Message-ID: <174385625883.111.5710608020798879272@patchwork.local>
Date: Sat, 05 Apr 2025 12:30:58 -0000
From: Joel Fernandes <joelagnelf@...dia.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Joel Fernandes <joelagnelf@...dia.com>, linux-kernel <linux-kernel@...r.kernel.org>, Frederic Weisbecker <frederic@...nel.org>, Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, Josh Triplett <josh@...htriplett.org>, Boqun Feng <boqun.feng@...il.com>, Uladzislau Rezki <urezki@...il.com>, Steven Rostedt <rostedt@...dmis.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>, Davidlohr Bueso <dave@...olabs.net>, rcu <rcu@...r.kernel.org>
Subject: Re: rcutorture: Perform more frequent testing of ->gpwrap

Hello, Paul,

On Sat, 5 Apr 2025 12:26:12 GMT, "Paul E. McKenney" wrote:
> On Sat, Mar 29, 2025 at 07:01:42PM -0400, Joel Fernandes wrote:
> > Currently, the ->gpwrap is not tested (at all per my testing) due to the
> > requirement of a large delta between a CPU's rdp->gp_seq and its node's
> > rnp->gpseq.
> > 
> > This results in no testing of ->gpwrap being set. This patch by default
> > adds 5 minutes of testing with ->gpwrap forced by lowering the delta
> > between rdp->gp_seq and rnp->gp_seq to just 8 GPs. All of this is
> > configurable, including the active time for the setting and a full
> > testing cycle.
> > 
> > By default, the first 25 minutes of a test will have the _default_
> > behavior there is right now (ULONG_MAX / 4) delta. Then for 5 minutes,
> > we switch to a smaller delta causing 1-2 wraps in 5 minutes. I believe
> > this is reasonable since we at least add a little bit of testing for
> > usecases where ->gpwrap is set.
> > 
> > Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> 
> I ran this as follows:
> 
> tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 10m --configs "TREE01" --bootargs "rcutorture.ovf_cycle_mins=7" --trust-make
> 
> Once I actually applied your patch, I did get this:
> 
> [  601.891042] gpwraps: 13745
> 
> Which seems to indicate some testing.  ;-)

Thanks a lot for running it. I am wondering if I should check in tree.c (only in
testing mode), if the wraps are too many and restrict testing if so. Otherwise,
it is hard to come up with a constant that ensures the wraps are under control.
On the other hand, since this is only for 5 minutes every 30 minutes, we can leave
it as is and avoid the complexity.

> 
> Additional comments inline.
> 
> 							Thanx, Paul
> 
> > ---
> >  kernel/rcu/rcu.h        |  4 +++
> >  kernel/rcu/rcutorture.c | 64 +++++++++++++++++++++++++++++++++++++++++
> >  kernel/rcu/tree.c       | 34 ++++++++++++++++++++--
> >  kernel/rcu/tree.h       |  1 +
> >  4 files changed, 101 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > index eed2951a4962..9a15e9701e02 100644
> > --- a/kernel/rcu/rcu.h
> > +++ b/kernel/rcu/rcu.h
> > @@ -572,6 +572,8 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
> >  			       unsigned long c_old,
> >  			       unsigned long c);
> >  void rcu_gp_set_torture_wait(int duration);
> > +void rcu_set_torture_ovf_lag(unsigned long lag);
> > +int rcu_get_gpwrap_count(int cpu);
> >  #else
> >  static inline void rcutorture_get_gp_data(int *flags, unsigned long *gp_seq)
> >  {
> > @@ -589,6 +591,8 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
> >  	do { } while (0)
> >  #endif
> >  static inline void rcu_gp_set_torture_wait(int duration) { }
> > +static inline void rcu_set_torture_ovf_lag(unsigned long lag) { }
> > +static inline int rcu_get_gpwrap_count(int cpu) { return 0; }
> 
> Very good, you did remember CONFIG_SMP=n.  And yes, I did try it.  ;-)
> 
> But shouldn't these be function pointers in rcu_torture_ops?  That way if
> some other flavor of RCU starts doing wrap protection for its grace-period
> sequence numbers, this testing can apply directly to that flavor as well.

These are here because 'rdp' is not accessible AFAIK from rcutorture.c.
I could add wrappers to these and include them as pointers the a struct as well.
But I think these will still stay to access rdp.

> 
> Then the pointers can simply be NULL in kernels built with CONFIG_SMP=n.
> 
> >  #endif
> >  unsigned long long rcutorture_gather_gp_seqs(void);
> >  void rcutorture_format_gp_seqs(unsigned long long seqs, char *cp, size_t len);
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index 895a27545ae1..79a72e70913e 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -118,6 +118,9 @@ torture_param(int, nreaders, -1, "Number of RCU reader threads");
> >  torture_param(int, object_debug, 0, "Enable debug-object double call_rcu() testing");
> >  torture_param(int, onoff_holdoff, 0, "Time after boot before CPU hotplugs (s)");
> >  torture_param(int, onoff_interval, 0, "Time between CPU hotplugs (jiffies), 0=disable");
> > +torture_param(int, ovf_cycle_mins, 30, "Total cycle duration for ovf lag testing (in minutes)");
> > +torture_param(int, ovf_active_mins, 5, "Duration for which ovf lag is active within each cycle (in minutes)");
> > +torture_param(int, ovf_lag_gps, 8, "Value to set for set_torture_ovf_lag during an active testing period.");
> 
> Given that "ovf" means just "overflow", would it make sense to get a "gp"
> in there?  Maybe just "gpwrap_..."?
> 
> "What is in a name?"  ;-)

Sure, makes sense I will rename.

> 
> I could argue with the defaults, but I run long tests often enough that
> I am not worried about coverage.  As long as we remember to either run
> long tests or specify appropriate rcutorture.ovf_cycle_mins when messing
> with ->gpwrap code.
> 
> >  torture_param(int, nocbs_nthreads, 0, "Number of NOCB toggle threads, 0 to disable");
> >  torture_param(int, nocbs_toggle, 1000, "Time between toggling nocb state (ms)");
> >  torture_param(int, preempt_duration, 0, "Preemption duration (ms), zero to disable");
> > @@ -2629,6 +2632,7 @@ rcu_torture_stats_print(void)
> >  	int i;
> >  	long pipesummary[RCU_TORTURE_PIPE_LEN + 1] = { 0 };
> >  	long batchsummary[RCU_TORTURE_PIPE_LEN + 1] = { 0 };
> > +	long n_gpwraps = 0;
> >  	struct rcu_torture *rtcp;
> >  	static unsigned long rtcv_snap = ULONG_MAX;
> >  	static bool splatted;
> > @@ -2639,6 +2643,7 @@ rcu_torture_stats_print(void)
> >  			pipesummary[i] += READ_ONCE(per_cpu(rcu_torture_count, cpu)[i]);
> >  			batchsummary[i] += READ_ONCE(per_cpu(rcu_torture_batch, cpu)[i]);
> >  		}
> > +		n_gpwraps += rcu_get_gpwrap_count(cpu);
> >  	}
> >  	for (i = RCU_TORTURE_PIPE_LEN; i >= 0; i--) {
> >  		if (pipesummary[i] != 0)
> > @@ -2672,6 +2677,7 @@ rcu_torture_stats_print(void)
> >  	pr_cont("read-exits: %ld ", data_race(n_read_exits)); // Statistic.
> >  	pr_cont("nocb-toggles: %ld:%ld\n",
> 
> The "\n" on the above line needs to be deleted.

Ok.

> >  		atomic_long_read(&n_nocb_offload), atomic_long_read(&n_nocb_deoffload));
> > +	pr_cont("gpwraps: %ld\n", n_gpwraps);
> >  
> >  	pr_alert("%s%s ", torture_type, TORTURE_FLAG);
> >  	if (atomic_read(&n_rcu_torture_mberror) ||
> > @@ -3842,6 +3848,58 @@ static int rcu_torture_preempt(void *unused)
> >  
> >  static enum cpuhp_state rcutor_hp;
> >  
> > +static struct hrtimer ovf_lag_timer;
> > +static bool ovf_lag_active;
> 
> Same "ovf" naming complaint as before.

Ok.

> > +}
> > +
> > +static int rcu_torture_ovf_lag_init(void)
> > +{
> > +	if (ovf_cycle_mins <= 0 || ovf_active_mins <= 0) {
> > +		pr_alert("rcu-torture: lag timing parameters must be positive\n");
> > +		return -EINVAL;
> > +	}
> 
> Why not refuse to start this portion of the test when testing CONFIG_SMP=n
> or something other than vanilla RCU?  No need to fail the test, just
> print something saying that this testing won't be happening.

Got it, will do.

> > +
> > +static void rcu_torture_ovf_lag_cleanup(void)
> > +{
> > +	hrtimer_cancel(&ovf_lag_timer);
> > +
> > +	if (ovf_lag_active) {
> > +		rcu_set_torture_ovf_lag(0);
> > +		ovf_lag_active = false;
> > +	}
> > +}
> 
> Did you try the modprobe/rmmod testing to verify that this
> cleans up appropriately?  You could use the drgn tool to check.
> See tools/rcu//rcu-cbs.py for an example drgn script that digs into the
> rcu_data structures.

Nice, will check!

Will work on this and provide v2.

thanks,

 - Joel

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