Received-SPF: pass (google.com: domain of linux-kernel+bounces-191574-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
From: Ankur Arora <ankur.a.arora@oracle.com>
To: linux-kernel@vger.kernel.org
Cc: tglx@linutronix.de, peterz@infradead.org, torvalds@linux-foundation.org,
        paulmck@kernel.org, rostedt@goodmis.org, mark.rutland@arm.com,
        juri.lelli@redhat.com, joel@joelfernandes.org, raghavendra.kt@amd.com,
        sshegde@linux.ibm.com, boris.ostrovsky@oracle.com,
        konrad.wilk@oracle.com, Ankur Arora <ankur.a.arora@oracle.com>,
        Ingo Molnar <mingo@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>
Subject: [PATCH v2 28/35] sched: support preempt=full under PREEMPT_AUTO
Date: Mon, 27 May 2024 17:35:14 -0700
Message-Id: <20240528003521.979836-29-ankur.a.arora@oracle.com>
In-Reply-To: <20240528003521.979836-1-ankur.a.arora@oracle.com>
References: <20240528003521.979836-1-ankur.a.arora@oracle.com>
Content-Transfer-Encoding: 8bit
Content-Type: text/plain
Precedence: bulk
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: 
	=?us-ascii?Q?GCnq7nuBMeYI/hsb3ErhsnQDXUZVOVq+7BwN18ClwXqO7UVrNl6W+xuUrZCT?=
 =?us-ascii?Q?PPTp92Axabv2IR5ISGA3A6QH8DK/FiDnqrOBHWtZTAk40yQNYhTJHzXUmFvH?=
 =?us-ascii?Q?bH6SRhc7L/4qfFXtQG1enYrKvXhbExVmF4LRAyTtUbzhRA5MQp1smS8y9dND?=
 =?us-ascii?Q?AydxrrJQvYMDfEvvD+MvpGs3po7uR6BTkIvzvkzhPHc1fewHYO4BnDaIPrky?=
 =?us-ascii?Q?4ogt7EUmOJJh98sfHrfPiEX5yreCmcvE0FCjbbtClnFJehU11BaXQI3wMFkK?=
 =?us-ascii?Q?Mx0HWMtLMibdn50Q0K0NguzoaDLypHcG8QjQG67W2u7bZQaP9iSg6lwtsS6H?=
 =?us-ascii?Q?XEzLZjJKyQZUT74sepQMPBn5Wgn6uSYpp0+AKx0D4uYqAjZYWH8A/b+XR94k?=
 =?us-ascii?Q?Go6x17YNzetnoIL75peamU5bIHZtxXoS03ohY/nTkSyfwvw+dUMtVia0xMlX?=
 =?us-ascii?Q?iJK0hNhJ/SoNRa2PsOLB76Nq9R7G6x3xoOwMaRUkIK3V/l5dorBe9aetBzgt?=
 =?us-ascii?Q?Noo9oqx9xnhLTem9mHjCOZEBUYvh2uHs8zzdnnMeYqJS4Z+INaymCZs9IZfY?=
 =?us-ascii?Q?AtKBEfwFdcf0BtwLHpoha4xiO1k+5SMKAtEq6Zg+9gtf0WM5YUOXizCcCTdo?=
 =?us-ascii?Q?GBkSA2KiboorwJNNelstbJ7MQifvdPuy+80o4FpVgTfnjgcSlu6a7FcwT4Wd?=
 =?us-ascii?Q?bePHeBRcCpZBV5w45kpXqcnGBS0OoSKeUHT35nAZtRm+zU6MfEMcOJK1ZeCG?=
 =?us-ascii?Q?qfoewcmEm6y8kIvewNIzTrAwjvZsITLUoueK3vHXhcZ0SWAVzsdoIg24E9AL?=
 =?us-ascii?Q?ElhweGqKZ3u2GVAuAUNAImI2Ie8ud0FlbAhCrdo5ob2ZxIFEJgeec4Pe4gfX?=
 =?us-ascii?Q?J9P8WmdQDQ/TJ1bb3RZ4nrkQ9W8g7FYlMJ9fp7hcRAQBC5R9iTf7K2C/A2x2?=
 =?us-ascii?Q?+qV4q++NTeCtE3LZSXoIvvvdFIDhjk67mtBbQXVcqkYRz76o9+F2cFqWd7F4?=
 =?us-ascii?Q?MKwJkAZptTXdineP4iOlridwgx4uHOmvDpWA48hbaHHkA39MjujHivAnt3VH?=
 =?us-ascii?Q?eMEbGd8aQBVZm39tJTGknJcSV40N4xDugM8f/9pc8nHZ1eva2TsiENMzUwNc?=
 =?us-ascii?Q?P8fId8FNhVnRJLbqCWPeZ2PGska03fbwzdiAos9cOn1kyl0pi7NtvhaVVKiw?=
 =?us-ascii?Q?skYLN3P9iML2xfpKDt7EaQtbFSGJrM5HAsXncOcLaUHVd6LAmRNc7Bbfxl45?=
 =?us-ascii?Q?A0D38yGr2jFcEEY9lDTtZnpCytrs3klNZBtH/fdaGVx2TRNu5RZ0w7B2uICT?=
 =?us-ascii?Q?ul+QWAS34gaGyKWE9bkANhs8sdUQohjoTPWxcEYIyfM9lFPbsWfIUugHwGXV?=
 =?us-ascii?Q?wIsIm//v9URprgLzzD1rA5t0GKUmk2B5jGPUMwl73rOw87E/uEoAmwU/nS7V?=
 =?us-ascii?Q?P8289jTk74bdMpy6za9KQrmlGAA08j0kR8iGOHcZi0hSNVkVNWCtbZlpNcO6?=
 =?us-ascii?Q?XVKT2jr8eRD8DA3qbz96jo/APZmWBY7qnqwmPCoxJ7B0PGdl7FxzQYVCvjbQ?=
 =?us-ascii?Q?AmU/GPaP825+mEDJmytSqdCXQAh9g2I27pVdSvuiHbyH/UO2MZKm0P69ic2y?=
 =?us-ascii?Q?Qw=3D=3D?=
X-MS-Exchange-AntiSpam-ExternalHop-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-ExternalHop-MessageData-0: 
	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
X-OriginatorOrg: oracle.com
X-MS-Exchange-CrossTenant-Network-Message-Id: ab5ab480-b927-43c9-7954-08dc7eae4316
X-MS-Exchange-CrossTenant-AuthSource: CO6PR10MB5409.namprd10.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 28 May 2024 00:36:49.5834
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: VOBNyPjaSX5giM4b5o7R3sH+CN1a26LDmrSWTb9iBLn+xhiyekJEVkTBbFqTyF94p8wRX0rsAS4eVhtr66W1QwaW6xcmi1IlWUiy7oXMf/w=
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS7PR10MB7177
X-Proofpoint-Virus-Version: vendor=baseguard
 engine=ICAP:2.0.293,Aquarius:18.0.1039,Hydra:6.0.650,FMLib:17.12.28.16
 definitions=2024-05-27_06,2024-05-27_01,2024-05-17_01
X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 phishscore=0 suspectscore=0
 adultscore=0 bulkscore=0 malwarescore=0 mlxscore=0 spamscore=0
 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1
 engine=8.12.0-2405010000 definitions=main-2405280003
X-Proofpoint-GUID: 0KFxodpnm3qf9ctrAqYtYsuRUXVX_EcN
X-Proofpoint-ORIG-GUID: 0KFxodpnm3qf9ctrAqYtYsuRUXVX_EcN

The default preemption policy for preempt-full under PREEMPT_AUTO is
to minimize latency, and thus to always schedule eagerly. This is
identical to CONFIG_PREEMPT, and so should result in similar
performance.

Comparing scheduling/IPC workload:

 # perf stat -a -e cs --repeat 10 -- perf bench sched messaging -g 20 -t -l 5000

 PREEMPT_AUTO, preempt=full

         3,080,508            context-switches      ( +-  0.64% )
	   3.65171 +- 0.00654 seconds time elapsed  ( +-  0.18% )

 PREEMPT_DYNAMIC, preempt=full

	 3,087,527            context-switches      ( +-  0.33% )
	   3.60163 +- 0.00633 seconds time elapsed  ( +-  0.18% )

Looking at the breakup between voluntary and involuntary
context-switches, we see almost identical behaviour as well.

 PREEMPT_AUTO, preempt=full

           2087910.00 +- 34720.95 voluntary context-switches   ( +- 1.660% )
            784437.60 +- 19827.79 involuntary context-switches ( +- 2.520% )

 PREEMPT_DYNAMIC, preempt=full

           2102879.60 +- 22767.11 voluntary context-switches   ( +- 1.080% )
            801189.90 +- 21324.18 involuntary context-switches ( +- 2.660% )

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Ziljstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Originally-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 kernel/sched/core.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c3ba33c77053..c25cccc09b65 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1035,9 +1035,10 @@ void wake_up_q(struct wake_q_head *head)
  * For preemption models other than PREEMPT_AUTO: always schedule
  * eagerly.
  *
- * For PREEMPT_AUTO: schedule idle threads eagerly, allow everything else
- * to finish its time quanta, and mark for rescheduling at the next exit
- * to user.
+ * For PREEMPT_AUTO: schedule idle threads eagerly, and under full
+ * preemption, all tasks eagerly. Otherwise, allow everything else
+ * to finish its time quanta, and mark for rescheduling at the next
+ * exit to user.
  */
 static resched_t resched_opt_translate(struct task_struct *curr,
 				       enum resched_opt opt)
@@ -1048,6 +1049,9 @@ static resched_t resched_opt_translate(struct task_struct *curr,
 	if (opt == RESCHED_FORCE)
 		return RESCHED_NOW;
 
+	if (preempt_model_preemptible())
+		return RESCHED_NOW;
+
 	if (is_idle_task(curr))
 		return RESCHED_NOW;
 
@@ -8997,7 +9001,9 @@ static void __sched_dynamic_update(int mode)
 			pr_warn("%s: preempt=full is not recommended with CONFIG_PREEMPT_RCU=n",
 				PREEMPT_MODE);
 
-		preempt_dynamic_mode = preempt_dynamic_undefined;
+		if (mode != preempt_dynamic_mode)
+			pr_info("%s: full\n", PREEMPT_MODE);
+		preempt_dynamic_mode = mode;
 		break;
 	}
 }
-- 
2.31.1