linux-kernel - Re: [RFC 0/2] target: Add TFO->complete_irq queue

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Date:	Thu, 04 Jun 2015 00:06:09 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Christoph Hellwig <hch@....de>
Cc:	"Nicholas A. Bellinger" <nab@...erainc.com>,
	target-devel <target-devel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Hannes Reinecke <hare@...e.de>,
	Sagi Grimberg <sagig@...lanox.com>
Subject: Re: [RFC 0/2] target: Add TFO->complete_irq queue_work bypass

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..?

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