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 07150C433EF for ; Fri, 10 Jun 2022 12:05:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243556AbiFJMFw (ORCPT ); Fri, 10 Jun 2022 08:05:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35250 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S245550AbiFJMFu (ORCPT ); Fri, 10 Jun 2022 08:05:50 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 339CF3F322; Fri, 10 Jun 2022 05:05:48 -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 ams.source.kernel.org (Postfix) with ESMTPS id 9C179B834DC; Fri, 10 Jun 2022 12:05:46 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D2574C3411E; Fri, 10 Jun 2022 12:05:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1654862745; bh=f9tCZ3dUGujHwZtgBBhgrGaWaeeMtU5w+k7egcKNfkU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=jR8trHrWTHYG7Z9ccS+GVbKyAVHH9oRp5ckreKkEj0zGsQL+sdaLJz/xs+JTOXidt +oEYLXGUcHLwrC8KnvQ8Ua2sMCxlC3qTniMSMxVVgVjRTH0AsnjchsRcXwmbvjNMLz 6k5SvVcwPRHnbw4bOV+Cyi3FNXjJzno+2ilZJU82Mi6tLJZ7CJcdxpM9WSTTdsvJF/ atC1RjwAyHrMA0l6ivT7fsoo9rVhuSvkHzKZJezQmPZOerlkPNxgfzcBehEnHmg7oA KDOe0t1Q1hhwJIJsWdeJpD8YbiA4r/r7fPAj63FCX/fbs14+yBZi3RIWXjTSHo7JeG hBYCeFocezpLQ== Date: Fri, 10 Jun 2022 14:05:42 +0200 From: Frederic Weisbecker To: Zqiang Cc: paulmck@kernel.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] rcu/nocb: Avoid polling when myrdp->nocb_head_rdp list is empty Message-ID: <20220610120542.GA1811980@lothringen> References: <20220607075057.909070-1-qiang1.zhang@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220607075057.909070-1-qiang1.zhang@intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 07, 2022 at 03:50:57PM +0800, Zqiang wrote: > Currently, If the 'rcu_nocb_poll' bootargs is enable, all rcuog kthreads > enter polling mode. however, due to only insert CPU's rdp which belong to > rcu_nocb_mask to 'nocb_head_rdp' list or all CPU's rdp served by rcuog > kthread have been de-offloaded, these cause the 'nocb_head_rdp' list > served by rcuog kthread is empty, when the 'nocb_head_rdp' is empty, > the rcuog kthread in polling mode not actually do anything. fix it by > exiting polling mode when the 'nocb_head_rdp'list is empty, otherwise > entering polling mode. > > Signed-off-by: Zqiang > --- > v1->v2: > Move rcu_nocb_poll flags check from rdp_offload_toggle() to > rcu_nocb_rdp_offload/deoffload(), avoid unnecessary setting of > rdp_gp->nocb_gp_sleep flags, because when rcu_nocb_poll is set > the rdp_gp->nocb_gp_sleep is not used. > > kernel/rcu/tree_nocb.h | 16 ++++++++++------ > 1 file changed, 10 insertions(+), 6 deletions(-) > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h > index fa8e4f82e60c..2a52c9abc681 100644 > --- a/kernel/rcu/tree_nocb.h > +++ b/kernel/rcu/tree_nocb.h > @@ -698,10 +698,14 @@ static void nocb_gp_wait(struct rcu_data *my_rdp) > TPS("WakeBypassIsDeferred")); > } > if (rcu_nocb_poll) { > - /* Polling, so trace if first poll in the series. */ > - if (gotcbs) > - trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll")); > - schedule_timeout_idle(1); > + if (list_empty(&my_rdp->nocb_head_rdp)) { > + rcu_wait(READ_ONCE(my_rdp->nocb_toggling_rdp)); I suspect you have based your patch on upstream tree which doesn't seem to have this one yet: "rcu/nocb: Add/del rdp to iterate from rcuog itself" With this patch you can't wait on changes to my_rdp->nocb_toggling_rdp because nocb_gp_wait() now performs the list_add/list_del itself. Please rebase your patch on top of latest rcu:dev from Paul's tree. Then all you need to do is to change the wait side, something like this (untested): diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index fa8e4f82e60c..f36d6be4f372 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -584,6 +584,15 @@ static int nocb_gp_toggle_rdp(struct rcu_data *rdp, return ret; } +static void nocb_gp_sleep(struct rdp *my_rdp, int cpu) +{ + trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep")); + swait_event_interruptible_exclusive(my_rdp->nocb_gp_wq, + !READ_ONCE(my_rdp->nocb_gp_sleep)); + trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EndSleep")); +} + + /* * No-CBs GP kthreads come here to wait for additional callbacks to show up * or for grace periods to end. @@ -701,13 +710,19 @@ static void nocb_gp_wait(struct rcu_data *my_rdp) /* Polling, so trace if first poll in the series. */ if (gotcbs) trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll")); - schedule_timeout_idle(1); + if (list_empty(&my_rdp->nocb_head_rdp)) { + raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags); + if (!my_rdp->nocb_toggling_rdp) + WRITE_ONCE(my_rdp->nocb_gp_sleep, true); + raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags); + /* Wait for any offloading rdp */ + rdp_gp_sleep(my_rdp, cpu); + } else { + schedule_timeout_idle(1); + } } else if (!needwait_gp) { /* Wait for callbacks to appear. */ - trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep")); - swait_event_interruptible_exclusive(my_rdp->nocb_gp_wq, - !READ_ONCE(my_rdp->nocb_gp_sleep)); - trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EndSleep")); + rdp_gp_sleep(my_rdp, cpu); } else { rnp = my_rdp->mynode; trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("StartWait"));