linux-kernel - Re: perf: PERF_EVENT_IOC_PERIOD on ARM vs everywhere else

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Date:	Thu, 31 Oct 2013 11:25:55 -0400 (EDT)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Will Deacon <will.deacon@....com>
cc:	Vince Weaver <vincent.weaver@...ne.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	"eranian@...gle.com" <eranian@...gle.com>
Subject: Re: perf: PERF_EVENT_IOC_PERIOD on ARM vs everywhere else

On Wed, 30 Oct 2013, Will Deacon wrote:
> On Wed, Oct 30, 2013 at 02:13:11PM +0000, Vince Weaver wrote:
> > Uncompiled too I guess?
> > 
> > kernel/events/core.c: In function ‘perf_event_period’:
> > kernel/events/core.c:3531: error: invalid type argument of ‘->’ (have ‘local64_t’)
> > make[3]: *** [kernel/events/core.o] Error 1
> 
> That's trivial to fix.

Probably, although Peter's been sending some "interesting" mostly untested 
perf patches to linux-kernel the past few days so before testing I'd 
rather have a patch that at least compiles cleanly.

> > I also won't be able to test the ARM change, as my pandaboard won't boot 
> > with recent kernels (can't find the MMC root filesystem) and I haven't had 
> > time to track down why.  Also even on a simple period changing test it 
> > often fails due to lost interrupts (the Cortex-A9 lost interrupt errata?).
> 
> The omap guys like trying to blame the A9 erratum for that (which doesn't
> even affect the cycle counter), but the reality is that the CTI never
> worked reliably with mainline, and now there are no developers working
> on that after the TI layoffs. I don't see the state of Pandaboard support
> improving over time.

So it might be OMAP4460/CTI specific and not related to the errata?
That might be nice especially if things might work on other cortex-a9 
boards.

A shame about the TI people, they were often semi-responsive to my
OMAP4460 questions even if it was usually just to say "ignore the 
documentation, that's broken, don't do that"

> If you have a canned testcase for this stuff, I'm happy to run it on my
> Chromebook.

There's the tests/ioctl/ioctl_period test in my perf_event_tests.git tree
but it's very simple.  Currently it will likely just return success on 
current x86 Linux but fail on newish ARM and patched x86.  I need to 
fix it to print old_behavior/new_behavior/fail I guess.

I have a cortex-a15 chromebook too, but I haven't managed to get a 
recent kernel booting on it yet.

Vince