Commit c02ebfdd authored Sep 28, 2016 by Gabriel Krisman Bertazi Committed by Jens Axboe Nov 06, 2016

blk-mq: Always schedule hctx->next_cpu



Commit 0e87e58b ("blk-mq: improve warning for running a queue on the
wrong CPU") attempts to avoid triggering the WARN_ON in
__blk_mq_run_hw_queue when the expected CPU is dead.  Problem is, in the
last batch execution before round robin, blk_mq_hctx_next_cpu can
schedule a dead CPU and also update next_cpu to the next alive CPU in
the mask, which will trigger the WARN_ON despite the previous
workaround.

The following patch fixes this scenario by always scheduling the value
in hctx->next_cpu.  This changes the moment when we round-robin the CPU
running the hctx, but it really doesn't matter, since it still executes
BLK_MQ_CPU_WORK_BATCH times in a row before switching to another CPU.

Fixes: 0e87e58b ("blk-mq: improve warning for running a queue on the wrong CPU")
Signed-off-by: Gabriel Krisman Bertazi <krisman@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

parent d278d4a8

block/blk-mq.c

+1 −3

Original line number	Diff line number	Diff line
		@@ -929,7 +929,7 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
		return WORK_CPU_UNBOUND;

		if (--hctx->next_cpu_batch <= 0) {
		int cpu = hctx->next_cpu, next_cpu;
		int next_cpu;

		next_cpu = cpumask_next(hctx->next_cpu, hctx->cpumask);
		if (next_cpu >= nr_cpu_ids)
		@@ -937,8 +937,6 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)

		hctx->next_cpu = next_cpu;
		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;

		return cpu;
		}

		return hctx->next_cpu;

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