Messages in this thread | | | Date | Tue, 31 May 2011 09:49:52 -0400 (EDT) | From | Vince Weaver <> | Subject | Re: perf: [patch] regression with PERF_EVENT_IOC_REFRESH |
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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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