Re: [PATCH 2/4] torture: Remove CONFIG_NR_CPUS configuration

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Jan 24, 2025 at 11:34:03AM -0800, Paul E. McKenney wrote:
> On Fri, Jan 24, 2025 at 06:48:40PM +0100, Uladzislau Rezki wrote:
> > On Fri, Jan 24, 2025 at 09:36:07AM -0800, Paul E. McKenney wrote:
> > > On Fri, Jan 24, 2025 at 06:21:30PM +0100, Uladzislau Rezki wrote:
> > > > On Fri, Jan 24, 2025 at 07:45:23AM -0800, Paul E. McKenney wrote:
> > > > > On Fri, Jan 24, 2025 at 12:41:38PM +0100, Uladzislau Rezki wrote:
> > > > > > On Thu, Jan 23, 2025 at 12:29:45PM -0800, Paul E. McKenney wrote:
> > > > > > > On Thu, Jan 23, 2025 at 07:58:26PM +0100, Uladzislau Rezki (Sony) wrote:
> > > > > > > > This configuration specifies the maximum number of CPUs which
> > > > > > > > is set to 8. The problem is that it can not be overwritten for
> > > > > > > > something higher.
> > > > > > > > 
> > > > > > > > Remove that configuration for TREE05, so it is possible to run
> > > > > > > > the torture test on as many CPUs as many system has.
> > > > > > > > 
> > > > > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > > > > > 
> > > > > > > You should be able to override this on the kvm.sh command line by
> > > > > > > specifying "--kconfig CONFIG_NR_CPUS=128" or whatever number you wish.
> > > > > > > For example, see the torture.sh querying the system's number of CPUs
> > > > > > > and then specifying it to a number of tests.
> > > > > > > 
> > > > > > > Or am I missing something here?
> > > > > > > 
> > > > > > It took me a while to understand what happens. Apparently there is this
> > > > > > 8 CPUs limitation. Yes, i can do it manually by passing --kconfig but
> > > > > > you need to know about that. I have not expected that.
> > > > > > 
> > > > > > Therefore i removed it from the configuration because i have not found
> > > > > > a good explanation why we need. It is confusing instead :)
> > > > > 
> > > > > Right now, if I do a run with --configs "TREE10 14*CFLIST", this will
> > > > > make use of 20 systems with 80 CPUs each.  If you remove that line from
> > > > > TREE05, won't each instance of TREE05 consume a full system, for a total
> > > > > of 33 systems?  Yes, I could use "--kconfig CONFIG_NR_CPUS=8" on the
> > > > > command line, but that would affect all the scenarios, not just TREE05.
> > > > > Including (say) TINY01, where I believe that it would cause kvm.sh
> > > > > to complain about a Kconfig conflict.
> > > > > 
> > > > > Hence me not being in favor of this change.  ;-)
> > > > > 
> > > > > Is there another way to make things work for both situations?
> > > > > 
> > > > OK, i see. Well. I will just go with --kconfig CONFIG_NR_CPUS=foo if i
> > > > need more CPUs for TREE05.
> > > > 
> > > > I will not resist, we just drop this patch :)
> > > 
> > > Thank you!
> > > 
> > > The bug you are chasing happens when a given synchonize_rcu() interacts
> > > with RCU readers, correct?
> > > 
> > Below one:
> > 
> > <snip>
> > /*
> >  * RCU torture fake writer kthread.  Repeatedly calls sync, with a random
> >  * delay between calls.
> >  */
> > static int
> > rcu_torture_fakewriter(void *arg)
> > {
> > ...
> > <snip>
> > 
> > > In rcutorture, only the rcu_torture_writer() call to synchronize_rcu()
> > > interacts with rcu_torture_reader().  So my guess is that running
> > > many small TREE05 guest OSes would reproduce this bug more quickly.
> > > So instead of this:
> > > 
> > > 	--kconfig CONFIG_NR_CPUS=128
> > > 
> > > Do this:
> > > 
> > > 	--configs "16*TREE05"
> > > 
> > > Or maybe even this:
> > > 
> > > 	--configs "16*TREE05" --kconfig CONFIG_NR_CPUS=4
> > Thanks for input.
> > 
> > > 
> > > Thoughts?
> > > 
> > If you mean below splat:
> 
> > 
> > i.e. with more nfakewriters.
> 
> Right, and large nfakewriters would help push the synchronize_rcu()
> wakeups off of the grace-period kthread.
> 
> > If you mean the one that has recently reported, i am not able to
> > reproduce it anyhow :)
> 
> Using larger numbers of smaller rcutorture guest OSes might help to
> reproduce it.  Maybe as small as three CPUs each.  ;-)
> 
OK. I will give a try this:

for (( i=0; i<$LOOPS; i++ )); do
	tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 5 --configs \
	'16*TREE05' --memory 10G --bootargs 'rcutorture.fwd_progress=1'
	echo "Done $i"
done

--
Uladzislau Rezki