From b7e94a1686c5daef4f649f7f4f839cc294f07710 Mon Sep 17 00:00:00 2001 From: Jun'ichi Nomura Date: Tue, 22 May 2012 18:57:17 +0900 Subject: [PATCH] [SCSI] Fix dm-multipath starvation when scsi host is busy block congestion control doesn't have any concept of fairness across multiple queues. This means that if SCSI reports the host as busy in the queue congestion control it can result in an unfair starvation situation in dm-mp if there are multiple multipath devices on the same host. For example: http://www.redhat.com/archives/dm-devel/2012-May/msg00123.html The fix for this is to report only the sdev busy state (and ignore the host busy state) in the block congestion control call back. The host is still congested, but the SCSI subsystem will sort out the congestion in a fair way because it knows the relation between the queues and the host. [jejb: fixed up trailing whitespace] Reported-by: Bernd Schubert Tested-by: Bernd Schubert Signed-off-by: Jun'ichi Nomura Cc: Signed-off-by: James Bottomley --- drivers/scsi/scsi_lib.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c index 62ddfd31d4ce..6dfb9785d345 100644 --- a/drivers/scsi/scsi_lib.c +++ b/drivers/scsi/scsi_lib.c @@ -1378,16 +1378,19 @@ static int scsi_lld_busy(struct request_queue *q) { struct scsi_device *sdev = q->queuedata; struct Scsi_Host *shost; - struct scsi_target *starget; if (!sdev) return 0; shost = sdev->host; - starget = scsi_target(sdev); - if (scsi_host_in_recovery(shost) || scsi_host_is_busy(shost) || - scsi_target_is_busy(starget) || scsi_device_is_busy(sdev)) + /* + * Ignore host/starget busy state. + * Since block layer does not have a concept of fairness across + * multiple queues, congestion of host/starget needs to be handled + * in SCSI layer. + */ + if (scsi_host_in_recovery(shost) || scsi_device_is_busy(sdev)) return 1; return 0; -- 2.20.1