Re: [PATCH 0/4] kernfs: proposed locking and concurrency improvement

From: Rick Lindsley
Date: Wed May 27 2020 - 08:45:31 EST

On 5/24/20 11:16 PM, Greg Kroah-Hartman wrote:

Independant of your kernfs changes, why do we really need to represent
all of this memory with that many different "memory objects"? What is
that providing to userspace?

I remember Ben Herrenschmidt did a lot of work on some of the kernfs and
other functions to make large-memory systems boot faster to remove some
of the complexity in our functions, but that too did not look into why
we needed to create so many objects in the first place.

That was my first choice too. Unfortunately, I was not consulted on this design decision, however, and now it's out there. It is, as you guessed, a hardware "feature". The hw believes there is value in identifying memory in 256MB chunks. There are, unfortunately, 2^18 or over 250,000 of those on a 64TB system, compared with dozens or maybe even hundreds of other devices.

We considered a revamping of the boot process - delaying some devices, reordering operations and such - but deemed that more dangerous to other architectures. Although this change is driven by a particular architecture, the changes we've identified are architecture independent. The risk of breaking something else is much lower than if we start reordering boot steps.

Also, why do you need to create the devices _when_ you create them? Can
you wait until after init is up and running to start populating the
device tree with them? That way boot can be moving on and disks can be
spinning up earlier?

I'm not a systemd expert, unfortunately, so I don't know if it needs to happen *right* then or not. I do know that upon successful boot, a ps reveals many systemd children still reporting in. It's not that we're waiting on everybody; the contention is causing a delay in the discovery of key devices like disks, and *that* leads to timeouts firing in systemd rules. Any workaround bent on dodging the problem tends to get exponentially worse when the numbers change. We noticed this problem at 32TB, designed some timeout changes and udev options to improve it, only to have both fail at 64TB. Worse, at 64TB, larger timeouts and udev options failed to work consistently anymore.

There are two times we do coldplugs - once in the initramfs, and then again after we switch over to the actual root. I did try omitting memory devices after the switchover. Much faster! So, why is the second one necessary? Are there some architectures that need that? I've not found anyone who can answer that, so going that route presents us with a different big risk.