linux-kernel - Re: [RFC PATCH 16/20] famfs: Add fault counters

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Date: Fri, 23 Feb 2024 13:19:01 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: John Groves <John@...ves.net>, Dan Williams <dan.j.williams@...el.com>
Cc: John Groves <jgroves@...ron.com>, Jonathan Corbet <corbet@....net>,
 Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Matthew Wilcox <willy@...radead.org>, linux-cxl@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev, john@...alactic.com,
 Dave Chinner <david@...morbit.com>, Christoph Hellwig <hch@...radead.org>,
 dave.hansen@...ux.intel.com, gregory.price@...verge.com
Subject: Re: [RFC PATCH 16/20] famfs: Add fault counters

On 2/23/24 12:39, John Groves wrote:
>> We had similar unit test regression concerns with fsdax where some
>> upstream change silently broke PMD faults. The solution there was trace
>> points in the fault handlers and a basic test that knows apriori that it
>> *should* be triggering a certain number of huge faults:
>>
>> https://github.com/pmem/ndctl/blob/main/test/dax.sh#L31
> Good approach, thanks Dan! My working assumption is that we'll be able to make
> that approach work in the famfs tests. So the fault counters should go away
> in the next version.

I do really suspect there's something more generic that should be done
here.  Maybe we need a generic 'huge_faults' perf event to pair up with
the good ol' faults that we already have:

# perf stat -e faults /bin/ls

 Performance counter stats for '/bin/ls':

               104      faults


       0.001499862 seconds time elapsed

       0.001490000 seconds user
       0.000000000 seconds sys