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 0BF54C88CB2 for ; Thu, 24 Aug 2023 13:29:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241279AbjHXN26 (ORCPT ); Thu, 24 Aug 2023 09:28:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59720 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241133AbjHXN2R (ORCPT ); Thu, 24 Aug 2023 09:28:17 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 671B6E78; Thu, 24 Aug 2023 06:28:14 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id E015566AFD; Thu, 24 Aug 2023 13:28:13 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 45369C433C8; Thu, 24 Aug 2023 13:28:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1692883693; bh=fOKlCHSaVPLOPPBaZK4SDgNZjhNH9UN9oiyiddAIt4I=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=OcLZe/StKYe9ABnIf2HXBFWSsQXdUCmeExfHwBWr1+DA/q6ZbbZYfNYcxY7efnK1q L1om8RwXQlMoYb1kC8B/HIkTBUxa0NL5WMWjGA9O/dwLKkVf9E1Qh2mGvu78AwAy6v UGBbkikDn2Sca7w0paUtzj5EZKFbBZqlyp4x1uJ/Q4mfR6GVEUF9U19ulKaZaNOp96 0H0yP3gTxe50+3D+7xMLn+l6pU73yD0cAW7CZ/Sku3IkW5xeUCwmGwDBsj2xaU+AVy vSZrZm2iCrQIAy0X8vVJ3QHtjqKfw9ora3UcdtVHHs2XOwUdDSq9Y9bpAJRUYYLUhS uBYM1Z1wrV7Uw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id D2F12CE035E; Thu, 24 Aug 2023 06:28:12 -0700 (PDT) Date: Thu, 24 Aug 2023 06:28:12 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Huacai Chen , Thomas Gleixner , Z qiang , Huacai Chen , Frederic Weisbecker , Neeraj Upadhyay , Josh Triplett , Boqun Feng , Ingo Molnar , John Stultz , Stephen Boyd , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Sergey Senozhatsky , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Binbin Zhou Subject: Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset() Message-ID: <69b47bd2-307d-41ae-ae5f-e18c7a6f0379@paulmck-laptop> Reply-To: paulmck@kernel.org References: <5777BD82-2C8D-4BAB-BDD3-C2C003DC57FB@joelfernandes.org> <87ttspct76.ffs@tglx> <03fe7084-0509-45fa-87ee-8f8705a221a6@paulmck-laptop> <16827b4e-9823-456d-a6be-157fbfae64c3@paulmck-laptop> <20230824130942.GA3810470@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20230824130942.GA3810470@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 24, 2023 at 01:09:42PM +0000, Joel Fernandes wrote: > On Thu, Aug 24, 2023 at 04:40:42AM -0700, Paul E. McKenney wrote: > > On Thu, Aug 24, 2023 at 10:50:41AM +0800, Huacai Chen wrote: > > > Hi, Paul, > > > > > > On Thu, Aug 24, 2023 at 6:41 AM Paul E. McKenney wrote: > > > > > > > > On Thu, Aug 24, 2023 at 12:03:25AM +0200, Thomas Gleixner wrote: > > > > > On Thu, Aug 17 2023 at 16:06, Huacai Chen wrote: > > > > > > On Thu, Aug 17, 2023 at 3:27 AM Joel Fernandes wrote: > > > > > >> > If do_update_jiffies_64() cannot be used in NMI context, > > > > > >> > > > > > >> Can you not make the jiffies update conditional on whether it is > > > > > >> called within NMI context? > > > > > > > > > > Which solves what? If KGDB has a breakpoint in the jiffies lock held > > > > > region then you still dead lock. > > > > > > > > > > >> I dislike that.. > > > > > > Is this acceptable? > > > > > > > > > > > > void rcu_cpu_stall_reset(void) > > > > > > { > > > > > > unsigned long delta; > > > > > > > > > > > > delta = nsecs_to_jiffies(ktime_get_ns() - ktime_get_coarse_ns()); > > > > > > > > > > > > WRITE_ONCE(rcu_state.jiffies_stall, > > > > > > jiffies + delta + rcu_jiffies_till_stall_check()); > > > > > > } > > > > > > > > > > > > This can update jiffies_stall without updating jiffies (but has the > > > > > > same effect). > > > > > > > > > > Now you traded the potential dead lock on jiffies lock for a potential > > > > > live lock vs. tk_core.seq. Not really an improvement, right? > > > > > > > > > > The only way you can do the above is something like the incomplete and > > > > > uncompiled below. NMI safe and therefore livelock proof time interfaces > > > > > exist for a reason. > > > > > > > > Just for completeness, another approach, with its own advantages > > > > and disadvantage, is to add something like ULONG_MAX/4 to > > > > rcu_state.jiffies_stall, but also set a counter indicating that this > > > > has been done. Then RCU's force-quiescent processing could decrement > > > > that counter (if non-zero) and reset rcu_state.jiffies_stall when it > > > > does reach zero. > > > > > > > > Setting the counter to three should cover most cases, but "live by the > > > > heuristic, die by the heuristic". ;-) > > > > > > > > It would be good to have some indication when gdb exited, but things > > > > like the gdb "next" command can make that "interesting" when applied to > > > > a long-running function. > > > > > > The original code is adding ULONG_MAX/2, so adding ULONG_MAX/4 may > > > make no much difference? The simplest way is adding 300*HZ, but Joel > > > dislikes that. > > > > I am not seeing the ULONG_MAX/2, so could you please point me to that > > original code? > > > > The advantage of ULONG_MAX/4 over ULONG_MAX/2 is that the time_after() > > and time_before() macros have ULONG_MAX/4 slop in either direction > > before giving you the wrong answer. You can get nearly the same result > > using ULONG_MAX/2, but it requires a bit more care. And even on 32-bit > > HZ=1000 systems, ULONG_MAX/4 gets you more than 12 days of gdb session > > or jiffies-update delay before you start getting false positives. > > > > Then things can be reset after (say) 3 calls to rcu_gp_fqs() and > > also the current reset at the beginning of a grace period, which > > is in record_gp_stall_check_time(). > > I like Paul's suggestion a lot except that if someone sets a breakpoint right > when the jiffies is being reset, so then we have to come back to doing > Thomas's suggestion. Please note that ULONG_MAX / 4 allows for jiffies not having been reset for more than 10 days on 32-bit systems and for many millions of years on 64-bit systems. ;-) > So maybe a combination of Paul's and Thomas's suggestions (of using > last_jiffies_update with the NMI-safe timestamp read) may work. I am absolutely not a fan of reworking all of the RCU CPU stall-warning code to use some other timebase, at least not without a very important reason to do so. Nothing mentioned in this thread even comes close to that level of importance. > > It would be better if RCU could get notified at both ends of the debug > > session, but given gdb commands such as "next", along with Thomas's > > point about gdb breakpoints being pretty much anywhere, this might or > > might not be so helpful in real life. But worth looking into. > > True, I was curious if rcu_cpu_stall_reset() can be called on a tickless > kernel as well before jiffies gets a chance to update, in which case I think > your suggestion of biasing the stall time and later resetting it would help a > lot for such situations. What code path can possibly invoke rcu_cpu_stall_reset() after an extended full-system nohz_full time period without first doing at least one context switch on the CPU that invokes rcu_cpu_stall_reset()? Thanx, Paul > thanks, > > - Joel > > > > Thanx, Paul > > > > > Huacai > > > > > > > > > > > Thanx, Paul > > > > > > > > > Thanks, > > > > > > > > > > tglx > > > > > --- > > > > > --- a/kernel/time/tick-sched.c > > > > > +++ b/kernel/time/tick-sched.c > > > > > @@ -51,6 +51,13 @@ struct tick_sched *tick_get_tick_sched(i > > > > > */ > > > > > static ktime_t last_jiffies_update; > > > > > > > > > > +unsigned long tick_estimate_stale_jiffies(void) > > > > > +{ > > > > > + ktime_t delta = ktime_get_mono_fast_ns() - READ_ONCE(last_jiffies_update); > > > > > + > > > > > + return delta < 0 ? 0 : div_s64(delta, TICK_NSEC); > > > > > +} > > > > > + > > > > > /* > > > > > * Must be called with interrupts disabled ! > > > > > */ > > > > > > > > > >