Date:   Wed, 10 Nov 2021 11:55:04 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     quic_neeraju <quic_neeraju@quicinc.com>
Cc:     josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        joel@joelfernandes.org, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, urezki@gmail.com,
        frederic@kernel.org, boqun.feng@gmail.com
Subject: Re: [PATCH] rcu-tasks: Inspect stalled task's trc state in locked
 state
Message-ID: <20211110195504.GE641268@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20211109112214.19618-1-quic_neeraju@quicinc.com>
 <20211109145235.GV641268@paulmck-ThinkPad-P17-Gen-1>
 <d099f078-eef7-1c44-7086-b67971218f3f@quicinc.com>
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On Wed, Nov 10, 2021 at 10:10:27AM +0530, quic_neeraju wrote:
> Hi Paul,
> 
> 
> On 11/9/2021 8:22 PM, Paul E. McKenney wrote:
> > On Tue, Nov 09, 2021 at 04:52:14PM +0530, Neeraj Upadhyay wrote:
> > > On RCU tasks trace stall, inspect the RCU-tasks-trace specific
> > > states of stalled task in locked down state, using try_invoke_
> > > on_locked_down_task(), to get reliable trc state of a non-running
> > > stalled task.
> > > 
> > > Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> > 
> > Queued for further review and testing, thank you!
> > 
> > Along those lines, what did you do to test this?  I would like to
> > add that to the commit message.  (The usual approach is to use the
> > rcutorture.stall_cpu module parameter, in case I have not yet passed
> > that along.)
> > 
> 
> Before sending the patch, I had tested with TRACE01 and adding long delay
> before rcu_read_unlock_trace() in tasks_tracing_torture_read_unlock()
> 
> static void tasks_tracing_torture_read_unlock(int idx)
> {
>         rcu_read_unlock_trace();
> }
> 
> I retested with below commandline (without adding delay in
> tasks_tracing_torture_read_unlock()):
> 
> tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 8 --configs TRACE01
> --bootargs "rcutorture.torture_type=tasks-tracing rcutorture.stall_cpu=10
> rcutorture.stall_cpu_block=1 rcupdate.rcu_task_stall_timeout=100"
> --trust-make
> 
> With this, I can see stalls for both RUNNING and WAITING state task:

Applied, thank you!

							Thanx, Paul

> [   21.520291] INFO: rcu_tasks_trace detected stalls on tasks:
> [   21.521292] P85: ... nesting: 1N cpu: 2
> [   21.521966] task:rcu_torture_sta state:D stack:15080 pid:   85 ppid:
> 2 flags:0x00004000
> [   21.523384] Call Trace:
> [   21.523808]  __schedule+0x273/0x6e0
> [   21.524428]  schedule+0x35/0xa0
> [   21.524971]  schedule_timeout+0x1ed/0x270
> [   21.525690]  ? del_timer_sync+0x30/0x30
> [   21.526371]  ? rcu_torture_writer+0x720/0x720
> [   21.527106]  rcu_torture_stall+0x24a/0x270
> [   21.527816]  kthread+0x115/0x140
> [   21.528401]  ? set_kthread_struct+0x40/0x40
> [   21.529136]  ret_from_fork+0x22/0x30
> [   21.529766]  1 holdouts
> [   21.632300] INFO: rcu_tasks_trace detected stalls on tasks:
> [   21.632345] rcu_torture_stall end.
> [   21.633293] P85: .
> [   21.633294] task:rcu_torture_sta state:R  running task stack:15080 pid:
> 85 ppid:     2 flags:0x00004000
> [   21.633299] Call Trace:
> [   21.633301]  ? vprintk_emit+0xab/0x180
> [   21.633306]  ? vprintk_emit+0x11a/0x180
> [   21.633308]  ? _printk+0x4d/0x69
> [   21.633311]  ? __default_send_IPI_shortcut+0x1f/0x40
> 
> 
> 
> Thanks
> Neeraj
> 
> > 							Thanx, Paul
> > 
> > > ---
> > >   kernel/rcu/tasks.h | 43 ++++++++++++++++++++++++++++++++++---------
> > >   1 file changed, 34 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > > index e4a32db9f712..4e49b847971b 100644
> > > --- a/kernel/rcu/tasks.h
> > > +++ b/kernel/rcu/tasks.h
> > > @@ -1073,25 +1073,50 @@ static void rcu_tasks_trace_postscan(struct list_head *hop)
> > >   	// Any tasks that exit after this point will set ->trc_reader_checked.
> > >   }
> > > +/* Communicate task state back to the RCU tasks trace stall warning request. */
> > > +struct trc_stall_chk_rdr {
> > > +	int nesting;
> > > +	int ipi_to_cpu;
> > > +	u8 needqs;
> > > +};
> > > +
> > > +static bool trc_check_slow_task(struct task_struct *t, void *arg)
> > > +{
> > > +	struct trc_stall_chk_rdr *trc_rdrp = arg;
> > > +
> > > +	if (task_curr(t))
> > > +		return false; // It is running, so decline to inspect it.
> > > +	trc_rdrp->nesting = READ_ONCE(t->trc_reader_nesting);
> > > +	trc_rdrp->ipi_to_cpu = READ_ONCE(t->trc_ipi_to_cpu);
> > > +	trc_rdrp->needqs = READ_ONCE(t->trc_reader_special.b.need_qs);
> > > +	return true;
> > > +}
> > > +
> > >   /* Show the state of a task stalling the current RCU tasks trace GP. */
> > >   static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
> > >   {
> > >   	int cpu;
> > > +	struct trc_stall_chk_rdr trc_rdr;
> > > +	bool is_idle_tsk = is_idle_task(t);
> > >   	if (*firstreport) {
> > >   		pr_err("INFO: rcu_tasks_trace detected stalls on tasks:\n");
> > >   		*firstreport = false;
> > >   	}
> > > -	// FIXME: This should attempt to use try_invoke_on_nonrunning_task().
> > >   	cpu = task_cpu(t);
> > > -	pr_alert("P%d: %c%c%c nesting: %d%c cpu: %d\n",
> > > -		 t->pid,
> > > -		 ".I"[READ_ONCE(t->trc_ipi_to_cpu) >= 0],
> > > -		 ".i"[is_idle_task(t)],
> > > -		 ".N"[cpu >= 0 && tick_nohz_full_cpu(cpu)],
> > > -		 READ_ONCE(t->trc_reader_nesting),
> > > -		 " N"[!!READ_ONCE(t->trc_reader_special.b.need_qs)],
> > > -		 cpu);
> > > +	if (!try_invoke_on_locked_down_task(t, trc_check_slow_task, &trc_rdr))
> > > +		pr_alert("P%d: %c\n",
> > > +			 t->pid,
> > > +			 ".i"[is_idle_tsk]);
> > > +	else
> > > +		pr_alert("P%d: %c%c%c nesting: %d%c cpu: %d\n",
> > > +			 t->pid,
> > > +			 ".I"[trc_rdr.ipi_to_cpu >= 0],
> > > +			 ".i"[is_idle_tsk],
> > > +			 ".N"[cpu >= 0 && tick_nohz_full_cpu(cpu)],
> > > +			 trc_rdr.nesting,
> > > +			 " N"[!!trc_rdr.needqs],
> > > +			 cpu);
> > >   	sched_show_task(t);
> > >   }
> > > -- 
> > > 2.17.1
> > >