From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AA4C9C4332B for ; Fri, 20 Mar 2020 02:49:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 65EA420753 for ; Fri, 20 Mar 2020 02:49:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584672585; bh=XhJ1LZqcmhPeZODYBScaf2tom9qUZ233rVvx76q+qms=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=cL+Z9XgSxhXfzHEuBOVzKq17JmXsvXD5E5PZPEIebBWzs+aRD5Hxj5vOdHd4JNarh ICG98JGfvy40YsgAX5S0RyOfLCD2RNuU8SA9svNhrESyPFehhdYpfcPu2jjLs+5nfW sOV+FgasYhK4IKnkukBmpl2VEDT2KQFxiEZ7JK3g= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726103AbgCTCtp (ORCPT ); Thu, 19 Mar 2020 22:49:45 -0400 Received: from mail.kernel.org ([198.145.29.99]:60462 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725856AbgCTCto (ORCPT ); Thu, 19 Mar 2020 22:49:44 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 528622071C; Fri, 20 Mar 2020 02:49:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584672583; bh=XhJ1LZqcmhPeZODYBScaf2tom9qUZ233rVvx76q+qms=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=b6H9M9PMVANwlE+I9R+lY+z102qor1d7e51mbhtZt2Vj0sHcU1S/AMAASQEtvBvAw S2UWWCfjQOoqQrw5jDtQE2xoxRvu34XJWrskvgjdkpgbudN7gOjcZb/k0LIXjdD055 KDhNsO3ZQ9bSzqjkrtg/vMUdW1UwXAlEQPkGtev0= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 22F1435226B9; Thu, 19 Mar 2020 19:49:43 -0700 (PDT) Date: Thu, 19 Mar 2020 19:49:43 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Ben Segall , Mel Gorman Subject: Re: [PATCH RFC v2 tip/core/rcu 01/22] sched/core: Add function to sample state of locked-down task Message-ID: <20200320024943.GA29649@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200319001024.GA28798@paulmck-ThinkPad-P72> <20200319001100.24917-1-paulmck@kernel.org> <20200319132238.75a034c3@gandalf.local.home> <20200319173525.GI3199@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200319173525.GI3199@paulmck-ThinkPad-P72> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Thu, Mar 19, 2020 at 10:35:25AM -0700, Paul E. McKenney wrote: > On Thu, Mar 19, 2020 at 01:22:38PM -0400, Steven Rostedt wrote: > > On Wed, 18 Mar 2020 17:10:39 -0700 > > paulmck@kernel.org wrote: [ . . . ] > > > /** > > > + * try_invoke_on_locked_down_task - Invoke a function on task in fixed state > > > + * @p: Process for which the function is to be invoked. > > > + * @func: Function to invoke. > > > + * @arg: Argument to function. > > > + * > > > + * If the specified task can be quickly locked into a definite state > > > + * (either sleeping or on a given runqueue), arrange to keep it in that > > > + * state while invoking @func(@arg). This function can use ->on_rq and > > > + * task_curr() to work out what the state is, if required. Given that > > > + * @func can be invoked with a runqueue lock held, it had better be quite > > > + * lightweight. > > > + * > > > + * Returns: > > > + * @false if the task slipped out from under the locks. > > > + * @true if the task was locked onto a runqueue or is sleeping. > > > + * However, @func can override this by returning @false. > > > > Should probably state that it will return false if the state could be > > locked, otherwise it returns the return code of the function. > > So like this? > > * Returns: > * @false if the task state could not be locked. > * Otherwise, the return value from @func(arg). > > > I'm wondering if we shouldn't have the function return code be something > > passed in by the parameter, and have this return either true (locked and > > function called), or false (not locked and function wasn't called). > > I was thinking of this as one of the possible uses of whatever arg > points to, which allows the caller of try_invoke_on_locked_down_task() > and the specified function to communicate whatever they wish. Then > the specified function could (for example) unconditionally return true > so that the return value from try_invoke_on_locked_down_task() indicated > whether or not the specified function was called. > > The current setup is very convenient for the use cases thus far. It > allows the function to say "Yeah, I was called, but I couldn't do > anything", thus allowing the caller to make exactly one check to know > that corrective action is required. And here is another use case that led me to take this approach. The trc_inspect_reader_notrunning() function in the patch below is passed to try_invoke_on_locked_down_task() whose caller can continue testing just the return value from try_invoke_on_locked_down_task() to work out what to do next. Thoughts? Other use cases? Thanx, Paul ------------------------------------------------------------------------ commit e26a234c1205bf02b62b62cd7f15f8086fc0b13b Author: Paul E. McKenney Date: Thu Mar 19 15:33:12 2020 -0700 rcu-tasks: Avoid IPIing userspace/idle tasks if kernel is so built Systems running CPU-bound real-time task do not want IPIs sent to CPUs executing nohz_full userspace tasks. Battery-powered systems don't want IPIs sent to idle CPUs in low-power mode. Unfortunately, RCU tasks trace can and will send such IPIs in some cases. Both of these situations occur only when the target CPU is in RCU dyntick-idle mode, in other words, when RCU is not watching the target CPU. This suggests that CPUs in dyntick-idle mode should use memory barriers in outermost invocations of rcu_read_lock_trace() and rcu_read_unlock_trace(), which would allow the RCU tasks trace grace period to directly read out the target CPU's read-side state. One challenge is that RCU tasks trace is not targeting a specific CPU, but rather a task. And that task could switch from one CPU to another at any time. This commit therefore uses try_invoke_on_locked_down_task() and checks for task_curr() in trc_inspect_reader_notrunning(). When this condition holds, the target task is running and cannot move. If CONFIG_TASKS_TRACE_RCU_READ_MB=y, the new rcu_dynticks_zero_in_eqs() function can be used to check if the specified integer (in this case, t->trc_reader_nesting) is zero while the target CPU remains in that same dyntick-idle sojourn. If so, the target task is in a quiescent state. If not, trc_read_check_handler() must indicate failure so that the grace-period kthread can take appropriate action or retry after an appropriate delay, as the case may be. With this change, given CONFIG_TASKS_TRACE_RCU_READ_MB=y, if a given CPU remains idle or a given task continues executing in nohz_full mode, the RCU tasks trace grace-period kthread will detect this without the need to send an IPI. Suggested-by: Mathieu Desnoyers Signed-off-by: Paul E. McKenney diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index e1089fd..296f926 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -501,6 +501,7 @@ void srcutorture_get_gp_data(enum rcutorture_type test_type, #endif #ifdef CONFIG_TINY_RCU +static inline bool rcu_dynticks_zero_in_eqs(int cpu, int *vp) { 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 @@ -510,6 +511,7 @@ static inline void show_rcu_gp_kthreads(void) { } static inline int rcu_get_gp_kthreads_prio(void) { return 0; } static inline void rcu_fwd_progress_check(unsigned long j) { } #else /* #ifdef CONFIG_TINY_RCU */ +bool rcu_dynticks_zero_in_eqs(int cpu, int *vp); 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 d31ed74..36f03d3 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -802,22 +802,38 @@ static void trc_read_check_handler(void *t_in) /* Callback function for scheduler to check non-running) task. */ static bool trc_inspect_reader_notrunning(struct task_struct *t, void *arg) { - if (task_curr(t)) - return false; // It is running, so decline to inspect it. + int cpu = task_cpu(t); + bool in_qs = false; + + if (task_curr(t)) { + // If no chance of heavyweight readers, do it the hard way. + if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) + return false; + + // If heavyweight readers are enabled on the remote task, + // we can inspect its state despite its currently running. + // However, we cannot safely change its state. + if (!rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting)) + return false; // No quiescent state, do it the hard way. + in_qs = true; + } else { + in_qs = likely(!t->trc_reader_nesting); + } // Mark as checked. Because this is called from the grace-period // kthread, also remove the task from the holdout list. t->trc_reader_checked = true; trc_del_holdout(t); - // If the task is in a read-side critical section, set up its + if (in_qs) + return true; // Already in quiescent state, done!!! + + // The task is in a read-side critical section, so set up its // its state so that it will awaken the grace-period kthread upon // exit from that critical section. - if (unlikely(t->trc_reader_nesting)) { - atomic_inc(&trc_n_readers_need_end); // One more to wait on. - WARN_ON_ONCE(t->trc_reader_special.b.need_qs); - WRITE_ONCE(t->trc_reader_special.b.need_qs, true); - } + atomic_inc(&trc_n_readers_need_end); // One more to wait on. + WARN_ON_ONCE(t->trc_reader_special.b.need_qs); + WRITE_ONCE(t->trc_reader_special.b.need_qs, true); return true; } diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index de6228a..4eb424e 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -239,6 +239,7 @@ static void rcu_dynticks_eqs_enter(void) * critical sections, and we also must force ordering with the * next idle sojourn. */ + rcu_dynticks_task_trace_enter(); // Before ->dynticks update! seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks); // RCU is no longer watching. Better be in extended quiescent state! WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && @@ -265,6 +266,7 @@ static void rcu_dynticks_eqs_exit(void) */ seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks); // RCU is now watching. Better not be in an extended quiescent state! + rcu_dynticks_task_trace_exit(); // After ->dynticks update! WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & RCU_DYNTICK_CTRL_CTR)); if (seq & RCU_DYNTICK_CTRL_MASK) { @@ -337,6 +339,28 @@ static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap) } /* + * Return true if the referenced integer is zero while the specified + * CPU remains within a single extended quiescent state. + */ +bool rcu_dynticks_zero_in_eqs(int cpu, int *vp) +{ + struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); + int snap; + + // If not quiescent, force back to earlier extended quiescent state. + snap = atomic_read(&rdp->dynticks) & ~(RCU_DYNTICK_CTRL_MASK | + RCU_DYNTICK_CTRL_CTR); + + smp_rmb(); // Order ->dynticks and *vp reads. + if (READ_ONCE(*vp)) + return false; // Non-zero, so report failure; + smp_rmb(); // Order *vp read and ->dynticks re-read. + + // If still in the same extended quiescent state, we are good! + return snap == (atomic_read(&rdp->dynticks) & ~RCU_DYNTICK_CTRL_MASK); +} + +/* * Set the special (bottom) bit of the specified CPU so that it * will take special action (such as flushing its TLB) on the * next exit from an extended quiescent state. Returns true if diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 44edd0a..43991a4 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -455,6 +455,8 @@ static void rcu_bind_gp_kthread(void); static bool rcu_nohz_full_cpu(void); static void rcu_dynticks_task_enter(void); static void rcu_dynticks_task_exit(void); +static void rcu_dynticks_task_trace_enter(void); +static void rcu_dynticks_task_trace_exit(void); /* Forward declarations for tree_stall.h */ static void record_gp_stall_check_time(void); diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 9355536..f4a344e 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -2553,3 +2553,21 @@ static void rcu_dynticks_task_exit(void) WRITE_ONCE(current->rcu_tasks_idle_cpu, -1); #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */ } + +/* Turn on heavyweight RCU tasks trace readers on idle/user entry. */ +static void rcu_dynticks_task_trace_enter(void) +{ +#ifdef CONFIG_TASKS_RCU_TRACE + if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) + current->trc_reader_special.b.need_mb = true; +#endif /* #ifdef CONFIG_TASKS_RCU_TRACE */ +} + +/* Turn off heavyweight RCU tasks trace readers on idle/user exit. */ +static void rcu_dynticks_task_trace_exit(void) +{ +#ifdef CONFIG_TASKS_RCU_TRACE + if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) + current->trc_reader_special.b.need_mb = false; +#endif /* #ifdef CONFIG_TASKS_RCU_TRACE */ +}