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=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 05F84C4363A for ; Wed, 28 Oct 2020 22:44:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B301D2072C for ; Wed, 28 Oct 2020 22:44:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1603925076; bh=yeuEHlayFmSooOIM4atMOm1X/2p7fFSFU1uBRzszkkg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=yL3ZaL3XNkMBvOd+FGn2Zm08WbECaVhgZluXpfPM16hupM+S3B9I64c8V2STc9mHl zS9oqYevjFoHEekvEv4n/GgmwJjtAbJIohU27BzS6Lhi9z+qjC2j63IHu9eiBkAcBT aN0prIfgChfGOL6+9liRzFFRtSlmECpZFc7MhYa8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389186AbgJ1Wod (ORCPT ); Wed, 28 Oct 2020 18:44:33 -0400 Received: from mail.kernel.org ([198.145.29.99]:53222 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389159AbgJ1WmX (ORCPT ); Wed, 28 Oct 2020 18:42:23 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-104-11.bvtn.or.frontiernet.net [50.39.104.11]) (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 A38EB247D4; Wed, 28 Oct 2020 20:15:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1603916154; bh=yeuEHlayFmSooOIM4atMOm1X/2p7fFSFU1uBRzszkkg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=hBUc6JcB0RsnYdPRgf4DTu6DEOetHuj7XfmsvGJ5Y5OUK7ACi488NbzzbGwCJKfEh rACnq/LMvlJDZzYy5pw4lyC10IPU2EJN6dS3KsOoIWgMZ6LhyXtMSCMbkR4jEsVQ2i i1oP/+mOgS6AIuGCAbj+Lrv/ubdHHHWYtXiR8dvw= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 469A835225B2; Wed, 28 Oct 2020 13:15:54 -0700 (PDT) Date: Wed, 28 Oct 2020 13:15:54 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, will@kernel.org, hch@lst.de, axboe@kernel.dk, chris@chris-wilson.co.uk, davem@davemloft.net, kuba@kernel.org, fweisbec@gmail.com, oleg@redhat.com, vincent.guittot@linaro.org Subject: Re: [RFC][PATCH v3 6/6] rcu/tree: Use irq_work_queue_remote() Message-ID: <20201028201554.GE3249@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20201028110707.971887448@infradead.org> <20201028111221.584884062@infradead.org> <20201028145428.GE2628@hirez.programming.kicks-ass.net> <20201028200243.GJ2651@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201028200243.GJ2651@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 28, 2020 at 09:02:43PM +0100, Peter Zijlstra wrote: > On Wed, Oct 28, 2020 at 03:54:28PM +0100, Peter Zijlstra wrote: > > On Wed, Oct 28, 2020 at 12:07:13PM +0100, Peter Zijlstra wrote: > > > AFAICT we only need/use irq_work_queue_on() on remote CPUs, since we > > > can directly access local state. So avoid the IRQ_WORK dependency and > > > use the unconditionally available irq_work_queue_remote(). > > > > > > This survives a number of TREE01 runs. > > > > OK, Paul mentioned on IRC that while it is extremely unlikely, this code > > does not indeed guarantee it will not try to IPI self. > > > > I'll try again. > > This is the best I could come up with.. :/ > > --- > Subject: rcu/tree: Use irq_work_queue_remote() > From: Peter Zijlstra > Date: Wed Oct 28 11:53:40 CET 2020 > > All sites that consume rcu_iw_gp_seq seem to have rcu_node lock held, > so setting it probably should too. Also the effect of self-IPI here > would be setting rcu_iw_gp_seq to the value we just set it to > (pointless) and clearing rcu_iw_pending, which we just set, so don't > set it. > > Passes TREE01. > > Signed-off-by: Peter Zijlstra (Intel) > --- > kernel/rcu/tree.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1308,14 +1308,16 @@ static int rcu_implicit_dynticks_qs(stru > resched_cpu(rdp->cpu); > WRITE_ONCE(rdp->last_fqs_resched, jiffies); > } > -#ifdef CONFIG_IRQ_WORK > + raw_spin_lock_rcu_node(rnp); The caller of rcu_implicit_dynticks_qs() already holds this lock. Please see the force_qs_rnp() function and its second call site, to which rcu_implicit_dynticks_qs() is passed as an argument. But other than that, this does look plausible. And getting rid of that #ifdef is worth something. ;-) Thanx, Paul > if (!rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq && > (rnp->ffmask & rdp->grpmask)) { > - rdp->rcu_iw_pending = true; > rdp->rcu_iw_gp_seq = rnp->gp_seq; > - irq_work_queue_on(&rdp->rcu_iw, rdp->cpu); > + if (likely(rdp->cpu != smp_processor_id())) { > + rdp->rcu_iw_pending = true; > + irq_work_queue_remote(rdp->cpu, &rdp->rcu_iw); > + } > } > -#endif > + raw_spin_unlock_rcu_node(rnp); > } > > return 0;