Re: perf: [patch] regression with PERF_EVENT_IOC_REFRESH

[Peter Zijlstra]

On Tue, 2011-05-31 at 09:49 -0400, Vince Weaver wrote:
> On Tue, 31 May 2011, Peter Zijlstra wrote:
> 
> > On Mon, 2011-05-30 at 21:33 -0400, Vince Weaver wrote:
> > > the problem was the mentioned commit tried to optimize the use of 
> > > watermark and wakeup_watermark without taking into account that 
> > > wakeup_watermark is a union with wakeup_events. 
> > 
> > Note that wake_events isn't related to IOC_REFRESH, wake_events is how
> > much events to buffer in the mmap-buffer before issuing a wakeup.
> > 
> > IOC_REFRESH increments event_limit, which is how many events to run
> > before disabling yourself.
> > 
> > What I gather is that due to that SIGIO bug (fixed by f506b3dc0e), you
> > had to have both an mmap and a wakeup in order for that signal to
> > arrive.
> 
> yes, but due to a bug in the mentioned changeset, the buffer watermark 
> value was being set to a low value even if *watermark* was 0.  So if you 
> were using IOC_REFRESH to set the *wakeup_events* value,

IOC_REFRESH sets event->event_limit, not wakeup_events.

>  it was also 
> setting the *wakeup_watermark* value (it's a union) and the buffer setup 
> was then unconditionally setting the buffer watermark to the value of the 
> supposedly unrelated *wakeup_watermark*.  Normally the wakeup watermark 
> would default to something like 2048, but if you were trying to set the 
> wakeup_events value to something like 3 then wakeup_watermark would be set 
> to that too, causing a lot more overflow events.

poll() wakeups, which were inadvertly linked to SIGIOs

> I verified all the above painfully using a lot of printks.

I prefer to use trace_printk() and /debug/tracing/, that doesn't slow
stuff down as much.

> I agree this does seem to be a combination of bugs, as even with a 
> properlyu set value on affected kernels you'd get spurious watermark 
> overflow events if you weren't consuming the ring buffer.

*nod*

> In any case, I can provide a cleaner patch than the one before that isn't 
> as intrusive.

Appreciated.

> 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.

> A power outage over the weekend has taken part of the 
> network down here though so my e-mail access is a bit limited, so I 
> apologize if I've been missing comments sent to my other e-mail address.

I'm afraid not, I've been mostly tied up with fixing some scheduler
regressions.

Also, it looks like I just broke stuff even worse in -tip, am bisecting
that now.
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