Re: [PATCH] oprofile: check whether oprofile perf enabled in op_overflow_handler()

[Will Deacon <will.deacon@arm.com>]

On Thu, Jan 16, 2014 at 11:52:45AM +0000, Robert Richter wrote:
> (cc'ing Will)

Thanks Robert,

> On 16.01.14 17:33:04, Weng Meiling wrote:
> > Using the same test case, the problem also exists in the same kernel with the new patch applied:
> > 
> > 
> > # opcontrol --start
> > 
> > Using 2.6+ OProfile kernel interface.
> > Using log file /var/lib/oprofile/samples/oprofiled.log
> > Daemon started.
> > [  508.456878] INFO: rcu_sched self-detected stall on CPU { 0}  (t=2100 jiffies g=685 c=684 q=83)
> > [  571.496856] INFO: rcu_sched self-detected stall on CPU { 0}  (t=8404 jiffies g=685 c=684 q=83)
> > [  634.526855] INFO: rcu_sched self-detected stall on CPU { 0}  (t=14707 jiffies g=685 c=684 q=83)
> 
> Yes, the patch does not prevent an interrupt storm. The same happened
> on x86 and was there solved also by limiting the minimum cycle period
> as the kernel was not able to ratelimit.
> 
> > ARM: events: increase minimum cycle period to 100k
> 
> > -event:0xFF counters:0 um:zero minimum:500 name:CPU_CYCLES : CPU cycle
> > +event:0xFF counters:0 um:zero minimum:100000 name:CPU_CYCLES : CPU cycle
> 
> However, an arbitrary hardcoded value migth not fit for all kind of
> cpus esp. on ARM where the variety is high. It also looks like there
> is no way other than patching the events file to force lower values
> than the minimum on cpus there this might be necessary.

Yeah, it's pretty much impossible to pick a one-size-fits-all value for ARM.

> The problem of too low sample periods could be solved on ARM by using
> perf's interrupt throttling, you might play around with:
> 
>  /proc/sys/kernel/perf_event_max_sample_rate:100000
> 
> I am not quite sure whether this works esp. for kernel counters and
> how userland can be notified about throttling. Throttling could be
> worth for operf too, not only for the oprofile kernel driver.
> 
> From a quick look it seems there is also code in x86 that dynamically
> adjusts the rate which might be worth being implemented for ARM too.

Are you referring to the perf_sample_event_took callback? If so, that
certainly looks worth persuing. I'll stick it on my list, thanks!

Will