Re: [driver-core PATCH v6 9/9] libnvdimm: Schedule device registration on node local to the device

From: Bart Van Assche <bvanassche@acm.org>
To: Dan Williams <dan.j.williams@intel.com>,
	alexander.h.duyck@linux.intel.com
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Tejun Heo <tj@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux-pm mailing list <linux-pm@vger.kernel.org>,
	jiangshanlai@gmail.com, "Rafael J. Wysocki" <rafael@kernel.org>,
	"Brown, Len" <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>,
	zwisler@kernel.org, Dave Jiang <dave.jiang@intel.com>
Subject: Re: [driver-core PATCH v6 9/9] libnvdimm: Schedule device registration on node local to the device
Date: Tue, 27 Nov 2018 12:33:00 -0800	[thread overview]
Message-ID: <1543350780.185366.81.camel@acm.org> (raw)
In-Reply-To: <CAPcyv4gWwsYOeXVNDoDy4v=SU6bRyBG1fgSu84y64jGApaJ91g@mail.gmail.com>

On Tue, 2018-11-27 at 11:34 -0800, Dan Williams wrote:
> On Tue, Nov 27, 2018 at 10:04 AM Alexander Duyck
> <alexander.h.duyck@linux.intel.com> wrote:
> > 
> > On Mon, 2018-11-26 at 18:21 -0800, Dan Williams wrote:
> > > On Thu, Nov 8, 2018 at 10:07 AM Alexander Duyck
> > > <alexander.h.duyck@linux.intel.com> wrote:
> > > > 
> > > > Force the device registration for nvdimm devices to be closer to the actual
> > > > device. This is achieved by using either the NUMA node ID of the region, or
> > > > of the parent. By doing this we can have everything above the region based
> > > > on the region, and everything below the region based on the nvdimm bus.
> > > > 
> > > > By guaranteeing NUMA locality I see an improvement of as high as 25% for
> > > > per-node init of a system with 12TB of persistent memory.
> > > > 
> > > 
> > > It seems the speed-up is achieved with just patches 1, 2, and 9 from
> > > this series, correct? I wouldn't want to hold up that benefit while
> > > the driver-core bits are debated.
> > 
> > Actually patch 6 ends up impacting things for persistent memory as
> > well. The problem is that all the async calls to add interfaces only do
> > anything if the driver is already loaded. So there are cases such as
> > the X86_PMEM_LEGACY_DEVICE case where the memory regions end up still
> > being serialized because the devices are added before the driver.
> 
> Ok, but is the patch 6 change generally useful outside of the
> libnvdimm case? Yes, local hacks like MODULE_SOFTDEP are terrible for
> global problems, but what I'm trying to tease out if this change
> benefits other async probing subsystems outside of libnvdimm, SCSI
> perhaps? Bart can you chime in with the benefits you see so it's clear
> to Greg that the driver-core changes are a generic improvement?

Hi Dan,

For SCSI asynchronous probing is really important because when scanning SAN
LUNs there is plenty of potential for concurrency due to the network delay.

I think the following quote provides the information you are looking for:

"This patch reduces the time needed for loading the scsi_debug kernel
module with parameters delay=0 and max_luns=256 from 0.7s to 0.1s. In
other words, this specific test runs about seven times faster."

Source: https://www.spinics.net/lists/linux-scsi/msg124457.html

Best regards,

Bart.