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 3225EC6FD1D for ; Mon, 20 Mar 2023 17:44:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229612AbjCTRoz (ORCPT ); Mon, 20 Mar 2023 13:44:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60698 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229497AbjCTRoV (ORCPT ); Mon, 20 Mar 2023 13:44:21 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 363241D921; Mon, 20 Mar 2023 10:40:14 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D2C8EFEC; Mon, 20 Mar 2023 10:40:57 -0700 (PDT) Received: from FVFF77S0Q05N.cambridge.arm.com (FVFF77S0Q05N.cambridge.arm.com [10.1.35.35]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6BB4A3F67D; Mon, 20 Mar 2023 10:40:12 -0700 (PDT) Date: Mon, 20 Mar 2023 17:40:01 +0000 From: Mark Rutland To: Peter Zijlstra Cc: Paul McKenney , linux-kernel@vger.kernel.org, frederic@kernel.org, quic_neeraju@quicinc.com, josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, joel@joelfernandes.org, rcu@vger.kernel.org Subject: Re: [PATCH] rcu: Remove RCU_NONIDLE() Message-ID: References: <20230320173751.GU2194297@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230320173751.GU2194297@hirez.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 20, 2023 at 06:37:51PM +0100, Peter Zijlstra wrote: > > Since there are now exactly _zero_ users of RCU_NONIDLE(), make it go > away before someone else decides to (ab)use it. > > Signed-off-by: Peter Zijlstra (Intel) Acked-by: Mark Rutland Mark. > --- > .../RCU/Design/Requirements/Requirements.rst | 36 +--------------------- > Documentation/RCU/whatisRCU.rst | 1 - > include/linux/rcupdate.h | 25 --------------- > 3 files changed, 1 insertion(+), 61 deletions(-) > > diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst > index 49387d823619..77155b51d4c2 100644 > --- a/Documentation/RCU/Design/Requirements/Requirements.rst > +++ b/Documentation/RCU/Design/Requirements/Requirements.rst > @@ -2071,41 +2071,7 @@ call. > > Because RCU avoids interrupting idle CPUs, it is illegal to execute an > RCU read-side critical section on an idle CPU. (Kernels built with > -``CONFIG_PROVE_RCU=y`` will splat if you try it.) The RCU_NONIDLE() > -macro and ``_rcuidle`` event tracing is provided to work around this > -restriction. In addition, rcu_is_watching() may be used to test > -whether or not it is currently legal to run RCU read-side critical > -sections on this CPU. I learned of the need for diagnostics on the one > -hand and RCU_NONIDLE() on the other while inspecting idle-loop code. > -Steven Rostedt supplied ``_rcuidle`` event tracing, which is used quite > -heavily in the idle loop. However, there are some restrictions on the > -code placed within RCU_NONIDLE(): > - > -#. Blocking is prohibited. In practice, this is not a serious > - restriction given that idle tasks are prohibited from blocking to > - begin with. > -#. Although nesting RCU_NONIDLE() is permitted, they cannot nest > - indefinitely deeply. However, given that they can be nested on the > - order of a million deep, even on 32-bit systems, this should not be a > - serious restriction. This nesting limit would probably be reached > - long after the compiler OOMed or the stack overflowed. > -#. Any code path that enters RCU_NONIDLE() must sequence out of that > - same RCU_NONIDLE(). For example, the following is grossly > - illegal: > - > - :: > - > - 1 RCU_NONIDLE({ > - 2 do_something(); > - 3 goto bad_idea; /* BUG!!! */ > - 4 do_something_else();}); > - 5 bad_idea: > - > - > - It is just as illegal to transfer control into the middle of > - RCU_NONIDLE()'s argument. Yes, in theory, you could transfer in > - as long as you also transferred out, but in practice you could also > - expect to get sharply worded review comments. > +``CONFIG_PROVE_RCU=y`` will splat if you try it.) > > It is similarly socially unacceptable to interrupt an ``nohz_full`` CPU > running in userspace. RCU must therefore track ``nohz_full`` userspace > diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst > index 2c5563a91998..c3b1cbfa1530 100644 > --- a/Documentation/RCU/whatisRCU.rst > +++ b/Documentation/RCU/whatisRCU.rst > @@ -1117,7 +1117,6 @@ in docbook. Here is the list, by category. > > RCU_LOCKDEP_WARN > rcu_sleep_check > - RCU_NONIDLE > > All: Unchecked RCU-protected pointer access:: > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index 094321c17e48..ddd42efc6224 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -156,31 +156,6 @@ static inline int rcu_nocb_cpu_deoffload(int cpu) { return 0; } > static inline void rcu_nocb_flush_deferred_wakeup(void) { } > #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */ > > -/** > - * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers > - * @a: Code that RCU needs to pay attention to. > - * > - * RCU read-side critical sections are forbidden in the inner idle loop, > - * that is, between the ct_idle_enter() and the ct_idle_exit() -- RCU > - * will happily ignore any such read-side critical sections. However, > - * things like powertop need tracepoints in the inner idle loop. > - * > - * This macro provides the way out: RCU_NONIDLE(do_something_with_RCU()) > - * will tell RCU that it needs to pay attention, invoke its argument > - * (in this example, calling the do_something_with_RCU() function), > - * and then tell RCU to go back to ignoring this CPU. It is permissible > - * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is > - * on the order of a million or so, even on 32-bit systems). It is > - * not legal to block within RCU_NONIDLE(), nor is it permissible to > - * transfer control either into or out of RCU_NONIDLE()'s statement. > - */ > -#define RCU_NONIDLE(a) \ > - do { \ > - ct_irq_enter_irqson(); \ > - do { a; } while (0); \ > - ct_irq_exit_irqson(); \ > - } while (0) > - > /* > * Note a quasi-voluntary context switch for RCU-tasks's benefit. > * This is a macro rather than an inline function to avoid #include hell.