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Thu, 09 Jul 2026 20:28:59 -0700 (PDT) From: Jing Wu Date: Fri, 10 Jul 2026 11:28:15 +0800 Subject: [PATCH v4 04/11] context_tracking: allow runtime per-CPU user tracking enable/disable Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260710-wujing-dhm-v4-4-2e912e5d9645@gmail.com> References: <20260710-wujing-dhm-v4-0-2e912e5d9645@gmail.com> In-Reply-To: <20260710-wujing-dhm-v4-0-2e912e5d9645@gmail.com> To: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Anna-Maria Behnsen , Tejun Heo , Jonathan Corbet , Shuah Khan , Shuah Khan , Thomas Gleixner Cc: Waiman Long , linux-kernel@vger.kernel.org, rcu@vger.kernel.org, cgroups@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, Jing Wu , Qiliang Yuan X-Mailer: b4 0.13.0 ct_cpu_track_user() and the context_tracking_key static key are currently restricted to boot-time use: the key is __ro_after_init and the function is __init with __initdata state. This prevents enabling nohz_full context tracking for CPUs isolated at runtime via cpuset partitions. Split ct_cpu_track_user() into three functions: ct_cpu_track_user(cpu) - sets per_cpu(context_tracking.active) and increments context_tracking_key; callable at runtime with the CPU offline. ct_cpu_untrack_user(cpu) - reverses the above; for de-isolation. ct_cpu_track_user_init(cpu) - __init wrapper; calls ct_cpu_track_user() and handles TIF_NOHZ / tasklist setup. Change context_tracking_key from DEFINE_STATIC_KEY_FALSE_RO to DEFINE_STATIC_KEY_FALSE so that static_branch_inc/dec() can be called after the __ro_after_init window closes. Update tick_nohz_init() to call ct_cpu_track_user_init() so boot behaviour is unchanged. This is a prerequisite for DHM (Dynamic Housekeeping Management) runtime CPU noise isolation without boot parameters. Co-developed-by: Qiliang Yuan Signed-off-by: Qiliang Yuan Signed-off-by: Jing Wu --- include/linux/context_tracking.h | 2 ++ kernel/context_tracking.c | 38 ++++++++++++++++++++++++++++++++++---- kernel/time/tick-sched.c | 2 +- 3 files changed, 37 insertions(+), 5 deletions(-) diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h index af9fe87a09225..735d353d87560 100644 --- a/include/linux/context_tracking.h +++ b/include/linux/context_tracking.h @@ -12,6 +12,8 @@ #ifdef CONFIG_CONTEXT_TRACKING_USER extern void ct_cpu_track_user(int cpu); +extern void ct_cpu_untrack_user(int cpu); +extern void __init ct_cpu_track_user_init(int cpu); /* Called with interrupts disabled. */ extern void __ct_user_enter(enum ctx_state state); diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c index a743e7ffa6c00..a81d9f8b85eed 100644 --- a/kernel/context_tracking.c +++ b/kernel/context_tracking.c @@ -411,7 +411,7 @@ static __always_inline void ct_kernel_enter(bool user, int offset) { } #define CREATE_TRACE_POINTS #include -DEFINE_STATIC_KEY_FALSE_RO(context_tracking_key); +DEFINE_STATIC_KEY_FALSE(context_tracking_key); EXPORT_SYMBOL_GPL(context_tracking_key); static noinstr bool context_tracking_recursion_enter(void) @@ -674,14 +674,44 @@ void user_exit_callable(void) } NOKPROBE_SYMBOL(user_exit_callable); -void __init ct_cpu_track_user(int cpu) +/** + * ct_cpu_track_user - enable context tracking for a CPU + * @cpu: target CPU (must be offline when called at runtime) + * + * Marks @cpu as actively tracking user/kernel transitions and increments + * the context_tracking_key refcount. Safe to call at runtime provided + * the CPU is offline so no context-tracking readers are active on it. + */ +void ct_cpu_track_user(int cpu) { - static __initdata bool initialized = false; - if (!per_cpu(context_tracking.active, cpu)) { per_cpu(context_tracking.active, cpu) = true; static_branch_inc(&context_tracking_key); } +} +EXPORT_SYMBOL_GPL(ct_cpu_track_user); + +/** + * ct_cpu_untrack_user - disable context tracking for a CPU + * @cpu: target CPU (must be offline when called) + * + * Reverses ct_cpu_track_user(). The CPU must be offline so that no + * context-tracking readers are active on it. + */ +void ct_cpu_untrack_user(int cpu) +{ + if (per_cpu(context_tracking.active, cpu)) { + per_cpu(context_tracking.active, cpu) = false; + static_branch_dec(&context_tracking_key); + } +} +EXPORT_SYMBOL_GPL(ct_cpu_untrack_user); + +void __init ct_cpu_track_user_init(int cpu) +{ + static __initdata bool initialized = false; + + ct_cpu_track_user(cpu); if (initialized) return; diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index cbbb87a0c6e7c..ba7adc671c580 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -677,7 +677,7 @@ void __init tick_nohz_init(void) } for_each_cpu(cpu, tick_nohz_full_mask) - ct_cpu_track_user(cpu); + ct_cpu_track_user_init(cpu); ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "kernel/nohz:predown", NULL, -- 2.43.0