From mboxrd@z Thu Jan 1 00:00:00 1970 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 9485718CC1C; Wed, 31 Jul 2024 12:28:19 +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=1722428899; cv=none; b=l5CkZWBVazU6nF+8tTAkoh1K/JvjZXJzxZJcQlm1iWrw7zxg/m7gWzKKmrNjPp8UXsunZQtA15k9+1v2CPE087hO8uW/cgQ9Nb1Att9wi/j/SQFSm0d+j2e2v9tPjBI1NLfB0ikIlG+W6XFDFYGi3L8hpabbedIGUTi//3AqVao= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722428899; c=relaxed/simple; bh=e41QtC3FwXyJ0F1j+sY5OXLnbQYN3h7oBWeSgwGs00w=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=T4zNL8+q9y3nm8EK7Q0pHQrv3rBXHyaTENyhXEMSmYC8V/Py4giqQ3jylfzO9EL0/uKT+9nciAf/Yscyn/F/XX6QMzNoIyDG++P76BEw/GDXxkahvA3JjtLin1gw3rLl4ltaw7AS+3UBhe3ZV9wXWLVBaJQ1qMAPi7hDEh4Du6s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Qa5zbo8V; 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="Qa5zbo8V" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AEBB8C116B1; Wed, 31 Jul 2024 12:28:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1722428899; bh=e41QtC3FwXyJ0F1j+sY5OXLnbQYN3h7oBWeSgwGs00w=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Qa5zbo8V4KSnB1pbQOP7lJzfVpBzM9D+OaZSES9vDPpg0zPZmcEFzgmSHAex/l935 7bv2GAxxdEdXghh6+uHS2fZzyOOUuUpFdfpDHOSuiRBvHNDyeDmiY0tQCLpPKUH+Tx QgijDl9mSMgaUqi95PZpm+l3SBnlQVeCakLg3BHyfXjE6xwGFuEJKZBENyAoCXKc8X KQFQUv1EXVSp8VpzuKijKi9BAByDDHkkEin7vAV0tPC6I3JHslNpCMkbSRgXE6kZSw st5a4aHwFTOVF6AvfURUGkd0Zb464o+7D4AelbePpvYr4PQ6ulgwYNeiESXlIBCfZy wF7+ULoXEy5bQ== Date: Wed, 31 Jul 2024 14:28:16 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: Valentin Schneider , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, Peter Zijlstra , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang Subject: Re: RCU-Task[-Trace] VS EQS (was Re: [PATCH v3 13/25] context_tracking, rcu: Rename rcu_dynticks_task*() into rcu_task*()) Message-ID: References: <20240724144325.3307148-1-vschneid@redhat.com> <20240724144325.3307148-14-vschneid@redhat.com> <31d78183-4526-41e8-90df-d03c95fdb9b2@paulmck-laptop> <30c6d4aa-7598-4dc1-8592-7533d64714c2@paulmck-laptop> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <30c6d4aa-7598-4dc1-8592-7533d64714c2@paulmck-laptop> Le Tue, Jul 30, 2024 at 03:39:44PM -0700, Paul E. McKenney a écrit : > On Wed, Jul 31, 2024 at 12:17:49AM +0200, Frederic Weisbecker wrote: > > Le Tue, Jul 30, 2024 at 07:23:58AM -0700, Paul E. McKenney a écrit : > > > On Thu, Jul 25, 2024 at 04:32:46PM +0200, Frederic Weisbecker wrote: > > > > Le Wed, Jul 24, 2024 at 04:43:13PM +0200, Valentin Schneider a écrit : > > > > > -/* Turn on heavyweight RCU tasks trace readers on idle/user entry. */ > > > > > -static __always_inline void rcu_dynticks_task_trace_enter(void) > > > > > +/* Turn on heavyweight RCU tasks trace readers on kernel exit. */ > > > > > +static __always_inline void rcu_task_trace_exit(void) > > > > > > > > Before I proceed on this last one, a few questions for Paul and others: > > > > > > > > 1) Why is rcu_dynticks_task_exit() not called while entering in NMI? > > > > Does that mean that NMIs aren't RCU-Task read side critical sections? > > > > > > Because Tasks RCU Rude handles that case currently. So good catch, > > > because this might need adjustment when we get rid of Tasks RCU Rude. > > > And both rcu_dynticks_task_enter() and rcu_dynticks_task_exit() look safe > > > to invoke from NMI handlers. Memory ordering needs checking, of course. > > > > > > Except that on architectures defining CONFIG_ARCH_WANTS_NO_INSTR, Tasks > > > RCU should instead check the ct_kernel_enter_state(RCU_DYNTICKS_IDX) > > > state, right? And on those architectures, I believe that > > > rcu_dynticks_task_enter() and rcu_dynticks_task_exit() can just be no-ops. > > > Or am I missing something here? > > > > I think rcu_dynticks_task_enter() and rcu_dynticks_task_exit() are > > still needed anyway because the target task can migrate. So unless the rq is locked, > > it's hard to match a stable task_cpu() with the corresponding RCU_DYNTICKS_IDX. > > Can it really migrate while in entry/exit or deep idle code? Or am I > missing a trick here? No but it can migrate before or after EQS. So we need to handle situations like: == CPU 0 == == CPU 1 == // TASK A is on rq set_task_cpu(TASK A, 0) // TASK B runs ct_user_enter() ct_user_exit() //TASK A runs It could be something like the following: int rcu_tasks_nohz_full_holdout(struct task_struct *t) { int cpu; int snap; cpu = task_cpu(t); /* Don't miss EQS exit if the task migrated out and in */ smp_rmb() snap = ct_dynticks_cpu(cpu); if (snap & RCU_DYNTICKS_IDX) return true; /* Check if it's the actual task running */ smp_rmb() if (rcu_dereference_raw(cpu_curr(cpu)) != t) return true; /* Make sure the task hasn't migrated in after the above EQS */ smp_rmb() return ct_dynticks_cpu(cpu) != snap; } But there is still a risk that ct_dynticks wraps before the last test. So we would need task_call_func() if task_cpu() is in nohz_full mode. > > > > > 2) Looking further into CONFIG_TASKS_TRACE_RCU_READ_MB=y, it seems to > > > > allow for uses of rcu_read_[un]lock_trace() while RCU is not watching > > > > (EQS). Is it really a good idea to support that? Are we aware of any > > > > such potential usecase? > > > > > > I hope that in the longer term, there will be no reason to support this. > > > Right now, architectures not defining CONFIG_ARCH_WANTS_NO_INSTR must > > > support this because tracers really can attach probes where RCU is > > > not watching. > > > > > > And even now, in architectures defining CONFIG_ARCH_WANTS_NO_INSTR, I > > > am not convinced that the early incoming and late outgoing CPU-hotplug > > > paths are handled correctly. RCU is not watching them, but I am not so > > > sure that they are all marked noinstr as needed. > > > > Ok I see... > > If need be, the outgoing-CPU transition to RCU-not-watching could be > delayed into arch-specific code. We already allow this for the incoming > transition. That's a lot of scary architectures code to handle :-) And how do we determine which place is finally safe to stop watching? Thanks. > > Thanx, Paul