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Date: Wed, 10 Nov 2021 11:55:04 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: quic_neeraju <quic_neeraju@...cinc.com>
Cc: josh@...htriplett.org, rostedt@...dmis.org,
mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
joel@...lfernandes.org, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org, urezki@...il.com,
frederic@...nel.org, boqun.feng@...il.com
Subject: Re: [PATCH] rcu-tasks: Inspect stalled task's trc state in locked
state
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@...cinc.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
> > >
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