From patchwork Wed Aug 7 16:35:05 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Neeraj Upadhyay X-Patchwork-Id: 13756497 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 76C832209B; Wed, 7 Aug 2024 16:35:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723048522; cv=none; b=sJfdzn7jAtbZFG7d7Iy+WWDxlIb3LJa/MQ4P6knj7u2TzPy7LXSDZvLhHOt5VjioTj8q7ZxaIibVh5ydDm6vezHRcugufFmxbcL/XgxR3b51jbTXEQpd84TWI95dA5nBe+X0m4ZmcDu0eFTk6PgSY9WiD6P4UrbwhJ9sTsBRiUs= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723048522; c=relaxed/simple; bh=YGXKpyiz2oqforJExEBds7DqYOCDlr1w1fpOqMNWp8M=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=o5k7h0nBLoSNPNLdNoWr6QXXIVA8eh9EkKozx+uAhdFtDlOivM/M/G0SBxTVYjQRp2uUvOon65Te3j1nJBfjZFBMQpZbjRkVKq8MkvOwZUp3oGRceN8GW3tsiseybtT2Y1lI9UdaY7BNaLE6CiYrlyLrwJVrfUBY7hkE+VakiL8= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TpPcXkE8; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="TpPcXkE8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 53C62C32781; Wed, 7 Aug 2024 16:35:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1723048522; bh=YGXKpyiz2oqforJExEBds7DqYOCDlr1w1fpOqMNWp8M=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=TpPcXkE8MTrU53O6U+9lmXL5tJxzNmlEt/LSLmNeP70PR1Y655hOOduWJX1OnaMrd sgsZfbr28WT9fAWAE0jXm2FAe79q3/6eCVBIQJUKlgEh6IomQeRNmrjPXYkIXLTGOD 4zcndlcx3HeJOSCxf/9Jbh7x6brREl6RUIPfboXLQ0W1WrfO9lflMuFT7RbT4yTfKa PoueWr1IdFrqgYdfQYUXA4mdSbKmyHf7QABjsnMxThYVSROL3XoR1qMHpo6XGvqJf8 diptmziORqIIFNzD5PwFG0tki0OYkJ9CCnr1DPKuChYuSYjQjy5Tqume8TZHovEb3f dsi8weOnvbh2g== From: neeraj.upadhyay@kernel.org To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, paulmck@kernel.org, joel@joelfernandes.org, frederic@kernel.org, boqun.feng@gmail.com, urezki@gmail.com, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, qiang.zhang1211@gmail.com, neeraj.iitr10@gmail.com, Neeraj Upadhyay Subject: [PATCH 1/2] rcu-tasks: Make RCU-Tasks pay attention to idle tasks Date: Wed, 7 Aug 2024 22:05:05 +0530 Message-Id: <20240807163506.434885-2-neeraj.upadhyay@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: <20240807163506.434885-1-neeraj.upadhyay@kernel.org> References: <20240807163506.434885-1-neeraj.upadhyay@kernel.org> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Neeraj Upadhyay Currently, idle tasks are ignored by Tasks RCU and idle tasks are in RCU-Tasks quiescent state for Tasks RCU. Change this to start paying attention to idle tasks except in deep-idle functions where RCU is not watching. With this, for architectures where kernel entry/exit and deep-idle functions have been properly tagged noinstr, Tasks Rude RCU can be disabled. [ neeraj.upadhyay: Frederic Weisbecker and Paul E. McKenney feedback. ] Signed-off-by: Neeraj Upadhyay --- .../RCU/Design/Requirements/Requirements.rst | 12 +- kernel/rcu/rcu.h | 4 + kernel/rcu/tasks.h | 107 ++++++++++++------ kernel/rcu/tree.c | 12 +- kernel/rcu/tree_exp.h | 2 +- 5 files changed, 89 insertions(+), 48 deletions(-) diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst index 6125e7068d2c..5016b85d53d7 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.rst +++ b/Documentation/RCU/Design/Requirements/Requirements.rst @@ -2611,8 +2611,8 @@ critical sections that are delimited by voluntary context switches, that is, calls to schedule(), cond_resched(), and synchronize_rcu_tasks(). In addition, transitions to and from userspace execution also delimit tasks-RCU read-side critical sections. -Idle tasks are ignored by Tasks RCU, and Tasks Rude RCU may be used to -interact with them. +Idle tasks which are idle from RCU's perspective are ignored by Tasks RCU, +and Tasks Rude RCU may be used to interact with them. Note well that involuntary context switches are *not* Tasks-RCU quiescent states. After all, in preemptible kernels, a task executing code in a @@ -2643,10 +2643,10 @@ moniker. And this operation is considered to be quite rude by real-time workloads that don't want their ``nohz_full`` CPUs receiving IPIs and by battery-powered systems that don't want their idle CPUs to be awakened. -Once kernel entry/exit and deep-idle functions have been properly tagged -``noinstr``, Tasks RCU can start paying attention to idle tasks (except -those that are idle from RCU's perspective) and then Tasks Rude RCU can -be removed from the kernel. +As Tasks RCU now pays attention to idle tasks (except those that are idle +from RCU's perspective), once kernel entry/exit and deep-idle functions have +been properly tagged ``noinstr``, Tasks Rude RCU can be removed from the +kernel. The tasks-rude-RCU API is also reader-marking-free and thus quite compact, consisting solely of synchronize_rcu_tasks_rude(). diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index feb3ac1dc5d5..5fec1b52039c 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -609,6 +609,8 @@ void srcutorture_get_gp_data(struct srcu_struct *sp, int *flags, #ifdef CONFIG_TINY_RCU static inline bool rcu_watching_zero_in_eqs(int cpu, int *vp) { return false; } +static inline bool rcu_watching_snap_in_eqs(int snap) { return false; } +static inline bool rcu_watching_snap_stopped_since(int cpu, int snap) { return false; } static inline unsigned long rcu_get_gp_seq(void) { return 0; } static inline unsigned long rcu_exp_batches_completed(void) { return 0; } static inline unsigned long @@ -622,6 +624,8 @@ static inline void rcu_gp_slow_register(atomic_t *rgssp) { } static inline void rcu_gp_slow_unregister(atomic_t *rgssp) { } #else /* #ifdef CONFIG_TINY_RCU */ bool rcu_watching_zero_in_eqs(int cpu, int *vp); +bool rcu_watching_snap_in_eqs(int snap); +bool rcu_watching_snap_stopped_since(int cpu, int snap); unsigned long rcu_get_gp_seq(void); unsigned long rcu_exp_batches_completed(void); unsigned long srcu_batches_completed(struct srcu_struct *sp); diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 2f8d6c8e3c4c..0c0367c8f5c8 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -35,6 +35,7 @@ typedef void (*postgp_func_t)(struct rcu_tasks *rtp); * @rtp_exit_list: List of tasks in the latter portion of do_exit(). * @cpu: CPU number corresponding to this entry. * @rtpp: Pointer to the rcu_tasks structure. + * @rcu_watching_snap: Per-GP rcu watching tracking for idle tasks. */ struct rcu_tasks_percpu { struct rcu_segcblist cblist; @@ -51,6 +52,7 @@ struct rcu_tasks_percpu { int cpu; int index; struct rcu_tasks *rtpp; + int rcu_watching_snap; }; /** @@ -902,14 +904,15 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp) //////////////////////////////////////////////////////////////////////// // // Simple variant of RCU whose quiescent states are voluntary context -// switch, cond_resched_tasks_rcu_qs(), user-space execution, and idle. -// As such, grace periods can take one good long time. There are no -// read-side primitives similar to rcu_read_lock() and rcu_read_unlock() -// because this implementation is intended to get the system into a safe -// state for some of the manipulations involved in tracing and the like. -// Finally, this implementation does not support high call_rcu_tasks() -// rates from multiple CPUs. If this is required, per-CPU callback lists -// will be needed. +// switch, cond_resched_tasks_rcu_qs(), user-space execution, and idle +// tasks which are in RCU-idle context. As such, grace periods can take +// one good long time. There are no read-side primitives similar to +// rcu_read_lock() and rcu_read_unlock() because this implementation is +// intended to get the system into a safe state for some of the +// manipulations involved in tracing and the like. Finally, this +// implementation does not support high call_rcu_tasks() rates from +// multiple CPUs. If this is required, per-CPU callback lists will be +// needed. // // The implementation uses rcu_tasks_wait_gp(), which relies on function // pointers in the rcu_tasks structure. The rcu_spawn_tasks_kthread() @@ -920,11 +923,13 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp) // Invokes synchronize_rcu() in order to wait for all in-flight // t->on_rq and t->nvcsw transitions to complete. This works because // all such transitions are carried out with interrupts disabled. -// rcu_tasks_pertask(), invoked on every non-idle task: -// For every runnable non-idle task other than the current one, use -// get_task_struct() to pin down that task, snapshot that task's -// number of voluntary context switches, and add that task to the -// holdout list. +// rcu_tasks_pertask(), invoked on every task: +// For idle task, snapshot the task's CPU's dynticks counter. If +// the CPU is in RCU-idle context, do not add it to the holdout list. +// For every runnable non-idle task other than the current one and +// idle tasks which are not in RCU-idle context, use get_task_struct() +// to pin down that task, snapshot that task's number of voluntary +// context switches, and add that task to the holdout list. // rcu_tasks_postscan(): // Gather per-CPU lists of tasks in do_exit() to ensure that all // tasks that were in the process of exiting (and which thus might @@ -983,27 +988,58 @@ static bool rcu_tasks_is_holdout(struct task_struct *t) if (!READ_ONCE(t->on_rq)) return false; - /* - * Idle tasks (or idle injection) within the idle loop are RCU-tasks - * quiescent states. But CPU boot code performed by the idle task - * isn't a quiescent state. - */ - if (is_idle_task(t)) - return false; - cpu = task_cpu(t); - /* Idle tasks on offline CPUs are RCU-tasks quiescent states. */ - if (t == idle_task(cpu) && !rcu_cpu_online(cpu)) + + if (t == idle_task(cpu)) { + /* Idle tasks on offline CPUs are RCU-tasks quiescent states. */ + if (!rcu_cpu_online(cpu)) + return false; +#ifdef CONFIG_SMP + /* + * As idle tasks cannot be involuntary preempted, non-running idle tasks + * are not in RCU-tasks critical section. + * synchronize_rcu() calls in rcu_tasks_pregp_step() and rcu_tasks_postgp() + * ensure that all ->on_cpu transitions are complete. + */ + if (!t->on_cpu) + return false; +#else + /* + * We are in rcu_tasks_kthread() context. Idle thread would + * have done a voluntary context switch. + */ return false; +#endif + } return true; } +void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func); +DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU Tasks"); + /* Per-task initial processing. */ static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop) { if (t != current && rcu_tasks_is_holdout(t)) { +#ifndef CONFIG_TINY_RCU + int cpu = task_cpu(t); + + if (t == idle_task(cpu)) { + struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, cpu); + + /* + * Do plain access. Ordering between remote CPU's pre idle accesses + * and post rcu-tasks grace period is provided by synchronize_rcu() + * in rcu_tasks_postgp(). + */ + rtpcp->rcu_watching_snap = ct_rcu_watching_cpu(cpu); + /* RCU-idle contexts are RCU-tasks quiescent state for idle tasks. */ + if (rcu_watching_snap_in_eqs(rtpcp->rcu_watching_snap)) + return; + } +#endif // #ifndef CONFIG_TINY_RCU get_task_struct(t); t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw); WRITE_ONCE(t->rcu_tasks_holdout, true); @@ -1011,9 +1047,6 @@ static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop) } } -void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func); -DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU Tasks"); - /* Processing between scanning taskslist and draining the holdout list. */ static void rcu_tasks_postscan(struct list_head *hop) { @@ -1082,10 +1115,13 @@ static void rcu_tasks_postscan(struct list_head *hop) static void check_holdout_task(struct task_struct *t, bool needreport, bool *firstreport) { - int cpu; + int cpu = task_cpu(t); + struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, cpu); if (!READ_ONCE(t->rcu_tasks_holdout) || t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) || + ((t == idle_task(cpu)) && + rcu_watching_snap_stopped_since(cpu, rtpcp->rcu_watching_snap)) || !rcu_tasks_is_holdout(t) || (IS_ENABLED(CONFIG_NO_HZ_FULL) && !is_idle_task(t) && READ_ONCE(t->rcu_tasks_idle_cpu) >= 0)) { @@ -1101,7 +1137,6 @@ static void check_holdout_task(struct task_struct *t, pr_err("INFO: rcu_tasks detected stalls on tasks:\n"); *firstreport = false; } - cpu = task_cpu(t); pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n", t, ".I"[is_idle_task(t)], "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)], @@ -1171,11 +1206,12 @@ static void tasks_rcu_exit_srcu_stall(struct timer_list *unused) * period elapses, in other words after all currently executing RCU * read-side critical sections have completed. call_rcu_tasks() assumes * that the read-side critical sections end at a voluntary context - * switch (not a preemption!), cond_resched_tasks_rcu_qs(), entry into idle, - * or transition to usermode execution. As such, there are no read-side - * primitives analogous to rcu_read_lock() and rcu_read_unlock() because - * this primitive is intended to determine that all tasks have passed - * through a safe state, not so much for data-structure synchronization. + * switch (not a preemption!), cond_resched_tasks_rcu_qs(), entry into + * RCU-idle context or transition to usermode execution. As such, there + * are no read-side primitives analogous to rcu_read_lock() and + * rcu_read_unlock() because this primitive is intended to determine + * that all tasks have passed through a safe state, not so much for + * data-structure synchronization. * * See the description of call_rcu() for more detailed information on * memory ordering guarantees. @@ -1193,8 +1229,9 @@ EXPORT_SYMBOL_GPL(call_rcu_tasks); * grace period has elapsed, in other words after all currently * executing rcu-tasks read-side critical sections have elapsed. These * read-side critical sections are delimited by calls to schedule(), - * cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls - * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched(). + * cond_resched_tasks_rcu_qs(), idle execution within RCU-idle context, + * userspace execution, calls to synchronize_rcu_tasks(), and (in theory, + * anyway) cond_resched(). * * This is a very specialized primitive, intended only for a few uses in * tracing and other situations requiring manipulation of function diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 930846f06bee..1f1574200bea 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -303,7 +303,7 @@ static void rcu_watching_online(void) * Return true if the snapshot returned from ct_rcu_watching() * indicates that RCU is in an extended quiescent state. */ -static bool rcu_watching_snap_in_eqs(int snap) +bool rcu_watching_snap_in_eqs(int snap) { return !(snap & CT_RCU_WATCHING); } @@ -312,16 +312,16 @@ static bool rcu_watching_snap_in_eqs(int snap) * rcu_watching_snap_stopped_since() - Has RCU stopped watching a given CPU * since the specified @snap? * - * @rdp: The rcu_data corresponding to the CPU for which to check EQS. + * @cpu: The CPU for which to check EQS. * @snap: rcu_watching snapshot taken when the CPU wasn't in an EQS. * - * Returns true if the CPU corresponding to @rdp has spent some time in an + * Returns true if the CPU corresponding has spent some time in an * extended quiescent state since @snap. Note that this doesn't check if it * /still/ is in an EQS, just that it went through one since @snap. * * This is meant to be used in a loop waiting for a CPU to go through an EQS. */ -static bool rcu_watching_snap_stopped_since(struct rcu_data *rdp, int snap) +bool rcu_watching_snap_stopped_since(int cpu, int snap) { /* * The first failing snapshot is already ordered against the accesses @@ -334,7 +334,7 @@ static bool rcu_watching_snap_stopped_since(struct rcu_data *rdp, int snap) if (WARN_ON_ONCE(rcu_watching_snap_in_eqs(snap))) return true; - return snap != ct_rcu_watching_cpu_acquire(rdp->cpu); + return snap != ct_rcu_watching_cpu_acquire(cpu); } /* @@ -826,7 +826,7 @@ static int rcu_watching_snap_recheck(struct rcu_data *rdp) * read-side critical section that started before the beginning * of the current RCU grace period. */ - if (rcu_watching_snap_stopped_since(rdp, rdp->watching_snap)) { + if (rcu_watching_snap_stopped_since(rdp->cpu, rdp->watching_snap)) { trace_rcu_fqs(rcu_state.name, rdp->gp_seq, rdp->cpu, TPS("dti")); rcu_gpnum_ovf(rnp, rdp); return 1; diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index c3266bf709d5..6202d321b2bb 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -400,7 +400,7 @@ static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp) unsigned long mask = rdp->grpmask; retry_ipi: - if (rcu_watching_snap_stopped_since(rdp, rdp->exp_watching_snap)) { + if (rcu_watching_snap_stopped_since(rdp->cpu, rdp->exp_watching_snap)) { mask_ofl_test |= mask; continue; }