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

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Date:   Fri, 19 Jun 2020 11:38:33 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Ian Kent <raven@...maw.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency
 improvement

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