Messages in this thread | | | Subject | Re: perf: [patch] regression with PERF_EVENT_IOC_REFRESH | From | Peter Zijlstra <> | Date | Tue, 31 May 2011 17:52:53 +0200 |
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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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