On Mon, Nov 16, 2020 at 10:44:54AM +0100, Hannes Reinecke wrote:

On 11/16/20 10:07 AM, Ming Lei wrote:

quoted

Limit scsi device's queue depth is less than max(host->can_queue, 1024)
in scsi_change_queue_depth(), and 1024 is big enough for saturating
current fast SCSI LUN(SSD, or raid volume on multiple SSDs).

We need this patch for replacing sdev->device_busy with sbitmap which
has to be pre-allocated with reasonable max depth.

Cc: Omar Sandoval <redacted>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Sumanesh Samanta <redacted>
Cc: Ewan D. Milne <redacted>
Tested-by: Sumanesh Samanta <redacted>
Signed-off-by: Ming Lei <redacted>
---
  drivers/scsi/scsi.c | 11 +++++++++++
  1 file changed, 11 insertions(+)

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 24619c3bebd5..a28d48c850cf 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c

@@ -214,6 +214,15 @@ void scsi_finish_command(struct scsi_cmnd *cmd)
  	scsi_io_completion(cmd, good_bytes);
  }
+
+/*
+ * 1024 is big enough for saturating the fast scsi LUN now
+ */
+static int scsi_device_max_queue_depth(struct scsi_device *sdev)
+{
+	return max_t(int, sdev->host->can_queue, 1024);
+}
+

Shouldn't this rather be initialized with scsi_host->can_queue?

Multiple queues may be used for one single LUN, so in theory we should
return max queue depth as host->can_queue * host->nr_hw_queues, but
this number can be too big for the sbitmap's pre-allocation.

That is why this patch introduces one reasonable limit on this value
of max(sdev->host->can_queue, 1024). Suppose single SSD can be saturated
by ~128 requests, we still can saturate one LUN with 8 SSDs behind if
the hw queue depth is set as too low.

These 'should be enough' settings inevitable turn out to be not enough in
the long run ...

I have provided the theory behind this idea, not just simple 'should be
enough'.


Thanks,
Ming