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Tue, 10 Sep 2024 09:16:42 +0000 Message-ID: Date: Tue, 10 Sep 2024 10:16:39 +0100 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 13/24] sched/fair: Prepare pick_next_task() for delayed dequeue To: Peter Zijlstra , mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, linux-kernel@vger.kernel.org Cc: kprateek.nayak@amd.com, wuyun.abel@bytedance.com, youssefesmat@chromium.org, tglx@linutronix.de, efault@gmx.de References: <20240727102732.960974693@infradead.org> <20240727105029.747330118@infradead.org> Content-Language: en-US From: Luis Machado In-Reply-To: <20240727105029.747330118@infradead.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: LO4P265CA0013.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:2ad::9) To PR3PR08MB5852.eurprd08.prod.outlook.com (2603:10a6:102:8e::21) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-TrafficTypeDiagnostic: PR3PR08MB5852:EE_|GV2PR08MB8051:EE_|AM4PEPF00027A6C:EE_|DB4PR08MB8175:EE_ X-MS-Office365-Filtering-Correlation-Id: 97417cfd-e41b-480a-a30d-08dcd1795304 x-checkrecipientrouted: true NoDisclaimer: true X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam-Untrusted: BCL:0;ARA:13230040|366016|7416014|376014|1800799024|921020; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 10 Sep 2024 09:16:59.3203 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 97417cfd-e41b-480a-a30d-08dcd1795304 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[63.35.35.123];Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: AM4PEPF00027A6C.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB4PR08MB8175 Hi Peter, On 7/27/24 11:27, Peter Zijlstra wrote: > Delayed dequeue's natural end is when it gets picked again. Ensure > pick_next_task() knows what to do with delayed tasks. > > Note, this relies on the earlier patch that made pick_next_task() > state invariant -- it will restart the pick on dequeue, because > obviously the just dequeued task is no longer eligible. > > Signed-off-by: Peter Zijlstra (Intel) > --- > kernel/sched/fair.c | 23 +++++++++++++++++++---- > 1 file changed, 19 insertions(+), 4 deletions(-) > > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5453,6 +5453,8 @@ set_next_entity(struct cfs_rq *cfs_rq, s > se->prev_sum_exec_runtime = se->sum_exec_runtime; > } > > +static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags); > + > /* > * Pick the next process, keeping these things in mind, in this order: > * 1) keep things fair between processes/task groups > @@ -5461,16 +5463,27 @@ set_next_entity(struct cfs_rq *cfs_rq, s > * 4) do not run the "skip" process, if something else is available > */ > static struct sched_entity * > -pick_next_entity(struct cfs_rq *cfs_rq) > +pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq) > { > /* > * Enabling NEXT_BUDDY will affect latency but not fairness. > */ > if (sched_feat(NEXT_BUDDY) && > - cfs_rq->next && entity_eligible(cfs_rq, cfs_rq->next)) > + cfs_rq->next && entity_eligible(cfs_rq, cfs_rq->next)) { > + /* ->next will never be delayed */ > + SCHED_WARN_ON(cfs_rq->next->sched_delayed); > return cfs_rq->next; > + } > + > + struct sched_entity *se = pick_eevdf(cfs_rq); > + if (se->sched_delayed) { > + dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED); > + SCHED_WARN_ON(se->sched_delayed); > + SCHED_WARN_ON(se->on_rq); While exercising the h_nr_delayed changes on Android/Pixel 6 (6.8-based), I ran into a situation where pick_eevdf seems to be returning NULL, and then we proceed to try to dereference it and crash during boot. I can fix it by guarding against a NULL se after the call to pick_eevdf, and then the code runs OK from there as pick_task_fair will have another go at trying to pick the next entity. I haven't checked exactly why we return NULL from pick_eevdf, but I recall seeing similar reports of pick_eevdf sometimes failing to pick any task. Anyway, I thought I'd point this out in case others see a similar situation. Back to testing the h_nr_delayed changes.