Re: [PATCH] rcuperf: Make rcuperf kernel test more robust for !expedited mode

[Joel Fernandes]

On Fri, Jul 05, 2019 at 12:52:31PM +0900, Byungchul Park wrote:
> On Thu, Jul 04, 2019 at 10:40:44AM -0700, Paul E. McKenney wrote:
> > On Thu, Jul 04, 2019 at 12:34:30AM -0400, Joel Fernandes (Google) wrote:
> > > It is possible that the rcuperf kernel test runs concurrently with init
> > > starting up.  During this time, the system is running all grace periods
> > > as expedited.  However, rcuperf can also be run for normal GP tests.
> > > Right now, it depends on a holdoff time before starting the test to
> > > ensure grace periods start later. This works fine with the default
> > > holdoff time however it is not robust in situations where init takes
> > > greater than the holdoff time to finish running. Or, as in my case:
> > > 
> > > I modified the rcuperf test locally to also run a thread that did
> > > preempt disable/enable in a loop. This had the effect of slowing down
> > > init. The end result was that the "batches:" counter in rcuperf was 0
> > > causing a division by 0 error in the results. This counter was 0 because
> > > only expedited GPs seem to happen, not normal ones which led to the
> > > rcu_state.gp_seq counter remaining constant across grace periods which
> > > unexpectedly happen to be expedited. The system was running expedited
> > > RCU all the time because rcu_unexpedited_gp() would not have run yet
> > > from init.  In other words, the test would concurrently with init
> > > booting in expedited GP mode.
> > > 
> > > To fix this properly, let us check if system_state if SYSTEM_RUNNING
> > > is set before starting the test. The system_state approximately aligns
> 
> Just minor typo..
> 
> To fix this properly, let us check if system_state if SYSTEM_RUNNING
> is set before starting the test. ...
> 
> Should be
> 
> To fix this properly, let us check if system_state is set to
> SYSTEM_RUNNING before starting the test. ...

That's a fair point. I wonder if Paul already fixed it up in his tree,
however I am happy to resend if he hasn't. Paul, how would you like to handle
this commit log nit?

it is just 'if ..' to 'is SYSTEM_RUNNING'