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

[Tejun Heo]

Hello, Ian.

On Wed, Jun 17, 2020 at 03:37:43PM +0800, Ian Kent wrote:
> The series here tries to reduce the locking needed during path walks
> based on the assumption that there are many path walks with a fairly
> large portion of those for non-existent paths, as described above.
> 
> That was done by adding kernfs negative dentry caching (non-existent
> paths) to avoid continual alloc/free cycle of dentries and a read/write
> semaphore introduced to increase kernfs concurrency during path walks.
> 
> With these changes we still need kernel parameters of udev.children-max=2048
> and systemd.default_timeout_start_sec=300 for the fastest boot times of
> under 5 minutes.

I don't have strong objections to the series but the rationales don't seem
particularly strong. It's solving a suspected problem but only half way. It
isn't clear whether this can be the long term solution for the problem
machine and whether it will benefit anyone else in a meaningful way either.

I think Greg already asked this but how are the 100,000+ memory objects
used? Is that justified in the first place?

Thanks.

-- 
tejun