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=-16.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 46818C47080 for ; Mon, 24 May 2021 03:46:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2358961151 for ; Mon, 24 May 2021 03:46:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232246AbhEXDsO (ORCPT ); Sun, 23 May 2021 23:48:14 -0400 Received: from mail.kernel.org ([198.145.29.99]:36464 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232132AbhEXDsM (ORCPT ); Sun, 23 May 2021 23:48:12 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 432E16024A; Mon, 24 May 2021 03:46:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1621828005; bh=tXCue5oDtxJNXtmLKNCXMQAsllqKJeAEtXjQYVXtDwo=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=VGvQZSWbMSudiLyCLjUtXFSZtADyeG5dOomugWnfHrwoqAbcgqd+b27U99kCzC9Y2 Tc7AwFJq/zI4rh/dI2RK2vK0SGc6K+ue9cJruIAGxJOzBrw6Zk8Vp+LyglNViyY7Ky 6p/T23Vw8jeh05a/XTxtfOp0QW1DSB7/xrjv/KvpraCs1unQSAX/qBxl8zMEPhtlCU PmAwr16fk3Atm0OosRFKGZ41lfne3Bv2j54d3faEXfL0IdqfXx8NcZ5UP0UaXWMcZF 53QeFe/jl7749NUuN2tsJ3jqb3Ub+gd3RWf5aGheY/g/vWFMSS/niWBcn/cerKd/i5 zLQ8d8S+hHe2w== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 0A56A5C018D; Sun, 23 May 2021 20:46:45 -0700 (PDT) Date: Sun, 23 May 2021 20:46:45 -0700 From: "Paul E. McKenney" To: Sergey Senozhatsky Cc: Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCHv2 1/2] rcu/tree: handle VM stoppage in stall detection Message-ID: <20210524034645.GH4441@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210521155624.174524-1-senozhatsky@chromium.org> <20210521180127.GD4441@paulmck-ThinkPad-P17-Gen-1> <20210521213855.GA3437356@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 Mon, May 24, 2021 at 10:56:44AM +0900, Sergey Senozhatsky wrote: > On (21/05/21 14:38), Paul E. McKenney wrote: > > > > And on that otherwise inexplicable refetch of the jiffies counter within > > check_cpu_stall(), the commit below makes it more effective. > > > > If check_cpu_stall() is delayed before or while printing the stall > > warning, we really want to wait the full time duration between the > > end of that stall warning and the start of the next one. > > > > Nice improvement! Thank you, glad you like it! > > Of course, if there is some way to learn whether printk() is overloaded, > > even more effective approaches could be taken. > > There is no better to do this. I was afraid of that. ;-) > > commit b9c5dc2856c1538ccf2d09246df2b58bede72cca > > Author: Paul E. McKenney > > Date: Fri May 21 14:23:03 2021 -0700 > > > > rcu: Start timing stall repetitions after warning complete > > > > Systems with low-bandwidth consoles can have very large printk() > > latencies, and on such systems it makes no sense to have the next RCU CPU > > stall warning message start output before the prior message completed. > > This commit therefore sets the time of the next stall only after the > > prints have completed. While printing, the time of the next stall > > message is set to ULONG_MAX/2 jiffies into the future. > > > > Signed-off-by: Paul E. McKenney > > FWIW, > > Reviewed-by: Sergey Senozhatsky Thank you again, I will apply this on my next rebase. > > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h > > index 05012a8081a1..ff239189a627 100644 > > --- a/kernel/rcu/tree_stall.h > > +++ b/kernel/rcu/tree_stall.h > > @@ -648,6 +648,7 @@ static void print_cpu_stall(unsigned long gps) > > > > static void check_cpu_stall(struct rcu_data *rdp) > > { > > + bool didstall = false; > > unsigned long gs1; > > unsigned long gs2; > > unsigned long gps; > > @@ -693,7 +694,7 @@ static void check_cpu_stall(struct rcu_data *rdp) > > ULONG_CMP_GE(gps, js)) > > return; /* No stall or GP completed since entering function. */ > > rnp = rdp->mynode; > > - jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3; > > + jn = jiffies + ULONG_MAX / 2; > > if (rcu_gp_in_progress() && > > (READ_ONCE(rnp->qsmask) & rdp->grpmask) && > > cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) { > > @@ -710,6 +711,7 @@ static void check_cpu_stall(struct rcu_data *rdp) > > print_cpu_stall(gps); > > if (READ_ONCE(rcu_cpu_stall_ftrace_dump)) > > rcu_ftrace_dump(DUMP_ALL); > > + didstall = true; > > > > } else if (rcu_gp_in_progress() && > > ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) && > > @@ -727,6 +729,11 @@ static void check_cpu_stall(struct rcu_data *rdp) > > print_other_cpu_stall(gs2, gps); > > if (READ_ONCE(rcu_cpu_stall_ftrace_dump)) > > rcu_ftrace_dump(DUMP_ALL); > > + didstall = true; > > + } > > + if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) { > > Can `rcu_state.jiffies_stall` change here? In theory, yes, sort of, anyway. In practice, highly unlikely. The most plausible way for this to happen is for this code path to be delayed for a long time on a 32-bit system, so that jiffies+ULONG_MAX/2 actually arrives. But in that case, all sorts of other complaints would happen first. But I could make this a cmpxchg(), if that is what you are getting at. Thanx, Paul > > + jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3; > > + WRITE_ONCE(rcu_state.jiffies_stall, jn); > > } > > }