linux-kernel - RE: blk-mq timeout handling fixes

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Date:	Wed, 17 Sep 2014 21:53:50 +0000
From:	"Elliott, Robert (Server Storage)" <Elliott@...com>
To:	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...com>
CC:	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: blk-mq timeout handling fixes



> -----Original Message-----
> From: Christoph Hellwig [mailto:hch@....de]
> Sent: Saturday, 13 September, 2014 6:40 PM
> To: Jens Axboe
> Cc: Elliott, Robert (Server Storage); linux-scsi@...r.kernel.org; linux-
> kernel@...r.kernel.org
> Subject: blk-mq timeout handling fixes
> 
> This series fixes various issues with timeout handling that Robert
> ran into when testing scsi-mq heavily.  He tested an earlier version,
> and couldn't reproduce the issues anymore, although the series changed
> quite significantly since and should probably be retested.
> 
> In summary we not only start the blk-mq timer inside the drivers
> ->queue_rq method after the request has been fully setup, and we
> also tell the drivers if we're timing out a reserved (internal)
> request or a real one.  Many drivers including will need to handle
> those internal ones differently, e.g. for scsi-mq we don't even
> have a scsi command structure allocated for the reserved commands.

I have rerun a variety of tests on:
* Jens' for-next tree that went into 3.17rc5
* plus this series
* plus two patches for infinite recursion on flushes from 
  Ming and then Christoph

and have not been able to trigger the scsi_times_out req->special
NULL pointer dereference that prompted this series.

Testing includes:
* concurrent heavy workload generators:
  * fio high iodepth direct 512 byte random reads (> 1M IOPS)
  * programs generating large bursts of paged writes
    * mkfs.ext4 (followed by e2fsck)
    * mkfs.xfs (followed by xfs_check)
    * ddpt
  * watch -n 0 sync to generate flushes
* scsi_logging_level MLCOMPLETE set to 0 or 1
  * scsi_lib.c patched to put all the ACTION_FAIL messages
    under level 1 so they can be squelched (massive error 
    prints cause more timeouts themselves)
* 4 hpsa and 16 mpt3sas devices (all made from SAS SSDs)
  * lockless hpsa driver
* injecting errors
  * device removal
  * device generating infinite errors
  * device generating a brief number of errors

The filesystems don't always recover properly, but nothing in 
the block or scsi midlayers crashed.

So, you may add this to the series:
Tested-by: Robert Elliott <elliott@...com>

---
Rob Elliott    HP Server Storage




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