Messages in this thread | | | Subject | Re: [RFC 0/2] target: Add TFO->complete_irq queue_work bypass | From | "Nicholas A. Bellinger" <> | Date | Thu, 04 Jun 2015 00:06:09 -0700 |
| |
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 basіcally 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..?
| |