Re: [PATCH v1 3/5] powerpc/pseries: Fix stolen time accounting when dtl debugfs is used

Paul Mackerras wrote:

On Fri, Oct 26, 2018 at 01:55:44AM +0530, Naveen N. Rao wrote:

quoted

When the dtl debugfs interface is used, we usually set the
dtl_enable_mask to 0x7 (DTL_LOG_ALL). When this happens, we start seeing
DTL entries for all preempt reasons, including CEDE. In
scan_dispatch_log(), we add up the times from all entries and account
those towards stolen time. However, we should only be accounting stolen
time when the preemption was due to HDEC at the end of our time slice.

It's always been the case that stolen time when idle has been
accounted as idle time, not stolen time.  That's why we didn't check
for this in the past.

Do you have a test that shows different results (as in reported idle
and stolen times) with this patch compared to without?

Ah ok, that makes sense now and explains why I couldn't observe much of 
a difference in practice. However, I also went by the fact that there 
are 7 other preemption reasons, which could impact our calculation.  
Looking at the list again, it looks like H_CONFER/H_PROD and some faults 
can also have an impact here, though they may be rare?

Thanks,
Naveen