Message-ID: <Z5fdaj2HnhpagC-A@pc636>
Date: Mon, 27 Jan 2025 20:24:26 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Uladzislau Rezki <urezki@...il.com>, Boqun Feng <boqun.feng@...il.com>,
	RCU <rcu@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Cheung Wall <zzqq0103.hey@...il.com>,
	Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH 2/4] torture: Remove CONFIG_NR_CPUS configuration

On Mon, Jan 27, 2025 at 10:15:21AM -0800, Paul E. McKenney wrote:
> On Mon, Jan 27, 2025 at 06:26:59PM +0100, Uladzislau Rezki wrote:
> > On Mon, Jan 27, 2025 at 08:51:01AM -0800, Paul E. McKenney wrote:
> > > On Mon, Jan 27, 2025 at 04:42:58PM +0100, Uladzislau Rezki wrote:
> > > > On Mon, Jan 27, 2025 at 06:51:44AM -0800, Paul E. McKenney wrote:
> > > > > On Mon, Jan 27, 2025 at 02:27:51PM +0100, Uladzislau Rezki wrote:
> > > > > > 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@...il.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
> > > > > 
> > > > > Making each guest OS smaller needs "--kconfig CONFIG_NR_CPUS=4" (or
> > > > > whatever) as well, perhaps also increasing the "16*TREE05".
> > > > > 
> > > > 
> > > > By default we have NR_CPUS=8, we we discussed. Providing to kvm "--cpus 5"
> > > > parameter will just set number of CPUs for a VM to 5:
> > > > 
> > > > <snip>
> > > > ...
> > > > [    0.060672] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=5, Nodes=1
> > > > ...
> > > > <snip>
> > > > 
> > > > so, for my test i do not see why i need to set --kconfig CONFIG_NR_CPUS=4.
> > > > 
> > > > Am i missing something? :)
> > > 
> > > Because that gets you more guest OSes running on your system, each with
> > > one RCU-update kthread that is being checked by RCU reader kthreads.
> > > Therefore, it might double the rate at which you are able to reproduce
> > > this issue.
> > > 
> > You mean that setting --kconfig CONFIG_NR_CPUS=4 and 16*TREE05 will run
> > 4 separate KVM instances?
> 
> Almost but not quite.
> 
> I am assuming that you have a system with a multiple of eight CPUs.
> 
> If so, and assuming that Cheung's bug is an interaction between a fast
> synchronize_rcu() grace period and a reader task that this grace period
> is waiting on, having more and smaller guest OSes might make the problem
> happen faster.  So instead of your:
> 
> tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 5 --configs \
> '16*TREE05' --memory 10G --bootargs 'rcutorture.fwd_progress=1'
> 
> You might be able to double the number of reproductions of the bug
> per unit time by instead using:
> 
> tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 5 --configs \
> '32*TREE05' --memory 10G --bootargs 'rcutorture.fwd_progress=1' \
> --kconfig "CONFIG_NR_CPUS=4"
> 
> Does that seem reasonable to you?
>
It only runs one instance for me:

tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 5 --configs 32*TREE05 --memory 10G --bootargs rcutorture.fwd_progress=1 --kconfig CONFIG_NR_CPUS=4
----Start batch 1: Mon Jan 27 08:20:17 PM CET 2025
TREE05 4: Starting build. Mon Jan 27 08:20:17 PM CET 2025
TREE05 4: Waiting for build to complete. Mon Jan 27 08:20:17 PM CET 2025
TREE05 4: Build complete. Mon Jan 27 08:21:26 PM CET 2025
---- TREE05 4: Kernel present. Mon Jan 27 08:21:26 PM CET 2025
---- Starting kernels. Mon Jan 27 08:21:26 PM CET 2025

with 4 CPUs inside VM :)

--
Uladzislau Rezki

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