From patchwork Thu Jan 30 18:55:18 2025 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Paul E. McKenney" X-Patchwork-Id: 13954867 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 82A8F1F0E47; Thu, 30 Jan 2025 18:55:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738263324; cv=none; b=dYKDdwiFpzIDB5Oz2GWJi6kOnVeWYNizDocl3ujn/guvKxvQRq/Qe3dpKFHozcU2Ul1ft5tfxqqqanFdtnXN1pSHFWUXBECofyx0Ow9Rwy7KX9yFKPbPzOI6edIkVZGI49rPZZDoUj0Jhtsz/DHPjiPPlUbO0caz15Q9oy5Ivw4= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738263324; c=relaxed/simple; bh=6fgztZJ4uUwMxEH3zc+IRUO+CqURXhb/5/6czN/rBXc=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=oU/k164GsAi3/M7IYI4OVz+/P0+SG7uC4ytcd9C3vOWPUqS5nHQ1P6LWr9XxUSQhoO8tC1p2zayA0npNK5MCIfSauqdDYV8KVHd5JBuzgo3ygY0WllQKJhugggOv26gznTN8XN99jzvreETqI/E9Xi2v5SRTeQN+toOIRK++atk= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kJYY6V5h; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kJYY6V5h" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 08C09C4CEE4; Thu, 30 Jan 2025 18:55:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1738263324; bh=6fgztZJ4uUwMxEH3zc+IRUO+CqURXhb/5/6czN/rBXc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=kJYY6V5hMiFAhmPT4Vpwwnxwl0r93v+ZxUmVVljOG8FwMxckDlpf1A9kZuPiuDXtM jKLMzbO/XUTZdEjkNdCJ0I1jL7Lc4VHgDKudDcVQ3S5asNSXkmi27Zu/vYYlBYVcLN rPnnkQNIOFbNMAuX27astZAvqLcZZPW3BLhJqwo/dMXoCbEUMDbgGTL4g/afTMJ37+ lUFHr2SFDBmbLTdSnVzhTMG/y+D4d4ZCtCEtQll7oEEkDzGHjjbhCsbZmrR3lEUCRk N/qt0mVJNwlCW51TDjvSkfE+LuuuoyXBYkZMdT8fIgI0g1WeI0QpBbqUhpN3evR9mA hnNVgFKk4ewtQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 90B94CE37DF; Thu, 30 Jan 2025 10:55:23 -0800 (PST) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, Ankur Arora , "Paul E . McKenney" , Sebastian Andrzej Siewior , Frederic Weisbecker Subject: [PATCH rcu v2] 5/9] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y Date: Thu, 30 Jan 2025 10:55:18 -0800 Message-Id: <20250130185522.1652093-5-paulmck@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: <9bc6e5fe-d49f-495a-9f55-537ed97a3615@paulmck-laptop> References: <9bc6e5fe-d49f-495a-9f55-537ed97a3615@paulmck-laptop> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Ankur Arora With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent states for read-side critical sections via rcu_all_qs(). One reason why this was needed: lacking preempt-count, the tick handler has no way of knowing whether it is executing in a read-side critical section or not. With (PREEMPT_LAZY=y, PREEMPT_DYNAMIC=n), we get (PREEMPT_COUNT=y, PREEMPT_RCU=n). In this configuration cond_resched() is a stub and does not provide quiescent states via rcu_all_qs(). (PREEMPT_RCU=y provides this information via rcu_read_unlock() and its nesting counter.) So, use the availability of preempt_count() to report quiescent states in rcu_flavor_sched_clock_irq(). Suggested-by: Paul E. McKenney Reviewed-by: Sebastian Andrzej Siewior Signed-off-by: Ankur Arora Reviewed-by: Frederic Weisbecker Signed-off-by: Paul E. McKenney --- kernel/rcu/tree_plugin.h | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index b025e95ede05..f444e8d3f68b 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -984,13 +984,16 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp) */ static void rcu_flavor_sched_clock_irq(int user) { - if (user || rcu_is_cpu_rrupt_from_idle()) { + if (user || rcu_is_cpu_rrupt_from_idle() || + (IS_ENABLED(CONFIG_PREEMPT_COUNT) && + (preempt_count() == HARDIRQ_OFFSET))) { /* * Get here if this CPU took its interrupt from user - * mode or from the idle loop, and if this is not a - * nested interrupt. In this case, the CPU is in - * a quiescent state, so note it. + * mode, from the idle loop without this being a nested + * interrupt, or while not holding the task preempt count + * (with PREEMPT_COUNT=y). In this case, the CPU is in a + * quiescent state, so note it. * * No memory barrier is required here because rcu_qs() * references only CPU-local variables that other CPUs