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McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang Cc: rcu@vger.kernel.org Subject: [PATCH RFC v1 1/6] rcu: introduce rcu_defer_qs_clear() helper Date: Fri, 22 May 2026 10:23:35 -0400 Message-Id: <20260522142342.1536533-2-joelagnelf@nvidia.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260522142342.1536533-1-joelagnelf@nvidia.com> References: <20260522142342.1536533-1-joelagnelf@nvidia.com> Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: MN2PR01CA0051.prod.exchangelabs.com (2603:10b6:208:23f::20) To DS0PR12MB6486.namprd12.prod.outlook.com (2603:10b6:8:c5::21) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR12MB6486:EE_|DS4PR12MB9793:EE_ X-MS-Office365-Filtering-Correlation-Id: 9e61e10a-3a0f-4cae-eead-08deb80dc3e0 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|366016|1800799024|921020|56012099003|22082099003|18002099003|11063799006; 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charset="utf-8" Currently rdp->defer_qs_pending transitions from DEFER_QS_PENDING to DEFER_QS_IDLE at two sites: rcu_preempt_deferred_qs_irqrestore() and rcu_preempt_deferred_qs_handler() (depth>0 reset). Both write the IDLE value directly. Introduce a single inline helper rcu_defer_qs_clear() in tree.h and route both sites through it. This becomes the single PENDING->IDLE transition point for upcoming work. Signed-off-by: Joel Fernandes --- kernel/rcu/tree.h | 5 +++++ kernel/rcu/tree_plugin.h | 4 ++-- 2 files changed, 7 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 7dfc57e9adb1..4069132f9d44 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -296,6 +296,11 @@ struct rcu_data { int cpu; }; =20 +static inline void rcu_defer_qs_clear(struct rcu_data *rdp) +{ + WRITE_ONCE(rdp->defer_qs_pending, DEFER_QS_IDLE); +} + /* Values for nocb_defer_wakeup field in struct rcu_data. */ #define RCU_NOCB_WAKE_NOT 0 #define RCU_NOCB_WAKE_BYPASS 1 diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 95ad967adcf3..8637f405cb47 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -488,7 +488,7 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *= t, unsigned long flags) =20 rdp =3D this_cpu_ptr(&rcu_data); if (rdp->defer_qs_pending =3D=3D DEFER_QS_PENDING) - rdp->defer_qs_pending =3D DEFER_QS_IDLE; + rcu_defer_qs_clear(rdp); =20 /* * If RCU core is waiting for this CPU to exit its critical section, @@ -645,7 +645,7 @@ static void rcu_preempt_deferred_qs_handler(struct irq_= work *iwp) * 5. 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McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang Cc: rcu@vger.kernel.org Subject: [PATCH RFC v1 2/6] rcu: clear defer_qs_pending when notifying GP changes Date: Fri, 22 May 2026 10:23:36 -0400 Message-Id: <20260522142342.1536533-3-joelagnelf@nvidia.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260522142342.1536533-1-joelagnelf@nvidia.com> References: <20260522142342.1536533-1-joelagnelf@nvidia.com> Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: MN2PR05CA0044.namprd05.prod.outlook.com (2603:10b6:208:236::13) To DS0PR12MB6486.namprd12.prod.outlook.com (2603:10b6:8:c5::21) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR12MB6486:EE_|MN0PR12MB5786:EE_ X-MS-Office365-Filtering-Correlation-Id: 46022ab6-c9c2-4cb5-c147-08deb80dc5ed X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016|921020|11063799006|5023799004|22082099003|56012099003|18002099003; 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charset="utf-8" Prior to this commit, defer_qs_pending was an unbalanced flag: rcu_read_unlock_special() set it to PENDING whenever a deferred-QS mechanism was scheduled, but the clear paths did not cover every up-tree quiescent-state reporting site. In those cases the flag stays PENDING after the QS is reported, and rcu_read_unlock_special()'s pending-gate then silently rejects all future arming attempts. A test patch confirms TREE03 can have get into the problematic stuck state very quickly (< 5 minutes). Clear the flag in __note_gp_changes(), right after the nothing-to-do early return. This is the natural per-CPU "GP transitioned, sync local state" hook, called from the GP-kthread's rcu_gp_init()/rcu_gp_cleanup() paths, and other GP advancement paths. For dynticks-idle CPUs, they do not call __note_gp_changes(), but they also do not arm new PENDING work (no readers running), and on wake-up, note_gp_changes() is called before any new reader runs. Signed-off-by: Joel Fernandes --- kernel/rcu/tree.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 55df6d37145e..d0816468ffee 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1281,6 +1281,8 @@ static bool __note_gp_changes(struct rcu_node *rnp, s= truct rcu_data *rdp) if (rdp->gp_seq =3D=3D rnp->gp_seq) return false; /* Nothing to do. */ =20 + rcu_defer_qs_clear(rdp); + /* Handle the ends of any preceding grace periods first. */ if (rcu_seq_completed_gp(rdp->gp_seq, rnp->gp_seq) || unlikely(rdp->gpwrap)) { --=20 2.34.1 From nobody Sun May 24 19:33:55 2026 Received: from DM5PR21CU001.outbound.protection.outlook.com (mail-centralusazon11011070.outbound.protection.outlook.com [52.101.62.70]) (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 C5E654219E3; Fri, 22 May 2026 14:24:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=fail smtp.client-ip=52.101.62.70 ARC-Seal: i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779459850; cv=fail; b=Id1CEtIGa8jTYbx6mh+32Y2UGDXXIJQLIcFGqoNizYcQqOlCQcyefaEDOpnAOv3EdnIBTFVp7ZA80DpZV2mmTJpIVsjBCNik7nXa1SDXDvCvBnMGDa6zwWZBUNVrjZzn9M6DUzgo+EhOJFJfbgsmFt8NB+gMTXxzWhhIox1h6m4= ARC-Message-Signature: i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779459850; c=relaxed/simple; bh=fbRjpXyN/E2RaoqyvhlPNuYovKwltiVbHlD9OAiwTP0=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: Content-Type:MIME-Version; b=haU0e5zNgCDH19WfvqiWfdEt6nIIc41VMXjwWhlhQL795e6gHn+QGVCR7SbChdk7ln5rbq4Gr+F48UWU5rn604yGjJCEkfS3TDXmzWrJnw4Nilyjt506suegmOXM6MfjW8ATffGRI0VlV2u7YYZ3k7HnKXnc5iC7nfc2YkNVsBA= ARC-Authentication-Results: i=2; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com; spf=fail smtp.mailfrom=nvidia.com; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b=dKKzalxY; arc=fail smtp.client-ip=52.101.62.70 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=nvidia.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b="dKKzalxY" ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=MIKSGopHFMomoUQYbNJERMG2FqIqYnJKQFlwJOSsTs0AYC6VcK7LPPgjXB78EIuFbHDvvlDz2xmMfHIqdKylDXN6a+0xwzfKXuC8NsR0P4YYFTlNtEdQA73WlgVPoJ98JO4OpN5aE10SPecHFzI2Pb6AN9+Y2TRWKlx1++mrDBoEQBoJKJ7z4iruQ/ewq59+jn2+Q6kDxoDvSWHumKSKQOVKfVkiEvAT4zLCWqqyCs0bAa1GoWlAUZymuJw125R08F/2ljPwq/xOVHjRV3/34Jv7qQM5aiJRcVGwPFUbsyYc/9Jos1bQaBiZ+19VdwPzzSURMZxHLlwhYOkMhXQcMw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector10001; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1; bh=2R2eohlshkyhTS7BzkyvjP9idrWHREPtQXCoqfqD+AE=; b=ZcKI884g0Bn4HzYGoSsyCkKZInT0uUO8m0Gk6T+4ef06dc9j8Q7G1902krSuAflDfxI3Nfrn7F6pAlSLxD8O1B9ATGZKqomzOVGJa/iWDvMq8jUlOVxLZt4ds1klSvozt07OSFiC2anqqoKz8uKZ4vyzcuZTcShSNDZfYziXFjJAwzvPR5m2Yrja0kH/QBsns0AQf17uND4/oIDIuKk++QuzBWHY2cPMJrDVFyDKUCIw0daUUrYTsl4uxfHHbImCpr/0l8M8vvS7pN5UPIX376ncH9QzRa6q0unR7SbbtnIgs12PVxolPUo/IIMDfkKfVycYsGt1XoXHcy14KYNMBQ== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=nvidia.com; dmarc=pass action=none header.from=nvidia.com; dkim=pass header.d=nvidia.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=Nvidia.com; s=selector2; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=2R2eohlshkyhTS7BzkyvjP9idrWHREPtQXCoqfqD+AE=; b=dKKzalxYjfhpMQDE0hDizXqpFkW7hljIphXNtY0YMMw14Qyi6ys8Bz8+qR8ClzOHOzk/jXZD9KvZfrdssQHrLcDG5EZURLZ5xHhfhZj+ozZUql2ttJlu48EmRXiUkaNNm9csEEsWrEM9dNwXKJIeOFLOuBJP9E7UQGk1VqnKYj6fCCuP7qr0Plbhdgxc0E8f3iWCSr67emq62BT/qvMBd/iDpSoK1h429bpv6luBAnAzOXAHkjPH0caHETtSkUKvgWZ0TZ6zsbOvzdSgP42zOtNxNGFbvwz3XczGH53eYio6IZjAe6I3t7s5WXYBNXVA+hOIYxLd4oxyUsqyxVuPJw== Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=nvidia.com; Received: from DS0PR12MB6486.namprd12.prod.outlook.com (2603:10b6:8:c5::21) by DS4PR12MB9793.namprd12.prod.outlook.com (2603:10b6:8:2a3::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.48.14; Fri, 22 May 2026 14:24:05 +0000 Received: from DS0PR12MB6486.namprd12.prod.outlook.com ([fe80::88a9:f314:c95f:8b33]) by DS0PR12MB6486.namprd12.prod.outlook.com ([fe80::88a9:f314:c95f:8b33%6]) with mapi id 15.21.0048.016; Fri, 22 May 2026 14:24:05 +0000 From: Joel Fernandes To: linux-kernel@vger.kernel.org, "Paul E. 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charset="utf-8" The deferred-QS irq-work handler previously cleared defer_qs_pending only when the handler ran inside an active rcu_read_lock() critical section (rcu_preempt_depth() > 0). Paul McKenney pointed out a common multi-segment compound pattern where the handler fires between segments and segment N+1's arming attempt is silently suppressed by the rcu_read_unlock_special() pending-gate: rcu_read_lock(); // segment 1 starts // may be preempted/boosted here local_irq_disable(); rcu_read_unlock(); // segment 1 ends; arms defer_qs_pending preempt_disable(); local_irq_enable(); // handler MAY fire here: depth=3D=3D0, but // but preempt is disabled, so it cant // nudge. rcu_read_lock(); // segment 2 starts preempt_enable(); local_irq_disable(); rcu_read_unlock(); // arming attempt suppressed incorrectly -- (1) local_irq_enable(); Waiting for the next __note_gp_changes() clear is too slow for the compound case, we need the deferred QS report sooner. Therefore, make the irq_work handler clears defer_qs_pending whenever rcu_in_compounded_section() is true so that (1) can do the arming. Signed-off-by: Joel Fernandes --- kernel/rcu/tree_plugin.h | 26 ++++++++++++++------------ 1 file changed, 14 insertions(+), 12 deletions(-) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 8637f405cb47..2da009dbe64c 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -621,6 +621,17 @@ notrace void rcu_preempt_deferred_qs(struct task_struc= t *t) rcu_preempt_deferred_qs_irqrestore(t, flags); } =20 +/* + * True if the current context is inside a compounded RCU read-side + * section, i.e. either in an active rcu_read_lock() (depth>0) or in an + * outer preempt-disabled / BH-disabled scope. + */ +static inline bool rcu_in_compounded_section(void) +{ + return rcu_preempt_depth() > 0 || + (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) !=3D 0; +} + /* * Minimal handler to give the scheduler a chance to re-evaluate. */ @@ -632,19 +643,10 @@ static void rcu_preempt_deferred_qs_handler(struct ir= q_work *iwp) rdp =3D container_of(iwp, struct rcu_data, defer_qs_iw); =20 /* - * If the IRQ work handler happens to run in the middle of RCU read-side - * critical section, it could be ineffective in getting the scheduler's - * attention to report a deferred quiescent state (the whole point of the - * IRQ work). For this reason, requeue the IRQ work. - * - * Basically, we want to avoid following situation: - * 1. rcu_read_unlock() queues IRQ work (state -> DEFER_QS_PENDING) - * 2. CPU enters new rcu_read_lock() - * 3. IRQ work runs but cannot report QS due to rcu_preempt_depth() > 0 - * 4. rcu_read_unlock() does not re-queue work (state still PENDING) - * 5. Deferred QS reporting does not happen. + * Clear defer_qs_pending when the handler fires inside a compounded + * section as we may need to rearm the irq_work. */ - if (rcu_preempt_depth() > 0) + if (rcu_in_compounded_section()) rcu_defer_qs_clear(rdp); } =20 --=20 2.34.1 From nobody Sun May 24 19:33:55 2026 Received: from CH4PR04CU002.outbound.protection.outlook.com (mail-northcentralusazon11013038.outbound.protection.outlook.com [40.107.201.38]) (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 8FF274534A2; Fri, 22 May 2026 14:24:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=fail smtp.client-ip=40.107.201.38 ARC-Seal: i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779459854; cv=fail; b=lG30AH268h3gGGfEVMrWkhiR+zi6xdtRNzAsELiPEL6t6XptjaVoYKI0BgSgi+bPfQW/71a3f8MppBMkPO7COphlRllEPEOGJy190ltID8zveqI8iLi/VsBUpkQivcsebSPNZiYFmPT2veq6ftXcOgyqScqjOpGaU7rNyBmCKhg= ARC-Message-Signature: i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779459854; c=relaxed/simple; bh=hsxPzI+av6oaL8CBIfvbAcFT672ooILCBEN5mfn38x8=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: Content-Type:MIME-Version; b=jh6nuuRBkdS0yPrvga7kmhgGR37lQjtN+R8oH2DgxGUSZsQY0kBjWLic5B+F5Bftlu4jnSiN1RFLzgP+r0UiAwgXy9rlcYKs01cPmjuvwDcoXRwkO7DPongL/UxQHarh9IYBxnw9tkG37/DMpbMb3gO52ph4KlAHcCXm0I1Eqmg= ARC-Authentication-Results: i=2; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com; spf=fail smtp.mailfrom=nvidia.com; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b=PgP6eNe5; arc=fail smtp.client-ip=40.107.201.38 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=nvidia.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b="PgP6eNe5" ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=QK/BbnrqPA+Z0MF7ANqpTskmeg9ZcbLs1PxdSlxgCx8C+4MHp2LP6MImhCS66jxvJhHG4fvYxLqbVQJWf8JjzN7EyEzHzS4+tfUky6Tkxq8yC3Z30m4l8hA6Jbdb7MhuGUhdoay/Mx7UcFGbLK7kpy2h6HxEu6g1B5CpytB0ZOzyQEMV+wZz8tBsqabmp4rACCwgqdFgatW0dsNWR3sabssC0NI3vJb8AldAHlpnoa/jBfYgcTJ13QnPm7NNAxIGL0xv8VdkVy2wisoIb0JaeUHeXtR8frEmlj3akrGUa/F++njWU0Yjrql3msqV1NVU9hrg0EXqX1VIzX1LbiW+lQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector10001; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1; bh=NOTcX9evTxo1FRB9J0hTdp18eLGUmzDvBQomLqMeZMY=; b=iAxvK4Ff9Vfq2cc0EKWrPgmN8XrxggQ8C8KTW4vDRfjLrmXU9ZlgYXcfkdD1loH3ZkvLH1OGJbpbJoJzDFIILIE1pQSdkzUru6We7hT1DiubhQrrRie51fRdSJsmOR49sFJtNhY4ww1Y3185WXhY8hXr/AaSDl20T92kRJgUtQmIt5zkSZMRZhZg1QV1h+AbuYI3lyzWD7YPlwsAe38bgT3kQ0yg2QS505LASobKyrbtho5FkgZPM2Xc1fLgN0f5chYqonxNS2SuUFgprQ3m3McLCTv8e9V3TQzC3ivNB+njST6SFtv+8CQtnjYkk92MdGRa9mrc93AScUReV7kv0w== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=nvidia.com; dmarc=pass action=none header.from=nvidia.com; dkim=pass header.d=nvidia.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=Nvidia.com; s=selector2; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=NOTcX9evTxo1FRB9J0hTdp18eLGUmzDvBQomLqMeZMY=; b=PgP6eNe5AUAU9QzTWoV9HQ4ZaNdIDArUHZArsi29DyzGwIGhHVO5U/lOKvWlCrQRlrW/dQ8rnFj4xG93wc5dsd+Ku+qgfCk/F5YqdkHy5JyLyjaUnAWFJcWxLNEh0TPJ3NN64p29wgE1VQ/MSReYIwIx28VW5Qv4/zMT2wkGjb0E5iI6VSI0l/S01ot+j7WViqVEq9Z0ZN1xGsHkckQizNTAXbpyh2Lb73E8kAiGOd/x6RGKN9pt0MJuMTFePv+G762XyRinkBKrfIudRya+LDOlt+OqZfnlP2PSwScbvNmCmf5Y9LTAe7lZY9hBNq8K09iajY9C1ZBd5yopoefq8Q== Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=nvidia.com; Received: from DS0PR12MB6486.namprd12.prod.outlook.com (2603:10b6:8:c5::21) by MN0PR12MB5786.namprd12.prod.outlook.com (2603:10b6:208:375::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.48.17; Fri, 22 May 2026 14:24:08 +0000 Received: from DS0PR12MB6486.namprd12.prod.outlook.com ([fe80::88a9:f314:c95f:8b33]) by DS0PR12MB6486.namprd12.prod.outlook.com ([fe80::88a9:f314:c95f:8b33%6]) with mapi id 15.21.0048.016; Fri, 22 May 2026 14:24:07 +0000 From: Joel Fernandes To: linux-kernel@vger.kernel.org, "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang Cc: rcu@vger.kernel.org Subject: [PATCH RFC v1 4/6] rcu: drop redundant defer_qs_pending clear in irqrestore handler Date: Fri, 22 May 2026 10:23:38 -0400 Message-Id: <20260522142342.1536533-5-joelagnelf@nvidia.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260522142342.1536533-1-joelagnelf@nvidia.com> References: <20260522142342.1536533-1-joelagnelf@nvidia.com> Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: BL1PR13CA0285.namprd13.prod.outlook.com (2603:10b6:208:2bc::20) To DS0PR12MB6486.namprd12.prod.outlook.com (2603:10b6:8:c5::21) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR12MB6486:EE_|MN0PR12MB5786:EE_ X-MS-Office365-Filtering-Correlation-Id: 2b11e240-b558-4ac5-82a0-08deb80dc885 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016|921020|11063799006|22082099003|56012099003|18002099003; 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charset="utf-8" With __note_gp_changes() now clearing defer_qs_pending at every per-CPU GP advance, the per-irqrestore clear is redundant. Remove it. Effect: PENDING now stays set from arming until the next per-CPU GP advance, future arming attempts on the same CPU within the same GP are gated by the rcu_read_unlock_special(). This serves both as an optimization (should not need new irq_work again this GP - for the compounded section case, we detect and clear it there), and reduces risks of recursion due to clearing too aggressively. Signed-off-by: Joel Fernandes --- kernel/rcu/tree_plugin.h | 2 -- 1 file changed, 2 deletions(-) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 2da009dbe64c..bb5f955a06df 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -487,8 +487,6 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *= t, unsigned long flags) union rcu_special special; =20 rdp =3D this_cpu_ptr(&rcu_data); - if (rdp->defer_qs_pending =3D=3D DEFER_QS_PENDING) - rcu_defer_qs_clear(rdp); =20 /* * If RCU core is waiting for this CPU to exit its critical section, --=20 2.34.1 From nobody Sun May 24 19:33:55 2026 Received: from CH4PR04CU002.outbound.protection.outlook.com (mail-northcentralusazon11013038.outbound.protection.outlook.com [40.107.201.38]) (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 1D8544611C7; Fri, 22 May 2026 14:24:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=fail smtp.client-ip=40.107.201.38 ARC-Seal: i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779459856; cv=fail; b=WJVKvLKN7zCa4qowBq7z8S8Eac1e6iKwCpf0m/MeuCOffWuZ+x2lvW/MjG50IbaYIy4ubvnoUMN6IPQ3dh2qiIU9SOYBu5CIR3kLmaGl8hJ/7LngUSuQsA0d9j8zj3UTLrduck3uMY+icbqLhkADj/lT3JfBASdC/yigbUZeUiY= ARC-Message-Signature: i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779459856; c=relaxed/simple; bh=i5ccyOM6NQzmv+Oq+eBJFpH1UlwsRjLK8QAgVcA9JUg=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: Content-Type:MIME-Version; b=H6H8y3jVtmg8aQaORrea2UVFdPdEafCRj6tHqqB4g1OIcgN3Alp9oTd4lGlDkk9brsdCFKkZbNdg7rcY4oB8Krs3qYAwCrxjcvJe8//ZI66m6maDK/mWggN8ErcJpqxufutHzVkx/0QwpfGTRDa3PMTuYNu4FhqKYq7uzsFYHd8= ARC-Authentication-Results: i=2; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com; spf=fail smtp.mailfrom=nvidia.com; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b=Ng1p9Mpl; arc=fail smtp.client-ip=40.107.201.38 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=nvidia.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b="Ng1p9Mpl" ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=h1NbjcdZ67sSUkurKaQPxLvikQNNYIFuER0wKd9Py2BvOU1Qx/vlL8Bw9DfD/EEE31c1s2ToEaTrWoFCi8uiSY05FtX/ENZiKG125NGbIodY/nxQYP1ZnuQXdnsn3eoJs5oyGbGd6GBzF30VNF5iqbYJceGj8jqayRmMky/YtX0ml3UozzfDKCnp9MuV0lomHIgE8Z/y+n+SpH9TL0KrcDeKZjv4PKJX0Rgu5Uej60BFTAUE3jlDibj6kxC3xmtSUUJjXeog1o9730p5CXraW771LTjp7FP8yzUvBQjDrtY+q6AnoW5Hy73rbXOFF7wovCz/iTf+LUu6Ej7aWIkh7A== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector10001; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1; bh=w8caone8W+O1koWf1I0fbJ31miabks0BK1rNItW5rdw=; b=Qgr3QQ3R4E0uQqxYukV374lKvbKKCSRDz7/3RNfzN36yEr5cnYe7St7VlhNHZuxWwypOCPPls4vRha2G6QJyTTtpX4JfInRtOKeCON7m4I9mpf58iqPoQuRTywB4+mAaiQWUratsyu/SZu6cwn5hNq4/wv6vAOGVjiPv0TPplVWrRaCX7BQPSpON4W15PFy17/nDaTqlcyf29sLkUfUfkz1X6IhUbDuju5AN1GqOBMFgDFlZB8VkMNGASbK+qAvwu0L9G7OsQegwGk4TJDfWraDook00/W4WM+olwt2iP+YQCD+XGEWirLqii5waZUNKngdemsVpX86rAjW1a4wW4A== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=nvidia.com; dmarc=pass action=none header.from=nvidia.com; dkim=pass header.d=nvidia.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=Nvidia.com; s=selector2; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=w8caone8W+O1koWf1I0fbJ31miabks0BK1rNItW5rdw=; b=Ng1p9Mplqx0zlt/+b955yjz+nUOqX5m31NxTKDD6sDsfk42dv85J5oDs89XGvBq9cW0LPRGm8xdHPC5yl8zBcRRocUngCBEi233BSyjMsqa6yHTMpE8vNQS/Kchqw34X3y64OjtoIcTctfi/vxOcUuTmncoDY4+atFg58CLyXE/i8NcHzvN9FxmCC62drHqirSZ2XyXzpsfL1E2wwby2jKhJBzuh5PSukC/4S5yOlM6XNiZyDw4vsBwSLth/C0nAMhzMcICXXtpl7dPc4zxfab5IR1dKjGF0odZA7ZiXeAYfeKwZI3t1mQochikVbPw+36GHyjWNujJQ3egFMKJt5A== Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=nvidia.com; Received: from DS0PR12MB6486.namprd12.prod.outlook.com (2603:10b6:8:c5::21) by MN0PR12MB5786.namprd12.prod.outlook.com (2603:10b6:208:375::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.48.17; Fri, 22 May 2026 14:24:09 +0000 Received: from DS0PR12MB6486.namprd12.prod.outlook.com ([fe80::88a9:f314:c95f:8b33]) by DS0PR12MB6486.namprd12.prod.outlook.com ([fe80::88a9:f314:c95f:8b33%6]) with mapi id 15.21.0048.016; Fri, 22 May 2026 14:24:09 +0000 From: Joel Fernandes To: linux-kernel@vger.kernel.org, "Paul E. 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charset="utf-8" The per-CPU clear in __note_gp_changes() (from an earlier commit) runs only when the local CPU notices a normal-GP advance. When an expedited GP arrives at a CPU whose defer_qs_pending is already PENDING, rcu_read_unlock_special() may skip irq_work queuing due to the pending gate. Clear defer_qs_pending on the IPI target right in rcu_exp_handler(). This makes it possible for any arming attempt that follows the IPI within the current GP to be able to queue irq_work again, allowing completion expedited GPs quickly than waiting for one scheduler tick. Signed-off-by: Joel Fernandes --- kernel/rcu/tree_exp.h | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 82cada459e5d..f8564a041879 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -763,6 +763,12 @@ static void rcu_exp_handler(void *unused) READ_ONCE(rdp->cpu_no_qs.b.exp))) return; =20 + /* + * Clear defer_qs_pending so arming attempts following this IPI + * within the current GP can queue irq_work again. + */ + rcu_defer_qs_clear(rdp); + /* * Second, the common case of not being in an RCU read-side * critical section. 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charset="utf-8" Debug-only stuck-state detector for upstream review. This commit is NOT for merge; it is a review aid. Reviewers can enable CONFIG_RCU_GP_CLEANUP_STALE_CHECK to gain runtime confidence in the preceding fix commits. Signed-off-by: Joel Fernandes --- kernel/rcu/Kconfig.debug | 11 ++++++++++ kernel/rcu/tree.c | 47 ++++++++++++++++++++++++++++++++++++++++ kernel/rcu/tree.h | 8 +++++++ 3 files changed, 66 insertions(+) diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug index 35218ba74eb5..5a40c4fe544c 100644 --- a/kernel/rcu/Kconfig.debug +++ b/kernel/rcu/Kconfig.debug @@ -98,6 +98,17 @@ config RCU_TORTURE_TEST_LOG_GP Say Y here if you want grace-period sequence numbers logged. Say N if you are unsure. =20 +config RCU_GP_CLEANUP_STALE_CHECK + bool "Detect stuck defer_qs_pending state at GP cleanup" + depends on RCU_TORTURE_TEST + default n + help + This option adds a per-CPU instrumentation counter on every + PENDING -> IDLE transition of rdp->defer_qs_pending, and a + detector in rcu_gp_cleanup(). + + Say N if you are unsure. + config RCU_REF_SCALE_TEST tristate "Scalability tests for read-side synchronization (RCU and others= )" depends on DEBUG_KERNEL diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index d0816468ffee..1307f3fb48ac 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2146,6 +2146,52 @@ static noinline_for_stack void rcu_gp_fqs_loop(void) } } =20 +#ifdef CONFIG_RCU_GP_CLEANUP_STALE_CHECK +/* + * Threshold of consecutive GPs with rdp->defer_qs_pending stuck at + * PENDING and no observed PENDING -> IDLE transition before WARN. + */ +#define RCU_DEFER_QS_STUCK_GPS_THRESHOLD 5 + +static void rcu_gp_cleanup_stale_check(void) +{ + int cpu; + unsigned long cur_gp_seq =3D READ_ONCE(rcu_state.gp_seq); + + for_each_online_cpu(cpu) { + struct rcu_data *rdp =3D per_cpu_ptr(&rcu_data, cpu); + s64 clears_now; + int p_now; + + if (READ_ONCE(rdp->gp_seq) !=3D cur_gp_seq) { + rdp->defer_qs_pending_stuck_gps =3D 0; + rdp->defer_qs_pending_clears_snap =3D + atomic64_read(&rdp->defer_qs_pending_clears); + continue; + } + + clears_now =3D atomic64_read(&rdp->defer_qs_pending_clears); + p_now =3D READ_ONCE(rdp->defer_qs_pending); + + if (p_now !=3D DEFER_QS_PENDING || + clears_now !=3D rdp->defer_qs_pending_clears_snap) { + rdp->defer_qs_pending_stuck_gps =3D 0; + rdp->defer_qs_pending_clears_snap =3D clears_now; + continue; + } + + rdp->defer_qs_pending_stuck_gps++; + WARN_ONCE(rdp->defer_qs_pending_stuck_gps >=3D + RCU_DEFER_QS_STUCK_GPS_THRESHOLD, + "RCU: defer_qs_pending STUCK on CPU %d for %u GPs (gp_seq=3D%lu, clears= =3D%lld)\n", + cpu, rdp->defer_qs_pending_stuck_gps, + cur_gp_seq, clears_now); + } +} +#else +static inline void rcu_gp_cleanup_stale_check(void) { } +#endif /* CONFIG_RCU_GP_CLEANUP_STALE_CHECK */ + /* * Clean up after the old grace period. */ @@ -2220,6 +2266,7 @@ static noinline void rcu_gp_cleanup(void) =20 /* Declare grace period done, trace first to use old GP number. */ trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("end")); + rcu_gp_cleanup_stale_check(); rcu_seq_end(&rcu_state.gp_seq); ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq); WRITE_ONCE(rcu_state.gp_state, RCU_GP_IDLE); diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 4069132f9d44..29d852bbe218 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -204,6 +204,11 @@ struct rcu_data { /* period it is aware of. */ struct irq_work defer_qs_iw; /* Obtain later scheduler attention. */ int defer_qs_pending; /* irqwork or softirq pending? */ +#ifdef CONFIG_RCU_GP_CLEANUP_STALE_CHECK + atomic64_t defer_qs_pending_clears; + s64 defer_qs_pending_clears_snap; + unsigned int defer_qs_pending_stuck_gps; +#endif struct work_struct strict_work; /* Schedule readers for strict GPs. */ =20 /* 2) batch handling */ @@ -299,6 +304,9 @@ struct rcu_data { static inline void rcu_defer_qs_clear(struct rcu_data *rdp) { WRITE_ONCE(rdp->defer_qs_pending, DEFER_QS_IDLE); +#ifdef CONFIG_RCU_GP_CLEANUP_STALE_CHECK + atomic64_inc(&rdp->defer_qs_pending_clears); +#endif } =20 /* Values for nocb_defer_wakeup field in struct rcu_data. */ --=20 2.34.1