linux-kernel - [PATCH 4.15 32/53] blk-mq: dont keep offline CPUs mapped to hctx 0

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Date:   Tue, 17 Apr 2018 17:58:57 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Stefan Haberland <sth@...ux.vnet.ibm.com>,
        Keith Busch <keith.busch@...el.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 4.15 32/53] blk-mq: dont keep offline CPUs mapped to hctx 0

4.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ming Lei <ming.lei@...hat.com>

commit bffa9909a6b48d8ca3398dec601bc9162a4020c4 upstream.

>>From commit 4b855ad37194 ("blk-mq: Create hctx for each present CPU),
blk-mq doesn't remap queue after CPU topo is changed, that said when
some of these offline CPUs become online, they are still mapped to
hctx 0, then hctx 0 may become the bottleneck of IO dispatch and
completion.

This patch sets up the mapping from the beginning, and aligns to
queue mapping for PCI device (blk_mq_pci_map_queues()).

Cc: Stefan Haberland <sth@...ux.vnet.ibm.com>
Cc: Keith Busch <keith.busch@...el.com>
Cc: stable@...r.kernel.org
Fixes: 4b855ad37194 ("blk-mq: Create hctx for each present CPU)
Tested-by: Christian Borntraeger <borntraeger@...ibm.com>
Reviewed-by: Christoph Hellwig <hch@....de>
Reviewed-by: Sagi Grimberg <sagi@...mberg.me>
Signed-off-by: Ming Lei <ming.lei@...hat.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 block/blk-mq-cpumap.c |    5 -----
 1 file changed, 5 deletions(-)

--- a/block/blk-mq-cpumap.c
+++ b/block/blk-mq-cpumap.c
@@ -16,11 +16,6 @@
 
 static int cpu_to_queue_index(unsigned int nr_queues, const int cpu)
 {
-	/*
-	 * Non present CPU will be mapped to queue index 0.
-	 */
-	if (!cpu_present(cpu))
-		return 0;
 	return cpu % nr_queues;
 }