From nobody Fri Apr 3 02:22:57 2026 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 F0B43ECAAA1 for ; Sat, 17 Sep 2022 16:42:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229733AbiIQQmm (ORCPT ); Sat, 17 Sep 2022 12:42:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42308 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229695AbiIQQmg (ORCPT ); Sat, 17 Sep 2022 12:42:36 -0400 Received: from mail-qv1-xf29.google.com (mail-qv1-xf29.google.com [IPv6:2607:f8b0:4864:20::f29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6AEB82E685 for ; Sat, 17 Sep 2022 09:42:33 -0700 (PDT) Received: by mail-qv1-xf29.google.com with SMTP id i15so18942024qvp.5 for ; Sat, 17 Sep 2022 09:42:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date; bh=YtXTLA3M+nWqodgxztJaMgLvSRyU03TOMEk16miDVSg=; b=ShOLZGx75Ii8OwDydiUuECJDaVrzXu7dm1LvXWu1uMAHXy8OWxy3+sY1KgJKUVy9sc lJGUEgu7gVKBhWs0Wz0ZhBtUO3JLjqMoq0kpgajDLoDILUKTL2HipbaT+ktS1fFKI787 zPzc24o/oNUffNEGEUH9lEkxMlvjJIA+zFR6g= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date; bh=YtXTLA3M+nWqodgxztJaMgLvSRyU03TOMEk16miDVSg=; b=RI4etayul/Fe+e0dZef6Cb9+Luujiv46tedd1dcMSE+fwzPNf0NwQyzjk3/e+9DEGD 3MTv85yJlF5yjHV1gYu5UVtwiUmnmJXICqgB9i4zu4YfEfwPhWhiV9PoAAWPvxwtJxrR Jat55L27fZg3joJgUSlLJSH84wQtfLXY6cvm3u6LZRlSQ3wpwTL/95FcanWq/Igq8KcF Bcewf2dxVHYBfKJO+M2AgWF6Jn5WWwVa+f6AGFzc39sDxUfnERFxYjM4gZ0cnFk2IOgk QNdrZWbX/Lzbm6aSOEp9mZvlCdwltTEb+FXRK2KZhjsIh8TNQkjGVgmOfG9rJHcBdlzQ Rnwg== X-Gm-Message-State: ACrzQf3riaLD3Qx7gGxwZerJGlLH8LNUeXd2UXXCZnZ96su5lzObFY85 AELkHZO2qOaT6eqCdTb8p+f5Nw== X-Google-Smtp-Source: AMsMyM4IYHlTU6MQt7EpgMPbONgN5h3lvYXkOHlxQ4yG2OYh9ORFH9JAXc2T1IMbliGqy+f9+mm10Q== X-Received: by 2002:a05:6214:1d21:b0:4ad:1361:befa with SMTP id f1-20020a0562141d2100b004ad1361befamr6419573qvd.111.1663432952613; Sat, 17 Sep 2022 09:42:32 -0700 (PDT) Received: from joelboxx.c.googlers.com.com (228.221.150.34.bc.googleusercontent.com. [34.150.221.228]) by smtp.gmail.com with ESMTPSA id q31-20020a05620a2a5f00b006bb0e5ca4bbsm9479239qkp.85.2022.09.17.09.42.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 17 Sep 2022 09:42:32 -0700 (PDT) From: "Joel Fernandes (Google)" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, rushikesh.s.kadam@intel.com, urezki@gmail.com, neeraj.iitr10@gmail.com, frederic@kernel.org, paulmck@kernel.org, rostedt@goodmis.org, "Joel Fernandes (Google)" Subject: [PATCH rcu/next 1/3] rcu/tree: Use READ_ONCE() for lockless read of rnp->qsmask Date: Sat, 17 Sep 2022 16:41:58 +0000 Message-Id: <20220917164200.511783-2-joel@joelfernandes.org> X-Mailer: git-send-email 2.37.3.968.ga6b4b080e4-goog In-Reply-To: <20220917164200.511783-1-joel@joelfernandes.org> References: <20220917164200.511783-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" The rnp->qsmask is locklessly accessed from rcutree_dying_cpu(). This may help avoid load tearing due to concurrent access, KCSAN issues, and preserve sanity of people reading the mask in tracing. Reviewed-by: Frederic Weisbecker Signed-off-by: Joel Fernandes (Google) --- kernel/rcu/tree.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 0ca21ac0f064..5ec97e3f7468 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2106,7 +2106,7 @@ int rcutree_dying_cpu(unsigned int cpu) if (!IS_ENABLED(CONFIG_HOTPLUG_CPU)) return 0; =20 - blkd =3D !!(rnp->qsmask & rdp->grpmask); + blkd =3D !!(READ_ONCE(rnp->qsmask) & rdp->grpmask); trace_rcu_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq), blkd ? TPS("cpuofl-bgp") : TPS("cpuofl")); return 0; --=20 2.37.3.968.ga6b4b080e4-goog From nobody Fri Apr 3 02:22:57 2026 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 08971ECAAA1 for ; Sat, 17 Sep 2022 16:42:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229748AbiIQQmp (ORCPT ); Sat, 17 Sep 2022 12:42:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42316 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229706AbiIQQmg (ORCPT ); Sat, 17 Sep 2022 12:42:36 -0400 Received: from mail-qv1-xf2d.google.com (mail-qv1-xf2d.google.com [IPv6:2607:f8b0:4864:20::f2d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 23B7B2F678 for ; Sat, 17 Sep 2022 09:42:34 -0700 (PDT) Received: by mail-qv1-xf2d.google.com with SMTP id l14so10485462qvq.8 for ; Sat, 17 Sep 2022 09:42:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date; bh=W20sVr4rHsYJL/FKcEAa6dXGu1RHDwQdwaDFAlx3ULo=; b=uHIj9ohx6d/ikjQUH9MJsZfu/j/LooTs088y2mOsZ7N/UW6p5VqrtfTdkxJu4dTLO9 dI+OHDQizo/UIdP8y7bRLnQMGJcr0bulMfrIjDxsnsn1rINWrRQ7Qc7W+R6iba6C9DUL /mZGnML0/7+6LWpSZGZC5FgZlRWwDkLl3avUQ= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date; bh=W20sVr4rHsYJL/FKcEAa6dXGu1RHDwQdwaDFAlx3ULo=; b=qhVbgU/+ZmqiydsJNW7XbTREgZTDQLGGZRI31JUQmjz5EFSx2w/bsr93NMMgtQp6Ny nyfYgAuRFxOZAFjSFDZcuReAJZCN8VGflhqZ3p8zp6cuDM+qiPGWFFOBpUNJhA3b/c0c AZzJttzggdg+AL8AlTUWNcv04NFlt92Lzsovo5eQ1aKRTxQ1IbI7naQnkcmX4C0qX2GI 1A/YEok2EAU8IK1+Bo7J4bs33TrybTTaLxBE3ChmKe/e6adsB8IjQUMExQjJ5Ni/Uc7N qHQNwRT9fQRWhRq7Ba5EPeVL9jnxzo8p1nyjXgi0AYXMoKtsmthn5cYbSO2/qp6sTrhf mZig== X-Gm-Message-State: ACrzQf02ZBr07z7k8Y2zLqViRBVCA1eFeM4A+ZJVEW0yakU0jy5RaS4B Xysj1oIeyTZozkFrlKvV3fzxcA== X-Google-Smtp-Source: AMsMyM6CVm//iVr2UMSIXomdq/E4kcyBPnoaIHoLb5h5yloPlLl+1C4yIDBz1aMUo9ZUdvIXPLlYig== X-Received: by 2002:a0c:f084:0:b0:49e:7bc1:26a3 with SMTP id g4-20020a0cf084000000b0049e7bc126a3mr8842257qvk.107.1663432953226; Sat, 17 Sep 2022 09:42:33 -0700 (PDT) Received: from joelboxx.c.googlers.com.com (228.221.150.34.bc.googleusercontent.com. [34.150.221.228]) by smtp.gmail.com with ESMTPSA id q31-20020a05620a2a5f00b006bb0e5ca4bbsm9479239qkp.85.2022.09.17.09.42.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 17 Sep 2022 09:42:32 -0700 (PDT) From: "Joel Fernandes (Google)" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, rushikesh.s.kadam@intel.com, urezki@gmail.com, neeraj.iitr10@gmail.com, frederic@kernel.org, paulmck@kernel.org, rostedt@goodmis.org, "Joel Fernandes (Google)" Subject: [PATCH rcu/next 2/3] rcu: Fix late wakeup when flush of bypass cblist happens (v6) Date: Sat, 17 Sep 2022 16:41:59 +0000 Message-Id: <20220917164200.511783-3-joel@joelfernandes.org> X-Mailer: git-send-email 2.37.3.968.ga6b4b080e4-goog In-Reply-To: <20220917164200.511783-1-joel@joelfernandes.org> References: <20220917164200.511783-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" When the bypass cblist gets too big or its timeout has occurred, it is flushed into the main cblist. However, the bypass timer is still running and the behavior is that it would eventually expire and wake the GP thread. Since we are going to use the bypass cblist for lazy CBs, do the wakeup soon as the flush for "too big or too long" bypass list happens. Otherwise, long delays can happen for callbacks which get promoted from lazy to non-lazy. This is a good thing to do anyway (regardless of future lazy patches), since it makes the behavior consistent with behavior of other code paths where flushing into the ->cblist makes the GP kthread into a non-sleeping state quickly. [ Frederic Weisbec: changes to not do wake up GP thread unless needed, comment changes ]. Reviewed-by: Frederic Weisbecker Signed-off-by: Joel Fernandes (Google) --- kernel/rcu/tree_nocb.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index 0a5f0ef41484..04c87f250e01 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -433,8 +433,9 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, s= truct rcu_head *rhp, if ((ncbs && j !=3D READ_ONCE(rdp->nocb_bypass_first)) || ncbs >=3D qhimark) { rcu_nocb_lock(rdp); + *was_alldone =3D !rcu_segcblist_pend_cbs(&rdp->cblist); + if (!rcu_nocb_flush_bypass(rdp, rhp, j)) { - *was_alldone =3D !rcu_segcblist_pend_cbs(&rdp->cblist); if (*was_alldone) trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FirstQ")); @@ -447,7 +448,12 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, = struct rcu_head *rhp, rcu_advance_cbs_nowake(rdp->mynode, rdp); rdp->nocb_gp_adv_time =3D j; } - rcu_nocb_unlock_irqrestore(rdp, flags); + + // The flush succeeded and we moved CBs into the regular list. + // Don't wait for the wake up timer as it may be too far ahead. + // Wake up the GP thread now instead, if the cblist was empty. + __call_rcu_nocb_wake(rdp, *was_alldone, flags); + return true; // Callback already enqueued. } =20 --=20 2.37.3.968.ga6b4b080e4-goog From nobody Fri Apr 3 02:22:57 2026 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 BF838ECAAA1 for ; Sat, 17 Sep 2022 16:42:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229724AbiIQQmt (ORCPT ); Sat, 17 Sep 2022 12:42:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42310 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229714AbiIQQmh (ORCPT ); Sat, 17 Sep 2022 12:42:37 -0400 Received: from mail-qk1-x72e.google.com (mail-qk1-x72e.google.com [IPv6:2607:f8b0:4864:20::72e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B0932303E3 for ; Sat, 17 Sep 2022 09:42:34 -0700 (PDT) Received: by mail-qk1-x72e.google.com with SMTP id h28so17920412qka.0 for ; Sat, 17 Sep 2022 09:42:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date; bh=LbqcEwrWK8xzGA015vucIgdiMEo2clH8LFFcAywvHQQ=; b=WyMg2FBbm/t2DCQzBs99+MvCJHW0x8rLhbPX/j2RGxjVYxVYRycoEUDAFecP1EX/fT PBhwMWhxq/aF+d2r1RTwRn4QXPhdGt495Yrkfd2kaQ6mz625FYHGPGTA4+tmP9W3cKNf 0qnLlsyWq/TNJi0sw1SE2aJ99lvvIKJkHci5M= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date; bh=LbqcEwrWK8xzGA015vucIgdiMEo2clH8LFFcAywvHQQ=; b=GIlonWvS0aY9lWgk2zqSgLCS5ywOf0dRTL8nmGPkEequY9xXR6v9zuUVoIJVxhPWz5 BVycgQPGneEOEgVJ5XBKMb7fDTyfsjN28DYt1pXqA5yVJk7MPFyXh9Pnh/JY67xt4hTJ J1dac/8mw04kpVXJXJvdY5E7kR/VQNd6AHyow14HPh0TgvqGgjCVKESK1eNVbGd0xQ/g 5TO9wnrYUNkwx1lmj0u+1F/HQO1iVgMGEJplJ8Wj1cGdk10Ia7GY3IZmqLik/aoIK2mQ K7Ww8IxGJMscMEF2X3lV0ug/o+kbmXGDV05BdK1s3mdcWv7boMxjwY6AJfVUONxlybK0 WKgQ== X-Gm-Message-State: ACrzQf39arL42dpz5Tgw7IvDavE765lOgGwELD5KF1ptqWx3HLFjuv7B xmNbM6jgfAvgV90BG+2uXULackA4qop33w== X-Google-Smtp-Source: AMsMyM7joNQtb0/LD1ROJWXNvXDA7dYnaBUzWEFaVKlNWDRFSERJUYlikz42Yc+wRk0MtQOPEq6Hwg== X-Received: by 2002:a05:620a:2681:b0:6b5:b60c:1e66 with SMTP id c1-20020a05620a268100b006b5b60c1e66mr7841048qkp.99.1663432953813; Sat, 17 Sep 2022 09:42:33 -0700 (PDT) Received: from joelboxx.c.googlers.com.com (228.221.150.34.bc.googleusercontent.com. [34.150.221.228]) by smtp.gmail.com with ESMTPSA id q31-20020a05620a2a5f00b006bb0e5ca4bbsm9479239qkp.85.2022.09.17.09.42.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 17 Sep 2022 09:42:33 -0700 (PDT) From: "Joel Fernandes (Google)" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, rushikesh.s.kadam@intel.com, urezki@gmail.com, neeraj.iitr10@gmail.com, frederic@kernel.org, paulmck@kernel.org, rostedt@goodmis.org, "Joel Fernandes (Google)" Subject: [PATCH rcu/next 3/3] rcu: Call trace_rcu_callback() also for bypass queuing (v2) Date: Sat, 17 Sep 2022 16:42:00 +0000 Message-Id: <20220917164200.511783-4-joel@joelfernandes.org> X-Mailer: git-send-email 2.37.3.968.ga6b4b080e4-goog In-Reply-To: <20220917164200.511783-1-joel@joelfernandes.org> References: <20220917164200.511783-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" If any callback is queued into the bypass list, then trace_rcu_callback() does not show it. This makes it unclear when a callback was actually queued, as the resulting trace only includes a rcu_invoke_callback event. Fix it by calling the tracing function even if queuing into bypass. This is needed for the future rcutop tool which monitors enqueuing of callbacks. Note that, in case of bypass queuing, the new tracing happens without the nocb_lock. This should be OK since on CONFIG_RCU_NOCB_CPU systems, the total callbacks is represented by an atomic counter. Also, other paths like rcu_barrier() also sample the total number of callback without the nocb_lock. Also, while at it, optimize the tracing so that rcu_state is not accessed if tracing is disabled, because that's useless if we are not tracing. A quick inspection of the generated assembler shows that rcu_state is accessed even if the jump label for the tracepoint is disabled. Here is gcc -S output of the bad asm (note that I un-inlined it just for testing and illustration however the final __trace_rcu_callback in the patch is marked static inline): __trace_rcu_callback: movq 8(%rdi), %rcx movq rcu_state+3640(%rip), %rax movq %rdi, %rdx cmpq $4095, %rcx ja .L3100 movq 192(%rsi), %r8 1:jmp .L3101 # objtool NOPs this .pushsection __jump_table, "aw" .balign 8 .long 1b - . .long .L3101 - . .quad __tracepoint_rcu_kvfree_callback+8 + 2 - . .popsection With this change, the jump label check which is NOOPed is moved to the beginning of the function. Signed-off-by: Joel Fernandes (Google) --- kernel/rcu/tree.c | 30 ++++++++++++++++++++++-------- 1 file changed, 22 insertions(+), 8 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 5ec97e3f7468..18f07e167d5e 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2728,6 +2728,22 @@ static void check_cb_ovld(struct rcu_data *rdp) raw_spin_unlock_rcu_node(rnp); } =20 +/* + * Trace RCU callback helper, call after enqueuing callback. + */ +static inline void __trace_rcu_callback(struct rcu_head *head, + struct rcu_data *rdp) +{ + if (trace_rcu_kvfree_callback_enabled() && + __is_kvfree_rcu_offset((unsigned long)head->func)) + trace_rcu_kvfree_callback(rcu_state.name, head, + (unsigned long)head->func, + rcu_segcblist_n_cbs(&rdp->cblist)); + else if (trace_rcu_callback_enabled()) + trace_rcu_callback(rcu_state.name, head, + rcu_segcblist_n_cbs(&rdp->cblist)); +} + /** * call_rcu() - Queue an RCU callback for invocation after a grace period. * @head: structure to be used for queueing the RCU updates. @@ -2809,17 +2825,15 @@ void call_rcu(struct rcu_head *head, rcu_callback_t= func) } =20 check_cb_ovld(rdp); - if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags)) + + if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags)) { + __trace_rcu_callback(head, rdp); return; // Enqueued onto ->nocb_bypass, so just leave. + } + // If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock. rcu_segcblist_enqueue(&rdp->cblist, head); - if (__is_kvfree_rcu_offset((unsigned long)func)) - trace_rcu_kvfree_callback(rcu_state.name, head, - (unsigned long)func, - rcu_segcblist_n_cbs(&rdp->cblist)); - else - trace_rcu_callback(rcu_state.name, head, - rcu_segcblist_n_cbs(&rdp->cblist)); + __trace_rcu_callback(head, rdp); =20 trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued")); =20 --=20 2.37.3.968.ga6b4b080e4-goog