linux-kernel - Re: [patch/rfc/rft] sd: allocate request

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Date:	Tue, 23 Oct 2012 08:45:30 +0200
From:	Bart Van Assche <bvanassche@....org>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	axboe@...nel.dk, linux-kernel@...r.kernel.org,
	SCSI Mailing List <linux-scsi@...r.kernel.org>
Subject: Re: [patch/rfc/rft] sd: allocate request_queue on device's local
 numa node

On 10/22/12 21:01, Jeff Moyer wrote:
> All of the infrastructure is available to allocate a request_queue on a
> particular numa node, but it isn't being utilized at all.  Wire up the
> sd driver to allocate the request_queue on the HBA's local numa node.
>
> This is a request for comments and testing (I've built and booted it,
> nothing more).  I believe that this should be a performance win, but I
> have no numbers to back it up as yet.  Suggestions for workloads to test
> are welcome.
>
> Cheers,
> Jeff
>
> Signed-off-by: Jeff Moyer <jmoyer@...hat.com>
>
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index da36a3a..7986483 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1664,7 +1664,8 @@ struct request_queue *__scsi_alloc_queue(struct Scsi_Host *shost,
>   	struct request_queue *q;
>   	struct device *dev = shost->dma_dev;
>
> -	q = blk_init_queue(request_fn, NULL);
> +	q = blk_init_queue_node(request_fn, NULL,
> +				dev_to_node(&shost->shost_dev));
>   	if (!q)
>   		return NULL;

Are you sure this approach will always result in the queue being 
allocated on the same NUMA node as the HCA ? If e.g. a user triggers LUN 
scanning via sysfs the above code may be invoked on another NUMA node 
than the node to which the HCA is connected. Also, if you have a look at 
e.g. scsi_request_fn() or scsi_device_unbusy() you will see that in 
order to avoid inter-node traffic it's important to allocate the sdev 
and shost data structures on the same NUMA node. How about the following 
approach ?
- Add a variant of scsi_host_alloc() that allows to specify on which
   NUMA node to allocate the shost structure and also that stores the
   identity of that node in the shost structure.
- Modify __scsi_alloc_queue() such that it allocates the sdev structure
   on the same NUMA node as the shost structure.
- Modify the SCSI LLD of your choice such that it uses the new
   scsi_host_alloc() call. According to what is appropriate the NUMA node
   on which to allocate the shost could be specified by the user or could
   be identical to the NUMA node of the HCA controlled by the SCSI LLD
   (see e.g. /sys/devices/pci*/*/numa_node). Please keep in mind that a
   single PCIe bus may have a minimal distance to more than one NUMA
   node. See e.g. the diagram at the top of page 8 in
 
http://bizsupport1.austin.hp.com/bc/docs/support/SupportManual/c03261871/c03261871.pdf
   for a system diagram of a NUMA system where each PCIe bus has a
   minimal distance to two different NUMA nodes.

Bart.
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