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 9A745C41621 for ; Tue, 24 Mar 2020 15:48:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6751320774 for ; Tue, 24 Mar 2020 15:48:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585064904; bh=/MscYPlcukixGjCLzA6QCZoh+oKv+nG1r6CprFDPL00=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=RfTAas95Bqzj9SzRFxgzThge9tTghNgZdpXQ+CiVanORhqXkC98Rv+3WuWpYpmQOC yPrzI7Twoum9VC9EPrLZxMK+kVOlxkQlwHpA6Sk4myYLiVZwcKY0O8fJYlCLy2DaMD cH+y1zkrAXkqU9BHTmvn3bM5POhntTO5gFwgsxPI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727733AbgCXPsY (ORCPT ); Tue, 24 Mar 2020 11:48:24 -0400 Received: from mail.kernel.org ([198.145.29.99]:41118 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727579AbgCXPsX (ORCPT ); Tue, 24 Mar 2020 11:48:23 -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 A63B22076F; Tue, 24 Mar 2020 15:48:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585064902; bh=/MscYPlcukixGjCLzA6QCZoh+oKv+nG1r6CprFDPL00=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=k18ujsH02+rC2T7/8lxQmLY5QiWNEXlhZkXR5/Bn6KGMrLmWdTMIUzAR9F4orBB0a 1zAn/8rzc5ashOxuK9Tu+jtffGFq8spJPpf3bPdjXLo9WctEGNHT2zyKxH8LNo6tkr iivRDrD3vKzWMo9fQuMjK86mwkUj+5SyuNl/V2ZU= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 7AAE83522AE1; Tue, 24 Mar 2020 08:48:22 -0700 (PDT) Date: Tue, 24 Mar 2020 08:48:22 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Steven Rostedt , 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, 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: <20200324154822.GC19865@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> <20200320024943.GA29649@paulmck-ThinkPad-P72> <20200324000639.GA29340@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200324000639.GA29340@google.com> 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 Mon, Mar 23, 2020 at 08:06:39PM -0400, Joel Fernandes wrote: > On Thu, Mar 19, 2020 at 07:49:43PM -0700, Paul E. McKenney wrote: > [...] > > 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) > > This function name is a bit confusing. The task could be running when this > function is called. Below you are detecting that the task is running, by > calling task_curr(). > > Maybe just trc_inspect_reader() is better? Sold! ;-) > [..] > > > 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; > > If this is every called from middle of a reader section (that is we > transition from IPI-mode to using heavier reader-sections), then is a memory > barrier needed here just to protect the reader section that already started? That memory barrier is provided by the memory ordering in the callers of rcu_dynticks_task_trace_enter() and rcu_dynticks_task_trace_exit(), namely, those callers' atomic_add_return() invocations. These barriers pair with the pair of smp_rmb() calls in rcu_dynticks_zero_in_eqs(), which is in turn invoked from the function formerly known as trc_inspect_reader_notrunning(), AKA trc_inspect_reader(). This same pair of smp_rmb() calls also pair with the conditional smp_mb() calls in rcu_read_lock_trace() and rcu_read_unlock_trace(). In your scenario, the calls in rcu_read_lock_trace() and rcu_read_unlock_trace() wouldn't happen, but in that case the ordering from atomic_add_return() would suffice. Does that work? Or is there an ordering bug in there somewhere? Thanx, Paul > thanks, > > - Joel > > > > +#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 */ > > +}