Re: [PATCH] Improve requeuing behavior

[Bart Van Assche <Bart.VanAssche@wdc.com>]

On Thu, 2017-08-24 at 14:09 +0200, Christoph Hellwig wrote:
> On Wed, Aug 23, 2017 at 02:05:35PM -0700, Bart Van Assche wrote:
> > Requests are unprepared and reprepared when being requeued. Avoid
> > that requeuing resets .jiffies_at_alloc and .retries by initializing
> > these two member variables from inside blk_get_request() and by
> > preserving both member variables when preparing a request. This patch
> > affects the requeuing behavior of scsi-sq and scsi-mq.
> > 
> > Reported-by: Brian King <brking@linux.vnet.ibm.com>
> > References: https://lkml.org/lkml/2017/8/18/923 ("Re: [BUG][bisected 270065e] linux-next fails to boot on powerpc")
> > Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
> > Cc: Brian King <brking@linux.vnet.ibm.com>
> > Cc: Hannes Reinecke <hare@suse.com>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Johannes Thumshirn <jthumshirn@suse.de>
> > ---
> >  drivers/scsi/scsi_lib.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > index ebc5c713ee37..8d1ec1e7b0e2 100644
> > --- a/drivers/scsi/scsi_lib.c
> > +++ b/drivers/scsi/scsi_lib.c
> > @@ -1122,6 +1122,8 @@ void scsi_initialize_rq(struct request *rq)
> >  	struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(rq);
> >  
> >  	scsi_req_init(&cmd->req);
> > +	cmd->jiffies_at_alloc = jiffies;
> > +	cmd->retries = 0;
> >  }
> >  EXPORT_SYMBOL(scsi_initialize_rq);
> 
> How is this working for non-passthrough commands where we don't
> call scsi_initialize_rq?

Hello Christoph,

Commit ca18d6f769d2 ("block: Make most scsi_req_init() calls implicit")
introduced the scsi_initialize_rq() function in such a way that it is not only
called for pass-through requests but also for FS requests. Because that commit
sets .initialize_rq_fn for both blk-sq and blk-mq blk_get_request() now calls
scsi_initialize_rq() for all SCSI requests. Or did I perhaps overlook something?

Bart.