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dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=nvidia.com; Received: from DM4PR12MB7742.namprd12.prod.outlook.com (2603:10b6:8:102::18) by CH1PPFC8B3B7859.namprd12.prod.outlook.com (2603:10b6:61f:fc00::622) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.592.11; Fri, 13 Nov 2026 21:20:47 +0000 Received: from DM4PR12MB7742.namprd12.prod.outlook.com ([fe80::8df:c519:298a:94f6]) by DM4PR12MB7742.namprd12.prod.outlook.com ([fe80::8df:c519:298a:94f6%6]) with mapi id 15.21.0615.007; Fri, 13 Nov 2026 21:20:47 +0000 Message-ID: Date: Fri, 13 Nov 2026 16:20:44 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 1/7] rcutorture: Rewrite RCU priority-boost test around a CPU-hungry reader To: Sebastian Andrzej Siewior Cc: paulmck@kernel.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, Frederic Weisbecker , Neeraj Upadhyay , Boqun Feng , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Davidlohr Bueso , Jonathan Corbet , Shuah Khan , linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20261111213826.2035045-1-joelagnelf@nvidia.com> <20261111213826.2035045-2-joelagnelf@nvidia.com> <20261113155301.Rs-PyiuR@linutronix.de> Content-Language: en-US From: Joel Fernandes In-Reply-To: <20261113155301.Rs-PyiuR@linutronix.de> Content-Type: text/plain; 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Instead, each interval one >> CPU runs a normal-priority reader that spins inside an RCU read-side >> critical section until it has consumed test_boost_reader_ms of CPU time >> (measured via sum_exec_runtime, so time spent preempted does not count), >> while a SCHED_FIFO hog on the same CPU tries to starve it. With RT >> throttling enabled, an unboosted reader gets only about 5% of the CPU, >> far less than it needs within test_boost_duration. If the reader >> finishes, boosting worked, otherwise it failed. > > What I see from tracing is: > > | rcu_torture_boo-2066 [001] d..2. 237.448002: sched_switch: prev_comm=rcu_torture_boo prev_pid=2066 prev_prio=120 prev_state=R+ ==> next_comm=rcu_torture_boo next_pid=93 next_prio=98 > > preempted as SCHED_OTHER > > | rcub/0-16 [003] d..3. 237.460085: sched_pi_setprio: comm=rcu_torture_boo pid=2066 oldprio=120 newprio=97 > > boosted shortly after > > | rcu_torture_boo-93 [001] d..2. 237.460108: sched_switch: prev_comm=rcu_torture_boo prev_pid=93 prev_prio=98 prev_state=R+ ==> next_comm=rcu_torture_boo next_pid=2066 next_prio=97 > | rcu_torture_boo-2066 [001] d.h1. 237.463994: sched_stat_runtime: comm=rcu_torture_boo pid=2066 runtime=3908069 [ns] > | rcu_torture_boo-2066 [001] d.h1. 237.467985: sched_stat_runtime: comm=rcu_torture_boo pid=2066 runtime=3991212 [ns] > | rcu_torture_boo-2066 [001] d.h1. 237.471995: sched_stat_runtime: comm=rcu_torture_boo pid=2066 runtime=4009726 [ns] > > back on CPU and not pushed away. > | rcu_torture_boo-2066 [001] dN.2. 238.397969: sched_stat_runtime: comm=rcu_torture_boo pid=2066 runtime=5311 [ns] > | rcu_torture_boo-2066 [001] d..2. 238.397975: sched_switch: prev_comm=rcu_torture_boo prev_pid=2066 prev_prio=97 prev_state=R+ ==> next_comm=kworker/1:3 next_pid=1386 next_prio=120 > | kworker/1:1H-745 [001] d..2. 238.398067: sched_switch: prev_comm=kworker/1:1H prev_pid=745 prev_prio=100 prev_state=I ==> next_comm=rcu_torture_boo next_pid=2066 next_prio=97 > | rcu_torture_boo-2066 [001] d.s3. 238.398122: sched_waking: comm=kworker/1:3 pid=1386 prio=120 target_cpu=001 > | rcu_torture_boo-2066 [001] d.s4. 238.398124: sched_stat_runtime: comm=rcu_torture_boo pid=2066 runtime=63431 [ns] > | rcu_torture_boo-2066 [001] dNs4. 238.398128: sched_wakeup: comm=kworker/1:3 pid=1386 prio=120 target_cpu=001 > | rcu_torture_boo-2066 [001] d..2. 238.398130: sched_switch: prev_comm=rcu_torture_boo prev_pid=2066 prev_prio=97 prev_state=R+ ==> next_comm=kworker/1:3 next_pid=1386 next_prio=120 > | kworker/1:3-1386 [001] d..2. 238.398137: sched_switch: prev_comm=kworker/1:3 prev_pid=1386 prev_prio=120 prev_state=I ==> next_comm=rcu_torture_boo next_pid=2066 next_prio=97 > | rcu_torture_boo-2066 [001] d.h1. 238.399986: sched_stat_runtime: comm=rcu_torture_boo pid=2066 runtime=1851337 [ns] > | rcu_torture_boo-2066 [001] d.h1. 238.400081: sched_stat_runtime: comm=rcu_torture_boo pid=2066 runtime=94860 [ns] > > The fair_server preempts now and then. > | rcu_torture_boo-2066 [001] d.h1. 239.455988: sched_stat_runtime: comm=rcu_torture_boo pid=2066 runtime=4002590 [ns] > | rcu_torture_boo-2066 [001] d..3. 239.455999: sched_waking: comm=rcu_preempt pid=15 prio=97 target_cpu=000 > | rcu_torture_boo-2066 [001] d..3. 239.456012: sched_waking: comm=rcu_exp_gp_kthr pid=18 prio=120 target_cpu=002 > | rcu_torture_boo-2066 [001] d..3. 239.456023: sched_pi_setprio: comm=rcu_torture_boo pid=2066 oldprio=97 newprio=120 > | rcu_torture_boo-2066 [001] d..3. 239.456024: sched_stat_runtime: comm=rcu_torture_boo pid=2066 runtime=35773 [ns] > | rcu_torture_boo-2066 [001] d..2. 239.456032: sched_switch: prev_comm=rcu_torture_boo prev_pid=2066 prev_prio=120 prev_state=R+ ==> next_comm=rcu_torture_boo next_pid=93 next_prio=98 > > Leaving the RCU section and getting de-boosted. > > This looks good. > Thank you for testing! I went through your other suggestions and they mostly look good to me, so I will revise the series and post a v2. > … > >> Success testing: with the series applied and no other changes, run >> TREE01 TREE02 TREE03 TREE08 TREE09 BUSTED-BOOST for 10 minutes each. >> Every scenario ended "SUCCESS" with rtbf (boost failures) of 0 in 59 to >> 83 boost intervals. (BUSTED-BOOST can no longer fail by construction >> because CONFIG_RCU_BOOST is now enabled automatically for preemptible >> RCU.) > > I didn't run the individual tests but checked in my rcu-tree setup that > boosting works as it should and that it complains once it does not. And > the throttling of the RT tasks has no visible impact. Excellent, thanks! Joel Fernandes