Re: 4.9.33-rt23 latency under high load

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2017-08-11 22:15:02 [+0000], Carol Wong wrote:
> Hi Sebastian,
Hi Carol,

> I can't say that this change is responsible, but I see a dramatic difference when this patch is removed. Perhaps this is a red herring. For sure in my test case, 4.9.30-rt21 reports small latency and 4.9.33-rt23 reports large latency.
> 
> > This change should be a nop if you don't change the task's CPU-mask
> > while the task is in a migrate-disable section. Do I miss something?
> > Is part of your workload changing cpu-affinity mask?
> > 
> 
> The test program does not alter affinity masks. 

interesting. Could you add a trace-printk to check if it gets into that
inner-loop if the mask changes and it needs to do the fixup? Other than
that I don't see anything that could make any difference.

> I performed the same test with 4.11.12-rt9 and observed large latencies under heavy cpu & net load.
> 
> [localhost:~]# cyclictest -S -p98 --policy=rr
> # /dev/cpu_dma_latency set to 0us
> policy: rr: loadavg: 0.65 0.61 0.56 1/204 2206
> 
> T: 0 ( 2131) P:98 I:1000 C:11132138 Min:      1 Act:    1 Avg:    1 Max:     375
> T: 1 ( 2132) P:98 I:1500 C:7421425 Min:      1 Act:    1 Avg:    1 Max:     105
> T: 2 ( 2133) P:98 I:2000 C:5566069 Min:      1 Act:    1 Avg:    1 Max:      68
> T: 3 ( 2134) P:98 I:2500 C:4452855 Min:      1 Act:    2 Avg:    1 Max:      64
> 
> 
> Cheers,
> Carol

Sebastian