linux-kernel - [PATCH v6 9/9] blk-mq: prevent offlining hk CPU with associated online isolated CPUs

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Message-Id: <20250424-isolcpus-io-queues-v6-9-9a53a870ca1f@kernel.org>
Date: Thu, 24 Apr 2025 20:19:48 +0200
From: Daniel Wagner <wagi@...nel.org>
To: Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>, 
 Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>, 
 "Michael S. Tsirkin" <mst@...hat.com>
Cc: "Martin K. Petersen" <martin.petersen@...cle.com>, 
 Thomas Gleixner <tglx@...utronix.de>, 
 Costa Shulyupin <costa.shul@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, 
 Valentin Schneider <vschneid@...hat.com>, Waiman Long <llong@...hat.com>, 
 Ming Lei <ming.lei@...hat.com>, Frederic Weisbecker <frederic@...nel.org>, 
 Mel Gorman <mgorman@...e.de>, Hannes Reinecke <hare@...e.de>, 
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
 linux-kernel@...r.kernel.org, linux-block@...r.kernel.org, 
 linux-nvme@...ts.infradead.org, megaraidlinux.pdl@...adcom.com, 
 linux-scsi@...r.kernel.org, storagedev@...rochip.com, 
 virtualization@...ts.linux.dev, GR-QLogic-Storage-Upstream@...vell.com, 
 Daniel Wagner <wagi@...nel.org>
Subject: [PATCH v6 9/9] blk-mq: prevent offlining hk CPU with associated
 online isolated CPUs

When isolcpus=io_queue is enabled, and the last housekeeping CPU for a
given hctx would go offline, there would be no CPU left which handles
the IOs. To prevent IO stalls, prevent offlining housekeeping CPUs which
are still severing isolated CPUs..

Signed-off-by: Daniel Wagner <wagi@...nel.org>
---
 block/blk-mq.c | 46 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c2697db591091200cdb9f6e082e472b829701e4c..aff17673b773583dfb2b01cb2f5f010c456bd834 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3627,6 +3627,48 @@ static bool blk_mq_hctx_has_requests(struct blk_mq_hw_ctx *hctx)
 	return data.has_rq;
 }

+static bool blk_mq_hctx_check_isolcpus_online(struct blk_mq_hw_ctx *hctx, unsigned int cpu)
+{
+	const struct cpumask *hk_mask;
+	int i;
+
+	if (!housekeeping_enabled(HK_TYPE_IO_QUEUE))
+		return true;
+
+	hk_mask = housekeeping_cpumask(HK_TYPE_IO_QUEUE);
+
+	for (i = 0; i < hctx->nr_ctx; i++) {
+		struct blk_mq_ctx *ctx = hctx->ctxs[i];
+
+		if (ctx->cpu == cpu)
+			continue;
+
+		/*
+		 * Check if this context has at least one online
+		 * housekeeping CPU in this case the hardware context is
+		 * usable.
+		 */
+		if (cpumask_test_cpu(ctx->cpu, hk_mask) &&
+		    cpu_online(ctx->cpu))
+			break;
+
+		/*
+		 * The context doesn't have any online housekeeping CPUs
+		 * but there might be an online isolated CPU mapped to
+		 * it.
+		 */
+		if (cpu_is_offline(ctx->cpu))
+			continue;
+
+		pr_warn("%s: trying to offline hctx%d but there is still an online isolcpu CPU %d mapped to it\n",
+			hctx->queue->disk->disk_name,
+			hctx->queue_num, ctx->cpu);
+		return true;
+	}
+
+	return false;
+}
+
 static bool blk_mq_hctx_has_online_cpu(struct blk_mq_hw_ctx *hctx,
 		unsigned int this_cpu)
 {
@@ -3647,7 +3689,7 @@ static bool blk_mq_hctx_has_online_cpu(struct blk_mq_hw_ctx *hctx,

 		/* this hctx has at least one online CPU */
 		if (this_cpu != cpu)
-			return true;
+			return blk_mq_hctx_check_isolcpus_online(hctx, this_cpu);
 	}

 	return false;
@@ -3659,7 +3701,7 @@ static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
 			struct blk_mq_hw_ctx, cpuhp_online);

 	if (blk_mq_hctx_has_online_cpu(hctx, cpu))
-		return 0;
+		return -EINVAL;

 	/*
 	 * Prevent new request from being allocated on the current hctx.

-- 
2.49.0