From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E462D411FB1; Wed, 7 Oct 2026 20:45:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791405916; cv=none; b=bxalMroJG8mSZQYXy08ZC9isRdAewD1fJ5ISFB+ENF7nLCjlfNEvvSA2uVy0sJsm4IvnHgzdgtO0aEUmtGCFOmPcHEO83KimQ06s6GjehnF1iyvAdJhMyOF3lyKtiLNUNw0BG+CurTsi2fb4orPSAXMUgpv6DvJK/tebi4VrwR8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791405916; c=relaxed/simple; bh=ngzssN2UBUf0qOB2JuRMxV46RPxwQq+1FbIFEPDF3g4=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=ugchK2k6NcRfUPswcfBvbgIPlnMMVBmZq4+qLv0VfPce+pXQH4XujSQ2GpXr3AJnkdjCVQENYY4u89tq6T8eLoKcmESG3cTUdIoXcaLst09t1MIcVSB/Xf3i1mo0uZYZn8AzXxP4EJi/DgZoAT4j/q/iJJNOQxk0gHf4wvw/SzY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=VW85xQrh; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="VW85xQrh" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D8EAA1F0089A; Wed, 7 Oct 2026 20:45:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791405913; bh=ly9arS0/WlmuK91h3tr4oHgSbTiK3NPyiNaNFFRcmnQ=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=VW85xQrh91CW4fQrn8TMTFByLerCAXRedNC1KZtWF8OAfQ7Od5o2Y+7XkR206WedG UO6jSw+7LrCXYhsjMWFXPD4EkjLHiT5FgFqnWPYMGCMVB5+x5P504w9Uau9J9JJDSc AyYEgequPByde0hMdOGi59G7Mwfe9RxAkMH4SClcVtf8aM9STPD1vuuvRecXJEbjdd lQOOjuzxyuMHNzo50d333+qj3XbM0iviksYwcsfnrNr1CGCC7eeaPpXXLwRYP/Sz1D TMUUQFfP0kUOYpC5RKrVyqLMk9/uwFfuigLifZhw1+IpJCvqDtT5i74T2876zfLcjl ntv9EVkilt9mg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 389AACE2744; Wed, 7 Oct 2026 13:45:13 -0700 (PDT) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, "Paul E. McKenney" , Breno Leitao , David Dai Subject: [PATCH v2 09/10] rcu-tasks: Disable callback contend/collapse messages by default Date: Wed, 7 Oct 2026 13:45:10 -0700 Message-Id: <20261007204511.1982646-9-paulmck@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit New workloads can do large bursts of call_rcu_tasks() invocations in a short time period, followed by a quiet time period long enough to drain all of the callbacks, followed by another burst of call_rcu_tasks() invocations. This can cause RCU Tasks to switch back and forth between queuing callbacks only on CPU 0 (during quiet periods) and on all CPUs (during bursts). Which is fine. Except for the fact that each cycle from CPU-0-only to all-CPUs queuing and back generates three console messages, one announcing the shift to all-CPUs queuing, another announcing the start of the shift back to CPU-0-only queuing, and the third announcing completion of this shift after an RCU grace period. And these console messages can overrun console-log communications channels and obscure other console-message-based debugging information. And the only known use for these console messages is debugging RCU Tasks itself. This commit therefore adds a rcupdate.rcu_task_collapse_debug module parameter that defaults to false (suppressing these console messages). Those debugging or otherwise playing with RCU Tasks callback queuing auto-adjustment can set this parameter to the value true. [ paulmck: Apply Breno Leitao feedback. ] Reported-by: Breno Leitao Reported-by: David Dai Signed-off-by: Paul E. McKenney Reviewed-by: Breno Leitao --- Documentation/admin-guide/kernel-parameters.txt | 7 +++++++ kernel/rcu/tasks.h | 15 ++++++++++++--- 2 files changed, 19 insertions(+), 3 deletions(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 914b65ae94134..6cc6d45b59d68 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -6405,6 +6405,13 @@ Kernel parameters period to instead use normal non-expedited grace-period processing. + rcupdate.rcu_task_collapse_debug= [KNL] + Enable debugging prints that record when RCU Tasks + and RCU Tasks Trace expand to per-CPU callback + queuing and collapse back to CPU-0 queuing. + This is default-disabled due to the fact that + some workloads can make it quite noisy. + rcupdate.rcu_task_collapse_lim= [KNL] Set the maximum number of callbacks present at the beginning of a grace period that allows diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 627295396cd91..fcac7361ec51e 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -178,6 +178,8 @@ static int rcu_task_contend_lim __read_mostly = 100; module_param(rcu_task_contend_lim, int, 0444); static int rcu_task_collapse_lim __read_mostly = 10; module_param(rcu_task_collapse_lim, int, 0444); +static bool rcu_task_collapse_debug __read_mostly; +module_param(rcu_task_collapse_debug, bool, 0644); static int rcu_task_lazy_lim __read_mostly = 32; module_param(rcu_task_lazy_lim, int, 0444); @@ -390,7 +392,8 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func, WRITE_ONCE(rtp->percpu_enqueue_shift, 0); WRITE_ONCE(rtp->percpu_dequeue_lim, rcu_task_cpu_ids); smp_store_release(&rtp->percpu_enqueue_lim, rcu_task_cpu_ids); - pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name); + if (data_race(rcu_task_collapse_debug)) + pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name); } raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags); } @@ -511,7 +514,9 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp) smp_store_release(&rtp->percpu_enqueue_lim, 1); rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu(); gpdone = false; - pr_info("Starting switch %s to CPU-0 callback queuing.\n", rtp->name); + if (data_race(rcu_task_collapse_debug)) + pr_info("Starting switch %s to CPU-0 callback queuing.\n", + rtp->name); } raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags); } @@ -519,7 +524,9 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp) raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags); if (rtp->percpu_enqueue_lim < rtp->percpu_dequeue_lim) { WRITE_ONCE(rtp->percpu_dequeue_lim, 1); - pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name); + if (data_race(rcu_task_collapse_debug)) + pr_info("Completing switch %s to CPU-0 callback queuing.\n", + rtp->name); } if (rtp->percpu_dequeue_lim == 1) { for (cpu = rtp->percpu_dequeue_lim; cpu < rcu_task_cpu_ids; cpu++) { @@ -704,6 +711,8 @@ static void __init rcu_tasks_bootup_oddness(void) pr_info("\tTasks-RCU CPU stall info multiplier clamped to %d (rcu_task_stall_info_mult).\n", rtsimc); rcu_task_stall_info_mult = rtsimc; } + if (rcu_task_collapse_debug) + pr_info("\tTasks-RCU callback contend/collapse debug enabled.\n"); #endif /* #ifdef CONFIG_TASKS_RCU */ #ifdef CONFIG_TASKS_RCU pr_info("\tTrampoline variant of Tasks RCU enabled.\n"); -- 2.40.1