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McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Josh Triplett , Boqun Feng , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , linux-kernel@vger.kernel.org, Joel Fernandes Subject: [PATCH] rcu: Reset stall detection after ftrace dump Date: Sun, 4 Jan 2026 15:36:42 -0500 Message-Id: <20260104203642.3171854-1-joelagnelf@nvidia.com> X-Mailer: git-send-email 2.34.1 Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: MN2PR19CA0026.namprd19.prod.outlook.com (2603:10b6:208:178::39) To SN7PR12MB8059.namprd12.prod.outlook.com (2603:10b6:806:32b::7) 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: SN7PR12MB8059:EE_|IA0PR12MB8839:EE_ X-MS-Office365-Filtering-Correlation-Id: 0a998da6-305e-46c8-35bc-08de4bd0fb41 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|7416014|1800799024; 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charset="utf-8" During some recent expedited RCU testing with rcutorture, I noticed RCU stalls when using ftrace_dump() to analyze traces. The issue that since ftrace_dump() can take a long time, any accumulated issue can show up just after the stall is unsupressed. Due to this a stall can be detected and reported. The problem is something like this: 1. rcu_ftrace_dump_stall_suppress() sets suppression value. 2. ftrace_dump() runs for a long time. 3. rcu_ftrace_dump_stall_unsuppress() clears suppression. 4. Immediately later, stall is detected. Fix this by calling rcu_cpu_stall_reset() when clearing suppression. This resets jiffies_stall to ULONG_MAX, preventing the issue. Tested with gp_exp=3Dtrue enabled with rcutorture and ftrace_dump() and no stall is reported, with only clean traces in the output. Signed-off-by: Joel Fernandes Reviewed-by: Paul E. McKenney --- This is a non-urgent fix and could go in either this or the next merge window (Boqun's call). kernel/rcu/rcu.h | 5 ++++- kernel/rcu/tree_stall.h | 1 + 2 files changed, 5 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 9cf01832a6c3..87fcf528b9ac 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -288,6 +288,7 @@ extern int rcu_cpu_stall_cputime; extern bool rcu_exp_stall_task_details __read_mostly; int rcu_jiffies_till_stall_check(void); int rcu_exp_jiffies_till_stall_check(void); +void rcu_cpu_stall_reset(void); =20 static inline bool rcu_stall_is_suppressed(void) { @@ -302,8 +303,10 @@ do { \ =20 #define rcu_ftrace_dump_stall_unsuppress() \ do { \ - if (rcu_cpu_stall_suppress =3D=3D 3) \ + if (rcu_cpu_stall_suppress =3D=3D 3) { \ rcu_cpu_stall_suppress =3D 0; \ + rcu_cpu_stall_reset(); \ + } \ } while (0) =20 #else /* #endif #ifdef CONFIG_RCU_STALL_COMMON */ diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index b67532cb8770..9a036fc27fed 100644 --- a/kernel/rcu/tree_stall.h +++ b/kernel/rcu/tree_stall.h @@ -192,6 +192,7 @@ void rcu_cpu_stall_reset(void) WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 3); WRITE_ONCE(rcu_state.jiffies_stall, ULONG_MAX); } +EXPORT_SYMBOL_GPL(rcu_cpu_stall_reset); =20 //////////////////////////////////////////////////////////////////////////= //// // --=20 2.34.1