Re: [RFC 0/2] target: Add TFO->complete_irq queue_work bypass

[Nicholas A. Bellinger]

On Wed, 2015-06-03 at 14:57 +0200, Christoph Hellwig wrote:
> This makes lockdep very unhappy, rightly so.  If you execute
> one end_io function inside another you basÑcally nest every possible
> lock taken in the I/O completion path.  Also adding more work
> to the hardirq path generally isn't a smart idea.  Can you explain
> what issues you were seeing and how much this helps?  Note that
> the workqueue usage in the target core so far is fairly basic, so
> there should some low hanging fruit.

So I've been using tcm_loop + RAMDISK backends for prototyping, but this
patch is intended for vhost-scsi so it can avoid the unnecessary
queue_work() context switch within target_complete_cmd() for all backend
driver types.

This is because vhost_work_queue() is just updating vhost_dev->work_list
and immediately wake_up_process() into a different vhost_worker()
process context.  For heavy small block workloads into fast IBLOCK
backends, avoiding this extra context switch should be a nice efficiency
win.

Also, AFAIK RDMA fabrics are allowed to do ib_post_send() response
callbacks directly from IRQ context as well.

Perhaps tcm_loop LLD code should just be limited to RAMDISK here..?

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/