valid idle duration and can immediately drive rq->avg_idle to its > clamp. > > Restore the idle_stamp validity check in update_rq_avg_idle() and skip > the rq->avg_idle update when there is no measured idle interval. > > Fixes: 4b603f1551a73 ("sched: Update rq->avg_idle when a task is moved to an idle CPU") > Signed-off-by: Shubhang Kaushik (Ampere) > --- > Temporary tracing under hackbench load confirmed that > update_rq_avg_idle() can be reached with rq->idle_stamp == 0. > Hackbench showed no material regression versus v7.2-rc5 mainline. > > Related discussion: > https://lore.kernel.org/r/20260423023322.1293923-1-firelzrd@gmail.com > > This is a narrower variant of the earlier proposal. It keeps the > rq->idle_stamp guard in update_rq_avg_idle(), but intentionally does not > stamp idle entry from set_next_task_idle(), preserving the existing > newidle accounting model and avoiding forced/proxy idle accounting > concerns. Makes sense since we switch to idle context without a newidle balance during find_proxy_task() and also during force-idling. Feel free to include: Reviewed-by: K Prateek Nayak > --- > kernel/sched/core.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 96226707c2f6135341aa779b8262f113e103d8ad..d8c9a80ffa83ac680bb479f67357ac1ec4a6d55e 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3732,11 +3732,17 @@ static inline void ttwu_do_wakeup(struct task_struct *p) > > void update_rq_avg_idle(struct rq *rq) > { > - u64 delta = rq_clock(rq) - rq->idle_stamp; > - u64 max = 2*rq->max_idle_balance_cost; > + u64 idle_stamp = rq->idle_stamp; > + u64 delta, max; > + > + if (unlikely(!idle_stamp)) > + return; > + > + delta = rq_clock(rq) - idle_stamp; > > update_avg(&rq->avg_idle, delta); > > + max = 2 * rq->max_idle_balance_cost; > if (rq->avg_idle > max) > rq->avg_idle = max; > rq->idle_stamp = 0; > > --- > base-commit: 3b5f4b83c4abc0c9b0a7b9e2b44e816611b7f2ec > change-id: 20260728-master-55cd7cc13290 > > Best regards, -- Thanks and Regards, Prateek