Re: Regression on rt kernel while using POSIX timers

["Patel, Vedang" <vedang.patel@intel.com>]

On Wed, 2017-02-15 at 20:05 -0600, Julia Cartwright wrote:
> On Wed, Feb 15, 2017 at 05:54:47PM +0100, bigeasy@linutronix.de
> wrote:
> > 
> > On 2017-02-13 18:48:33 [+0000], Patel, Vedang wrote:
> > > 
> > > I am getting very similar results even if I change the priority
> > > of
> > > ktimersoftd to 99. Are there any recent rt patches which might
> > > have
> > > changed the behaviour of POSIX timers? 
> > I had this running
> > 
> > > 
> > > ~# cyclictest -t1 -p 80 -i 500 -l 100000
> > > # /dev/cpu_dma_latency set to 0us
> > > policy: fifo: loadavg: 627.48 661.98 543.65 406/2254
> > > 18149          
> > > 
> > > T: 0 (16148) P:80 I:500 C: 100000 Min:      5 Act:   11 Avg:   11
> > > Max: 23
> > on a AMD-A10 box and the latest v4.9-RT. This does not look that
> > bad.
> > From tracing it doesn't look too good either. The -m option should
> > be
> > your friend. Since the cyclictest isn't pinned to a CPU and I have
> > four
> > of them, the scheduler decides to migrate cyclictest on each wake
> > up.
> > yay. cyclictest itself gets woken up via a signal from the
> > ktimersoftirq.
> Perhaps a separate off-shoot topic, but has there been significant
> changes in v4.9 upstream (or v4.9-rt) which impact how often
> migration
> is performed?  We've seen what appears to be much more aggressive
> migration of SCHED_FAIR tasks, (aggressive in the sense that more
> threads are migrated more often), which appears to cause contention
> on
> rq-locks impacting latencies in the wake_up paths.
> 
> We're still working to further characterize at this
> point....hopefully
> more information forthcoming.
I tried conducting similar experiments on v4.4.47-rt59. But, the
results are same as v4.9 kernel.

My machine also has 8 cores and migration might cause regressions. Let
me try pinning the processes to a single CPU and see if that is making
any difference. 

More information to follow soon...

Thanks for all the inputs, 
Vedang Patel
Software Engineer
Intel Corporation
>    Julia