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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 6 Jun 2016 14:20:32 -0700
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Subject: Re: performance delta after VFS i_mutex=>i_rwsem conversion
To: Waiman Long <waiman.long@hpe.com>
Cc: Dave Hansen <dave.hansen@intel.com>, "Chen, Tim C" <tim.c.chen@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Davidlohr Bueso <dbueso@suse.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Jason Low <jason.low2@hp.com>, Michel Lespinasse <walken@google.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Waiman Long <waiman.long@hp.com>, Al Viro <viro@zeniv.linux.org.uk>,
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On Mon, Jun 6, 2016 at 2:13 PM, Waiman Long <waiman.long@hpe.com> wrote:
>
> The tricky part about optimistic spinning in rwsem is that we don't know for
> sure if any of the lock holding readers is running or not.

I'm notm sure how common the reader-vs-writer contention is, at least
for the new inode use. I'm sure you can trigger it with crazy
benchmarks, but I wouldn't worry about it unless people start
complaining.

The writer-writer case is easy to trigger with write-heavy loads (ok,
rename/unlink in this case). Are there real loads where there are lots
of concurrent lookup and writes? I really don't know (note that
"lookup" needs to be uncached and actually hit the lowlevel filesystem
for the locking to even trigger in the first place).

I guess some "concurrent readdir with unlink" load would show that
behavior, but is it _realistic_? No idea. Let's not worry about it too
much until somebody shows a reason to worry.

               Linus