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Message-ID: <20220613175600.GG1790663@paulmck-ThinkPad-P17-Gen-1>
Date: Mon, 13 Jun 2022 10:56:00 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Waiman Long <longman@...hat.com>
Cc: Frederic Weisbecker <frederic@...nel.org>,
Neeraj Upadhyay <quic_neeraju@...cinc.com>,
Josh Triplett <josh@...htriplett.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org,
"Uladzislau Rezki (Sony)" <urezki@...il.com>
Subject: Re: [PATCH] rcu-tasks: Delay rcu_tasks_verify_self_tests() to avoid
missed callbacks
On Mon, Jun 13, 2022 at 12:01:24PM -0400, Waiman Long wrote:
> On 6/10/22 16:58, Paul E. McKenney wrote:
> > On Fri, Jun 10, 2022 at 02:42:12PM -0400, Waiman Long wrote:
> > > Even though rcu_tasks selftest is initiated early in the boot process,
> > > the verification done at late initcall time may not be late enough to
> > > catch all the callbacks especially on systems with just a few cpus and
> > > small memory.
> > >
> > > After 12 bootup's On a s390x system, 1 of them had failed rcu_tasks
> > > verification test.
> > >
> > > [ 8.183013] call_rcu_tasks() has been failed.
> > > [ 8.183041] WARNING: CPU: 0 PID: 1 at kernel/rcu/tasks.h:1696 rcu_tasks_verify_self_tests+0x64/0xd0
> > > [ 8.203246] Callback from call_rcu_tasks() invoked.
> > >
> > > In this particular case, the callback missed the check by about
> > > 20ms. Similar rcu_tasks selftest failures are also seen in ppc64le
> > > systems.
> > >
> > > [ 0.313391] call_rcu_tasks() has been failed.
> > > [ 0.313407] WARNING: CPU: 0 PID: 1 at kernel/rcu/tasks.h:1696 rcu_tasks_verify_self_tests+0x5c/0xa0
> > > [ 0.335569] Callback from call_rcu_tasks() invoked.
> > >
> > > Avoid this missed callback by delaying the verification using
> > > delayed_work. The delay is set to be about 0.1s which hopefully will
> > > be long enough to catch all the callbacks on systems with few cpus and
> > > small memory.
> > >
> > > Fixes: bfba7ed084f8 ("rcu-tasks: Add RCU-tasks self tests")
> > > Signed-off-by: Waiman Long <longman@...hat.com>
> > Good catch, thank you!
> >
> > A few days ago, I queued this:
> >
> > 2585014188d5 ("rcu-tasks: Be more patient for RCU Tasks boot-time testing")
> >
> > This is shown in full at the end of this email. Does this fix this
> > problem for you?
>
> I think your patch should fix the false positive warning and it give plenty
> of time for this to happen.
>
> I do have one question though. rcu_tasks_verify_selft_tests() is called from
> do_initcalls(). Since it may not be the last late initcall, does that mean
> other late initcalls queued after that may be delayed by a second or more?
Indeed. Which is why I would welcome the workqueues portion of your
patch on top of the above patch in -rcu. ;-)
Thanx, Paul
> > > ---
> > > kernel/rcu/tasks.h | 19 ++++++++++++++++---
> > > 1 file changed, 16 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > > index 3925e32159b5..25f964a671ba 100644s
> > > --- a/kernel/rcu/tasks.h
> > > +++ b/kernel/rcu/tasks.h
> > > @@ -1735,7 +1735,7 @@ static void rcu_tasks_initiate_self_tests(void)
> > > #endif
> > > }
> > > -static int rcu_tasks_verify_self_tests(void)
> > > +static void rcu_tasks_verify_self_tests(struct work_struct *work __maybe_unused)
> > > {
> > > int ret = 0;
> > > int i;
> > > @@ -1749,10 +1749,23 @@ static int rcu_tasks_verify_self_tests(void)
> > > if (ret)
> > > WARN_ON(1);
> > > +}
> > > +
> > > +static struct delayed_work rcu_tasks_verify_work;
> > > - return ret;
> > > +/*
> > > + * The rcu_tasks verification is done indirectly via the work queue to
> > > + * introduce an additional 0.1s delay to catch all the callbacks before
> > > + * the verification is done as late_initcall time may not be late enough
> > > + * to have all the callbacks fired.
> > > + */
> > > +static int rcu_tasks_verify_schedule_work(void)
> > > +{
> > > + INIT_DELAYED_WORK(&rcu_tasks_verify_work, rcu_tasks_verify_self_tests);
> > > + schedule_delayed_work(&rcu_tasks_verify_work, HZ/10);
> > > + return 0;
> > > }
> > > -late_initcall(rcu_tasks_verify_self_tests);
> > > +late_initcall(rcu_tasks_verify_schedule_work);
> > > #else /* #ifdef CONFIG_PROVE_RCU */
> > > static void rcu_tasks_initiate_self_tests(void) { }
> > > #endif /* #else #ifdef CONFIG_PROVE_RCU */
> > > --
> > > 2.31.1
> > ------------------------------------------------------------------------
> >
> > commit 2585014188d5e66052b4226b42602b6f3d921389
> > Author: Paul E. McKenney <paulmck@...nel.org>
> > Date: Tue Jun 7 15:23:52 2022 -0700
> >
> > rcu-tasks: Be more patient for RCU Tasks boot-time testing
> > The RCU-Tasks family of grace-period primitives can take some time to
> > complete, and the amount of time can depend on the exact hardware and
> > software configuration. Some configurations boot up fast enough that the
> > RCU-Tasks verification process gets false-positive failures. This commit
> > therefore allows up to 30 seconds for the grace periods to complete, with
> > this value adjustable downwards using the rcupdate.rcu_task_stall_timeout
> > kernel boot parameter.
> > Reported-by: Matthew Wilcox <willy@...radead.org>
> > Reported-by: Zhouyi Zhou <zhouzhouyi@...il.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > Tested-by: Zhouyi Zhou <zhouzhouyi@...il.com>
> >
> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > index df6b2cb2f205d..fcbd0ec33c866 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -145,6 +145,7 @@ static int rcu_task_ipi_delay __read_mostly = RCU_TASK_IPI_DELAY;
> > module_param(rcu_task_ipi_delay, int, 0644);
> > /* Control stall timeouts. Disable with <= 0, otherwise jiffies till stall. */
> > +#define RCU_TASK_BOOT_STALL_TIMEOUT (HZ * 30)
> > #define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
> > static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
> > module_param(rcu_task_stall_timeout, int, 0644);
> > @@ -1776,23 +1777,24 @@ struct rcu_tasks_test_desc {
> > struct rcu_head rh;
> > const char *name;
> > bool notrun;
> > + unsigned long runstart;
> > };
> > static struct rcu_tasks_test_desc tests[] = {
> > {
> > .name = "call_rcu_tasks()",
> > /* If not defined, the test is skipped. */
> > - .notrun = !IS_ENABLED(CONFIG_TASKS_RCU),
> > + .notrun = IS_ENABLED(CONFIG_TASKS_RCU),
> > },
> > {
> > .name = "call_rcu_tasks_rude()",
> > /* If not defined, the test is skipped. */
> > - .notrun = !IS_ENABLED(CONFIG_TASKS_RUDE_RCU),
> > + .notrun = IS_ENABLED(CONFIG_TASKS_RUDE_RCU),
> > },
> > {
> > .name = "call_rcu_tasks_trace()",
> > /* If not defined, the test is skipped. */
> > - .notrun = !IS_ENABLED(CONFIG_TASKS_TRACE_RCU)
> > + .notrun = IS_ENABLED(CONFIG_TASKS_TRACE_RCU)
> > }
> > };
> > @@ -1803,23 +1805,28 @@ static void test_rcu_tasks_callback(struct rcu_head *rhp)
> > pr_info("Callback from %s invoked.\n", rttd->name);
> > - rttd->notrun = true;
> > + rttd->notrun = false;
> > }
> > static void rcu_tasks_initiate_self_tests(void)
> > {
> > + unsigned long j = jiffies;
> > +
> > pr_info("Running RCU-tasks wait API self tests\n");
> > #ifdef CONFIG_TASKS_RCU
> > + tests[0].runstart = j;
> > synchronize_rcu_tasks();
> > call_rcu_tasks(&tests[0].rh, test_rcu_tasks_callback);
> > #endif
> > #ifdef CONFIG_TASKS_RUDE_RCU
> > + tests[1].runstart = j;
> > synchronize_rcu_tasks_rude();
> > call_rcu_tasks_rude(&tests[1].rh, test_rcu_tasks_callback);
> > #endif
> > #ifdef CONFIG_TASKS_TRACE_RCU
> > + tests[2].runstart = j;
> > synchronize_rcu_tasks_trace();
> > call_rcu_tasks_trace(&tests[2].rh, test_rcu_tasks_callback);
> > #endif
> > @@ -1829,11 +1836,18 @@ static int rcu_tasks_verify_self_tests(void)
> > {
> > int ret = 0;
> > int i;
> > + unsigned long bst = rcu_task_stall_timeout;
> > + if (bst <= 0 || bst > RCU_TASK_BOOT_STALL_TIMEOUT)
> > + bst = RCU_TASK_BOOT_STALL_TIMEOUT;
> > for (i = 0; i < ARRAY_SIZE(tests); i++) {
> > - if (!tests[i].notrun) { // still hanging.
> > - pr_err("%s has been failed.\n", tests[i].name);
> > - ret = -1;
> > + while (tests[i].notrun) { // still hanging.
> > + if (time_after(jiffies, tests[i].runstart + bst)) {
> > + pr_err("%s has failed boot-time tests.\n", tests[i].name);
> > + ret = -1;
> > + break;
> > + }
> > + schedule_timeout_uninterruptible(1);
> > }
> > }
> >
>
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