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Date: Tue, 31 May 2011 12:39:37 -0400 (EDT)
From: Vince Weaver <vweaver1@eecs.utk.edu>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
cc: Vince Weaver <vince@deater.net>, linux-kernel@vger.kernel.org,
        mingo@elte.hu, paulus@samba.org, acme@redhat.com
Subject: Re: perf: [patch] regression with PERF_EVENT_IOC_REFRESH
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On Tue, 31 May 2011, Peter Zijlstra wrote:
> 
> IOC_REFRESH sets event->event_limit, not wakeup_events.

ahh, yes. 

So it's a userspace "bug".

The test code called a "IOC_REFRESH, 3" whenever it got signalled.
It didn't distinguish between whether it was a plain overflow or if it was 
a ring-buffer overflow (can it distinguish?).

Thus the watermark bug was confusing the user-space code into refreshing 
when it was not necessary.

> > I'm also bisecting the other problem I mentioned, the one where overflows 
> > are 10x too large on 3.0-rc1.  I'm at work with a Nehalem machine so the 
> > bisect should go faster than the bisect I had to do on an atom machine 
> > this weekend.  
> 
> It wouldn't be the SIGIO fix would it?, with that every overflow
> generates a SIGIO, not only the poll() wakeups. And ouch at bisecting
> (or even building a kernel) on an Atom, those things are horridly slow.

Oh, it could be the SIGIO fix.  I hadn't realized that got merged already.

And yes, bisect on atom is painful, but my alternatives were a 1.7GHz P4 
(with small disk, so would have been over NFS), a 600MHz G3 iBook, or an 
armv6 machine.  So atom it was.

Vince
vweaver1@eecs.utk.edu