Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency improvement

Tejun Heo
2020-06-22T17:53:49Z

On Fri, Jun 19, 2020 at 07:44:29PM -0700, Rick Lindsley wrote:
> echo 0 > /sys/devices//system/memory/memory10374/online
>
> and boom - you've taken memory chunk 10374 offline.
>
> These changes are not just a whim. I used lockstat to measure contention
> during boot. The addition of 250,000 "devices" in parallel created
> tremendous contention on the kernfs_mutex and, it appears, on one of the
> directories within it where memory nodes are created. Using a mutex means
> that the details of that mutex must bounce around all the cpus ... did I
> mention 1500+ cpus? A whole lot of thrash ...

I don't know. The above highlights the absurdity of the approach itself to
me. You seem to be aware of it too in writing: 250,000 "devices".

Thanks.

--
tejun