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client-ip=165.204.84.17; helo=SATLEXMB04.amd.com; pr=C Received: from SATLEXMB04.amd.com (165.204.84.17) by BN2PEPF00004FC0.mail.protection.outlook.com (10.167.243.186) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.20.8207.12 via Frontend Transport; Thu, 28 Nov 2024 07:30:06 +0000 Received: from [10.252.205.52] (10.180.168.240) by SATLEXMB04.amd.com (10.181.40.145) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.39; Thu, 28 Nov 2024 01:29:57 -0600 Message-ID: <670a0d54-e398-4b1f-8a6e-90784e2fdf89@amd.com> Date: Thu, 28 Nov 2024 12:59:54 +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 v2 1/3] sched/fair: Fix warning if NEXT_BUDDY enabled To: Adam Li , , , , CC: , , , , , , , , , References: <20241127055610.7076-1-adamli@os.amperecomputing.com> <20241127055610.7076-2-adamli@os.amperecomputing.com> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <20241127055610.7076-2-adamli@os.amperecomputing.com> Content-Type: text/plain; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 28 Nov 2024 07:30:06.8002 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: fcaa8143-a8e2-4d84-58d0-08dd0f7e7b80 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=[SATLEXMB04.amd.com] X-MS-Exchange-CrossTenant-AuthSource: BN2PEPF00004FC0.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: LV8PR12MB9082 Hello Adam, On 11/27/2024 11:26 AM, Adam Li wrote: > Enabling NEXT_BUDDY triggers warning, and rcu stall: > > [ 124.977300] cfs_rq->next->sched_delayed I could reproduce this with a run of "perf bench sched messaging" but given that we hit this warning, it also means that either set_next_buddy() has incorrectly set a delayed entity as next buddy, or clear_next_buddy() did not clear a delayed entity. I also see PSI splats like: psi: inconsistent task state! task=2524:kworker/u1028:2 cpu=154 psi_flags=10 clear=14 set=0 but the PSI flags it has set "(TSK_MEMSTALL_RUNNING | TSK_MEMSTALL)" and the flags it is trying to clear "(TSK_MEMSTALL_RUNNING | TSK_MEMSTALL | TSK_RUNNING)" seem to be only possible if you have picked a dequeued entity for running before its wakeup, which is also perhaps why the "nr_running" computation goes awry and pick_eevdf() returns NULL (which it should never since pick_next_entity() is only called when rq->cfs.nr_running is > 0) > [ 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=15003 jiffies, g=5629, q=26516 ncpus=384) > > > 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; I tried to put a SCHED_WARN_ON() here to track where this comes from and seems like it is usually from attach_task() in the load balancing path pulling a delayed task which is set as the next buddy in check_preempt_wakeup_fair() Can you please try the following diff instead of the first two patches and see if you still hit these warnings, stalls, and pick_eevdf() returning NULL? diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index ff7cae9274c5..61e74eb5af22 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5478,6 +5478,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) bool sleep = flags & DEQUEUE_SLEEP; update_curr(cfs_rq); + clear_buddies(cfs_rq, se); if (flags & DEQUEUE_DELAYED) { SCHED_WARN_ON(!se->sched_delayed); @@ -5520,8 +5521,6 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) update_stats_dequeue_fair(cfs_rq, se, flags); - clear_buddies(cfs_rq, se); - update_entity_lag(cfs_rq, se); if (sched_feat(PLACE_REL_DEADLINE) && !sleep) { se->deadline -= se->vruntime; @@ -8767,7 +8766,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int if (unlikely(throttled_hierarchy(cfs_rq_of(pse)))) return; - if (sched_feat(NEXT_BUDDY) && !(wake_flags & WF_FORK)) { + if (sched_feat(NEXT_BUDDY) && !(wake_flags & WF_FORK) && !pse->sched_delayed) { set_next_buddy(pse); } -- If you are still encountering pick_eevdf() returning NULL, there could be a larger issues (with eligibility computation, etc.) that the second patch can hide which can lead to bigger problems later. Thank you. > cfs_rq_of(se)->next = se; > } > } -- Thanks and Regards, Prateek