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Date:   Mon, 22 Jun 2020 13:53:43 -0400
From:   Tejun Heo <tj@kernel.org>
To:     Rick Lindsley <ricklind@linux.vnet.ibm.com>
Cc:     Ian Kent <raven@themaw.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency
 improvement
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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