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charset="utf-8" Enabling NEXT_BUDDY triggers warning, and rcu stall: [ 124.977300] cfs_rq->next->sched_delayed [ 124.977310] WARNING: CPU: 51 PID: 2150 at kernel/sched/fair.c:5621 pick_= task_fair+0x130/0x150 [ 125.049547] CPU: 51 UID: 0 PID: 2150 Comm: kworker/51:1 Tainted: G = E 6.12.0.adam+ #1 [ 125.163561] Call trace: [ 125.165996] pick_task_fair+0x130/0x150 (P) [ 125.170167] pick_task_fair+0x130/0x150 (L) [ 125.174338] pick_next_task_fair+0x48/0x3c0 [ 125.178512] __pick_next_task+0x4c/0x220 [ 125.182426] pick_next_task+0x44/0x980 [ 125.186163] __schedule+0x3d0/0x628 [ 125.189645] schedule+0x3c/0xe0 [ 125.192776] worker_thread+0x1a4/0x368 [ 125.196516] kthread+0xfc/0x110 [ 125.199647] ret_from_fork+0x10/0x20 [ 125.203213] ---[ end trace 0000000000000000 ]--- [ 211.151849] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks: [ 211.159759] rcu: (detected by 141, t=3D15003 jiffies, g=3D5629, q=3D= 26516 ncpus=3D384) Do not set next buddy if sched_delayed is set. Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue") Signed-off-by: Adam Li --- kernel/sched/fair.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index fbdca89c677f..cd1188b7f3df 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8748,6 +8748,8 @@ static void set_next_buddy(struct sched_entity *se) return; if (se_is_idle(se)) return; + if (se->sched_delayed) + return; cfs_rq_of(se)->next =3D se; } } --=20 2.25.1 From nobody Sat Feb 7 05:01:30 2026 Received: from CY7PR03CU001.outbound.protection.outlook.com (mail-westcentralusazon11022121.outbound.protection.outlook.com [40.93.200.121]) (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 7DC3428EB for ; Wed, 27 Nov 2024 05:57:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=fail smtp.client-ip=40.93.200.121 ARC-Seal: i=2; a=rsa-sha256; d=subspace.kernel.org; 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charset="utf-8" pick_eevdf() may return NULL, which triggers NULL pointer dereference at pick_next_entity(): struct sched_entity *se =3D pick_eevdf(cfs_rq); if (se->sched_delayed) [ 297.371198] Unable to handle kernel NULL pointer dereference at virt= ual address 0000000000000051 [ 297.406112] CPU: 116 UID: 0 PID: 10328 Comm: Grizzly-worker( Tainted= : G W E 6.12.0.adam+ #1 [ 297.597362] Call trace: [ 297.599795] pick_task_fair+0x50/0x150 (P) [ 297.603879] pick_task_fair+0x50/0x150 (L) [ 297.607963] pick_next_task_fair+0x30/0x3c0 [ 297.612134] __pick_next_task+0x4c/0x220 [ 297.616045] pick_next_task+0x44/0x980 [ 297.619782] __schedule+0x3d0/0x628 [ 297.623259] do_task_dead+0x50/0x60 [ 297.626736] do_exit+0x28c/0x410 [ 297.629955] do_group_exit+0x3c/0xa0 [ 297.633518] get_signal+0x8c4/0x8d0 [ 297.636996] do_signal+0x9c/0x270 [ 297.640299] do_notify_resume+0xe0/0x198 [ 297.644212] el0_svc+0xf4/0x170 [ 297.647342] el0t_64_sync_handler+0x10c/0x138 [ 297.651687] el0t_64_sync+0x1ac/0x1b0 [ 297.655339] Code: d503201f 1400002a aa1403e0 97ffda0b (39414401) [ 297.661439] ---[ end trace 0000000000000000 ]--- [ 297.726593] Kernel panic - not syncing: Oops: Fatal exception Signed-off-by: Adam Li --- kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index cd1188b7f3df..d5a3b5589e4e 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5623,7 +5623,7 @@ pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq) } =20 struct sched_entity *se =3D pick_eevdf(cfs_rq); 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Wed, 27 Nov 2024 05:57:26 +0000 From: Adam Li To: peterz@infradead.org, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org Cc: dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, linux-kernel@vger.kernel.org, patches@amperecomputing.com, cl@linux.com, christian.loehle@arm.com, vineethr@linux.ibm.com, Adam Li Subject: [PATCH v2 3/3] sched/fair: Update comments regarding last and skip buddy Date: Wed, 27 Nov 2024 05:56:10 +0000 Message-Id: <20241127055610.7076-4-adamli@os.amperecomputing.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20241127055610.7076-1-adamli@os.amperecomputing.com> References: <20241127055610.7076-1-adamli@os.amperecomputing.com> Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: SGBP274CA0018.SGPP274.PROD.OUTLOOK.COM (2603:1096:4:b0::30) To SJ0PR01MB6445.prod.exchangelabs.com (2603:10b6:a03:2a1::14) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MW4PR01MB6436:EE_|DS1PR01MB9086:EE_ X-MS-Office365-Filtering-Correlation-Id: 7e50385f-3cfa-4b83-2d7c-08dd0ea85538 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|52116014|7416014|376014|38350700014; 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charset="utf-8" Commit 5e963f2bd465 ("sched/fair: Commit to EEVDF") removed the "last" and "skip" buddy. Update comments in pick_next_entity(). Signed-off-by: Adam Li Reviewed-by: Madadi Vineeth Reddy Reviewed-by: Adam Li --- kernel/sched/fair.c | 8 +------- 1 file changed, 1 insertion(+), 7 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d5a3b5589e4e..259c56dcdff6 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5602,17 +5602,11 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched= _entity *se) =20 static int dequeue_entities(struct rq *rq, struct sched_entity *se, int fl= ags); =20 -/* - * Pick the next process, keeping these things in mind, in this order: - * 1) keep things fair between processes/task groups - * 2) pick the "next" process, since someone really wants that to run - * 3) pick the "last" process, for cache locality - * 4) do not run the "skip" process, if something else is available - */ static struct sched_entity * pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq) { /* + * Pick the "next" buddy, since someone really wants that to run. * Enabling NEXT_BUDDY will affect latency but not fairness. */ if (sched_feat(NEXT_BUDDY) && --=20 2.25.1