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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 88D64C433F5 for ; Thu, 31 Mar 2022 23:13:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242878AbiCaXPE (ORCPT ); Thu, 31 Mar 2022 19:15:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43590 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242867AbiCaXPC (ORCPT ); Thu, 31 Mar 2022 19:15:02 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0A2C724B5C9 for ; Thu, 31 Mar 2022 16:13:14 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 9827A6152D for ; Thu, 31 Mar 2022 23:13:13 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id ED6A3C340ED; Thu, 31 Mar 2022 23:13:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1648768393; bh=ki8Jcuqk/wwUM9+LuyWgTTyWSrQjUthx1u+VD8E3pZQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=Nm83YWZGcHxcvZezZl8gLrjWLsrCcodUjMJfLLTUK90RAH/s3Akn/DWKzJCQRHo/l ohvleongY23a925PbJBLKyEvk/gWXiFMgvW6eBSgYtM5TOsgAPyx7btOwaec705jjw sKOYr/FkgKYAIlVKU3UYhbsgdDfbqzyW1CiWN5lGjr2rvOFJo/Zgb7Y21NEJ6bWGG3 DAuPvhiIEzhT443a5EuTNPyYzm65dxYZYYvBrjtWt0e2g9VwSqISKYFrVaEXpWUM3+ 1Ow5b87PpzuikXf9UuijJzKhFTbn24D23uvq5MVpxQ6mW2zOpQRKDkZeXj3bf6yMeb dV/0kjNatKYvQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 892015C0A0E; Thu, 31 Mar 2022 16:13:12 -0700 (PDT) Date: Thu, 31 Mar 2022 16:13:12 -0700 From: "Paul E. McKenney" To: Eric Dumazet Cc: LKML Subject: Re: [BUG] rcu-tasks : should take care of sparse cpu masks Message-ID: <20220331231312.GA4285@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220331224222.GY4285@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 31, 2022 at 03:57:36PM -0700, Eric Dumazet wrote: > On Thu, Mar 31, 2022 at 3:54 PM Eric Dumazet wrote: > > > > On Thu, Mar 31, 2022 at 3:42 PM Paul E. McKenney wrote: > > > > > > On Thu, Mar 31, 2022 at 02:45:25PM -0700, Eric Dumazet wrote: > > > > Hi Paul > > > > > > > > It seems you assume per cpu ptr for arbitrary indexes (< nr_cpu_ids) are valid. > > > > > > Gah! I knew I was forgetting something... > > > > > > But just to check, is this a theoretical problem or something you hit > > > on real hardware? (For the rest of this email, I am assuming the latter.) > > > > Code review really... > > > > > > > > > What do you think of the (untested) following patch ? > > > > > > One issue with this patch is that the contention could be unpredictable, > > > or worse, vary among CPU, especially if the cpu_possible_mask was oddly > > > distributed. > > > > > > So might it be better to restrict this to all on CPU 0 on the one hand > > > and completely per-CPU on the other? (Or all on the boot CPU, in case > > > I am forgetting some misbegotten architecture that can run without a > > > CPU 0.) > > > > If I understand correctly, cblist_init_generic() could setup > > percpu_enqueue_shift > > to something smaller than order_base_2(nr_cpu_ids) > > > > Meaning that we could reach a non zero idx in (smp_processor_id() >> > > percpu_enqueue_shift) > > > > So even if CPU0 is always present (I am not sure this is guaranteed, > > but this seems reasonable), > > we could still attempt a per_cpu_ptr(PTR, not_present_cpu), and get garbage. > > Also you mention CPU 0, but I do not see where cpu binding is > performed on the kthread ? The initial setting of ->percpu_enqueue_shift forces all in-range CPU IDs to shift down to zero. The grace-period kthread is allowed to run where it likes. The callback lists are protected by locking, even in the case of local access, so this should be safe. Or am I missing your point? Thanx, Paul > > > > Thanks. > > > > > > > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h > > > > index 99cf3a13954cfb17828fbbeeb884f11614a526a9..df3785be4022e903d9682dd403464aa9927aa5c2 > > > > 100644 > > > > --- a/kernel/rcu/tasks.h > > > > +++ b/kernel/rcu/tasks.h > > > > @@ -273,13 +273,17 @@ static void call_rcu_tasks_generic(struct > > > > rcu_head *rhp, rcu_callback_t func, > > > > bool needadjust = false; > > > > bool needwake; > > > > struct rcu_tasks_percpu *rtpcp; > > > > + int ideal_cpu, chosen_cpu; > > > > > > > > rhp->next = NULL; > > > > rhp->func = func; > > > > local_irq_save(flags); > > > > rcu_read_lock(); > > > > - rtpcp = per_cpu_ptr(rtp->rtpcpu, > > > > - smp_processor_id() >> > > > > READ_ONCE(rtp->percpu_enqueue_shift)); > > > > + > > > > + ideal_cpu = smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift); > > > > + chosen_cpu = cpumask_next(ideal_cpu - 1, cpu_online_mask); > > > > + > > > > + rtpcp = per_cpu_ptr(rtp->rtpcpu, chosen_cpu); > > > > if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled. > > > > raw_spin_lock_rcu_node(rtpcp); // irqs already disabled. > > > > j = jiffies;