[PATCH] ARM: imx6q: work around faulty PMU irq routing

[l.stach@pengutronix.de (Lucas Stach)]

Am Dienstag, den 29.04.2014, 13:28 +0800 schrieb Shawn Guo:
> On Fri, Apr 25, 2014 at 11:13:25AM +0200, Lucas Stach wrote:
> > Am Freitag, den 25.04.2014, 07:37 +0200 schrieb Dirk Behme:
> > > On 24.04.2014 22:23, Lucas Stach wrote:
> > > > The i.MX6 PMU has a design errata where the interrupts of all cores are
> > > > wired together into a single irq line. To work around this we have to
> > > > bounce the interrupt around all cores until we find the one where the PMU
> > > > counter overflow has happened.
> > > >
> > > > This causes the perf measurements to be less accurate and we can't really
> > > > handle the case where two cores fire a PMU irq at the same time. The
> > > > implemented woraround makes perf at least somewhat useable on imx6 SoCs
> > > > with more than one core.
> > > >
[...]
> > > Do you have anything like a test case which shows that it works (at 
> > > least better) on a !single core with this patch? Compared to a 
> > > non-patched system?
> > > 
> > Without this patch, running perf top completely kills the system on
> > i.MX6q, most likely because of the sheer number of spurious interrupts
> > hitting the system from 4 cores. Even the spurious killer doesn't work
> > sometimes, so perf is completely busted right now.
> > 
> > With this patch perf has to reduce the sample frequency in order to
> > compensate the added irq latency, but at least we get some plausible
> > numbers out. Though I won't take any blame if the amount of salt you
> > have to apply while looking at those numbers is already a deadly
> > dose. ;)
> > 
> > I don't yet have any numbers on how accurate the measurement is, but at
> > least things didn't look completely off.
> 
> If it cannot provide correct/accurate data, I'd say let's not fake it
> to, and just let it be completely broken there, so that people can be
> aware of the brokenness, and not take inaccurate data as accurate one.
> 
The data isn't bogus, it just isn't as accurate as it could be with a
properly working PMU. I'll run some tests with a defined load on
Solo/Quad to see how far the measurements are off. I'm fine with holding
this patch until then.

But the thing is this patch also fixes a serious userspace triggerable
DoS on i.MX6q. Just running perf top completely locks up the system
because of the sheer number of stray irqs. This isn't the case anymore
with this patch applied.

Maybe we can just print a warning into dmesg to make the users aware of
the imprecise measurement.

Regards,
Lucas
-- 
Pengutronix e.K.             | Lucas Stach                 |
Industrial Linux Solutions   | http://www.pengutronix.de/  |