Re: [BUG nohz]: wrong user and system time accounting

[Luiz Capitulino]

On Thu, 30 Mar 2017 16:18:17 +0200
Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:

> On Thu, Mar 30, 2017 at 09:59:54PM +0800, Wanpeng Li wrote:
> > 2017-03-30 21:38 GMT+08:00 Frederic Weisbecker <fweisbec@xxxxxxxxx>:  
> > > If it works, we may want to take that solution, likely less performance sensitive
> > > than using sched_clock(). In fact sched_clock() is fast, especially as we require it to
> > > be stable for nohz_full, but using it involves costly conversion back and forth to jiffies.  
> > 
> > So both Rik and you agree with the skew tick solution, I will try it
> > tomorrow. Btw, if we should just add random offset to the cpu in the
> > nohz_full mode or add random offset to all cpus like the codes above?  
> 
> Lets just keep it to all CPUs for simplicty.
> Also please add a comment that explains why we need that skew_tick on nohz_full.

I've tried all the test-cases we discussed in this thread with skew_tick=1
and it worked as expected in bare-metal and KVM guests.

However, I found a test-case that works in bare-metal but show problems
in KVM guests. It could something that's KVM specific, or it could be
something that's harder to reproduce in bare-metal.

The reproducer is (not sure all the steps are necessary):

1. Isolate 8 cores in the host with isolcpus= and nohz_full= (and skew_tick=1)

2. Create a KVM guest with 8 vCPUs and pin each vCPU to an isolated
   host core

3. Boot the guest with isolcpus=2,3,4,5,6,7 nohz_full=2,3,4,5,6,7 skew_tick=1

4. Once the guest is booted, run:

# for i in $(seq 2 7); do taskset -c $i hog& ;done
# taskset -c 2,3,4,5,6,7 \
  cyclictest -m -n -q -p95 -D 1m -h60 -i 200 -t 6 -a 2,3,4,5,6,7

  (where hog is a program taking 100% of the CPU, and cyclictest
   is RT's cyclictest)

5. Run top -d1

In a few minutes into this test-case, I see one isolated CPU in the
guest reporting around 95% system time (where the expected is close
to 100% user time, which the others isolated CPUs correctly report).