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

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Message-ID: <l66vdkefx4ut73jis52wvn4j6hzj5omvrtpsoda6gbl27d4uwg@yolm6jx4yitn>
Date: Fri, 23 Feb 2024 13:56:42 -0600
From: John Groves <John@...ves.net>
To: Dave Hansen <dave.hansen@...el.com>
Cc: John Groves <jgroves@...ron.com>, Jonathan Corbet <corbet@....net>, 
	Dan Williams <dan.j.williams@...el.com>, 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 24/02/23 10:23AM, Dave Hansen wrote:
> On 2/23/24 09:42, John Groves wrote:
> > One of the key requirements for famfs is that it service vma faults
> > efficiently. Our metadata helps - the search order is n for n extents,
> > and n is usually 1. But we can still observe gnarly lock contention
> > in mm if PTE faults are happening. This commit introduces fault counters
> > that can be enabled and read via /sys/fs/famfs/...
> > 
> > These counters have proved useful in troubleshooting situations where
> > PTE faults were happening instead of PMD. No performance impact when
> > disabled.
> 
> This seems kinda wonky.  Why does _this_ specific filesystem need its
> own fault counters.  Seems like something we'd want to do much more
> generically, if it is needed at all.
> 
> Was the issue here just that vm_ops->fault() was getting called instead
> of ->huge_fault()?  Or something more subtle?

Thanks for your reply Dave!

First, I'm willing to pull the fault counters out if the brain trust doesn't
like them.

I put them in because we were running benchmarks of computational data
analytics and and noted that jobs took 3x as long on famfs as raw dax -
which indicated I was doing something wrong, because it should be equivalent
or very close.

The the solution was to call thp_get_unmapped_area() in
famfs_file_operations, and performance doesn't vary significantly from raw
dax now. Prior to that I wasn't making sure the mmap address was PMD aligned.

After that I wanted a way to be double-secret-certain that it was servicing
PMD faults as intended. Which it basically always is, so far. (The smoke
tests in user space check this.)

John