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SubjectRe: perf: [patch] regression with PERF_EVENT_IOC_REFRESH
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, 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.

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

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.

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

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.

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.

Vince


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