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

[Uladzislau Rezki <urezki@gmail.com>]

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:

<snip>
[   32.107748] =============================
[   32.108512] WARNING: suspicious RCU usage
[   32.109232] 6.12.0-rc4-dirty #66 Not tainted
[   32.110058] -----------------------------
[   32.110817] kernel/events/core.c:13962 RCU-list traversed in non-reader section!!
[   32.111221] kworker/u34:2 (251) used greatest stack depth: 12112 bytes left
[   32.112125]
[   32.112125] other info that might help us debug this:
[   32.112125]
[   32.112130]
[   32.112130] rcu_scheduler_active = 2, debug_locks = 1
[   32.116039] 3 locks held by cpuhp/1/20:
[   32.116758]  #0: ffffffff93a6a750 (cpu_hotplug_lock){++++}-{0:0}, at: cpuhp_thread_fun+0x50/0x220
[   32.118410]  #1: ffffffff93a6ce00 (cpuhp_state-down){+.+.}-{0:0}, at: cpuhp_thread_fun+0x50/0x220
[   32.120091]  #2: ffffffff93b7eb68 (pmus_lock){+.+.}-{3:3}, at: perf_event_exit_cpu_context+0x32/0x2d0
[   32.121723]
[   32.121723] stack backtrace:
[   32.122413] CPU: 1 UID: 0 PID: 20 Comm: cpuhp/1 Not tainted 6.12.0-rc4-dirty #66
[   32.123666] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1.1 04/01/2014
[   32.125302] Call Trace:
[   32.125769]  <TASK>
[   32.126148]  dump_stack_lvl+0x83/0xa0
[   32.126823]  lockdep_rcu_suspicious+0x113/0x180
[   32.127652]  perf_event_exit_cpu_context+0x2c4/0x2d0
[   32.128593]  ? __pfx_perf_event_exit_cpu+0x10/0x10
[   32.129489]  perf_event_exit_cpu+0x9/0x10
[   32.130243]  cpuhp_invoke_callback+0x187/0x6e0
[   32.131065]  ? cpuhp_thread_fun+0x50/0x220
[   32.131800]  cpuhp_thread_fun+0x185/0x220
[   32.132560]  ? __pfx_smpboot_thread_fn+0x10/0x10                                                                                                                           [   32.133394]  smpboot_thread_fn+0xd8/0x1d0
[   32.134050]  kthread+0xd0/0x100
[   32.134592]  ? __pfx_kthread+0x10/0x10
[   32.135270]  ret_from_fork+0x2f/0x50
[   32.135896]  ? __pfx_kthread+0x10/0x10
[   32.136610]  ret_from_fork_asm+0x1a/0x30
[   32.137356]  </TASK>
[   32.140997] smpboot: CPU 1 is now offline
<snip>

I reproduced that using:

+rcutorture.nfakewriters=128
+rcutorture.gp_sync=1
+rcupdate.rcu_expedited=0
+rcupdate.rcu_normal=1
+rcutree.rcu_normal_wake_from_gp=1
<snip>

The test script:

for (( i=0; i<$LOOPS; i++ )); do
        tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 64 --configs \
        '100*TREE05' --memory 20G --bootargs 'rcutorture.fwd_progress=1'
        echo "Done $i"
done

i.e. with more nfakewriters.

If you mean the one that has recently reported, i am not able to
reproduce it anyhow :)

--
Uladzislau Rezki