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SubjectRe: Boot warning at rcu_check_gp_start_stall()
On Tue, Jan 21, 2020 at 02:09:05PM -0500, Qian Cai wrote:
> > On Jan 21, 2020, at 11:15 AM, Paul E. McKenney <paulmck@kernel.org> wrote:
> > On Tue, Jan 21, 2020 at 09:37:13AM -0500, Qian Cai wrote:
> >>> On Jan 21, 2020, at 9:19 AM, Paul E. McKenney <paulmck@kernel.org> wrote:
> >>>
> >>> One approach would be to boot with rcupdate.rcu_cpu_stall_timeout=300,
> >>> which would allow more time.
> >>
> >> It works for me if once that warning triggered, give a bit information about adjusting the parameter when debugging options are on to suppress the warning due to expected long boot.
> >
> > Indeed. 300 seconds as shown above is currently the maximum, but
> > please let me know if it needs to be increased. This module parameter
> > is writable after boot via sysfs, so maybe that could be part of the
> > workaround.
> >
> >>> Longer term, I could suppress this warning during boot when
> >>> CONFIG_EFI_PGT_DUMP=y, but that sounds quite specific. Alternatively,
> >>> I could provide a Kconfig option that suppressed this during boot
> >>> that was selected by whatever long-running boot-time Kconfig option
> >>> needed it. Yet another approach would be for long-running operations
> >>> like efi_dump_pagetable() to suppress stalls on entry and re-enable them
> >>> upon exit.
> >>>
> >>> Thoughts?
> >>
> >> None of the options sounds particularly better for me because there could come up with other options may trigger this, memtest comes in mind, for example. Then, it is a bit of pain to maintain of unknown.
> >
> > I was afraid of that. ;-)
> >
> > Could you please send me the full dmesg up to that point? No promises,
> > but it might well be that I can make some broad-spectrum adjustment
> > within RCU. Only one way to find out…
>
> https://cailca.github.io/files/dmesg.txt

Interesting.

Does the following (very lightly tested) patch help?

Thanx, Paul

------------------------------------------------------------------------

commit fb21277f8f1c5cc40a8d41da2db4b0c499459821
Author: Paul E. McKenney <paulmck@kernel.org>
Date: Tue Jan 21 12:30:22 2020 -0800

rcu: Don't flag non-starting GPs before GP kthread is running

Currently rcu_check_gp_start_stall() complains if a grace period takes
too long to start, where "too long" is roughly one RCU CPU stall-warning
interval. This has worked well, but there are some debugging Kconfig
options (such as CONFIG_EFI_PGT_DUMP=y) that can make booting take a
very long time, so much so that the stall-warning interval has expired
before RCU's grace-period kthread has even been spawned.

This commit therefore resets the rcu_state.gp_req_activity and
rcu_state.gp_activity timestamps just before the grace-period kthread
is spawned, and modifies the checks and adds ordering to ensure that
if rcu_check_gp_start_stall() sees that the grace-period kthread
has been spawned, that it will also see the resets applied to the
rcu_state.gp_req_activity and rcu_state.gp_activity timestamps.

Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 04718bc..d9d619d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1209,7 +1209,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedroot"));
WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_INIT);
WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
- if (!rcu_state.gp_kthread) {
+ if (!READ_ONCE(rcu_state.gp_kthread)) {
trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread"));
goto unlock_out;
}
@@ -1259,10 +1259,10 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
*/
static void rcu_gp_kthread_wake(void)
{
- if ((current == rcu_state.gp_kthread &&
+ if ((current == READ_ONCE(rcu_state.gp_kthread) &&
!in_irq() && !in_serving_softirq()) ||
!READ_ONCE(rcu_state.gp_flags) ||
- !rcu_state.gp_kthread)
+ !READ_ONCE(rcu_state.gp_kthread))
return;
WRITE_ONCE(rcu_state.gp_wake_time, jiffies);
WRITE_ONCE(rcu_state.gp_wake_seq, READ_ONCE(rcu_state.gp_seq));
@@ -3619,7 +3619,10 @@ static int __init rcu_spawn_gp_kthread(void)
}
rnp = rcu_get_root();
raw_spin_lock_irqsave_rcu_node(rnp, flags);
- rcu_state.gp_kthread = t;
+ WRITE_ONCE(rcu_state.gp_activity, jiffies);
+ WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
+ // Reset .gp_activity and .gp_req_activity before setting .gp_kthread.
+ smp_store_release(&rcu_state.gp_kthread, t); /* ^^^ */
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
wake_up_process(t);
rcu_spawn_nocb_kthreads();
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 476458c..75f6e9f 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -578,6 +578,7 @@ void show_rcu_gp_kthreads(void)
unsigned long jw;
struct rcu_data *rdp;
struct rcu_node *rnp;
+ struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);

j = jiffies;
ja = j - READ_ONCE(rcu_state.gp_activity);
@@ -585,8 +586,7 @@ void show_rcu_gp_kthreads(void)
jw = j - READ_ONCE(rcu_state.gp_wake_time);
pr_info("%s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_flags %#x\n",
rcu_state.name, gp_state_getname(rcu_state.gp_state),
- rcu_state.gp_state,
- rcu_state.gp_kthread ? rcu_state.gp_kthread->state : 0x1ffffL,
+ rcu_state.gp_state, t ? t->state : 0x1ffffL,
ja, jr, jw, (long)READ_ONCE(rcu_state.gp_wake_seq),
(long)READ_ONCE(rcu_state.gp_seq),
(long)READ_ONCE(rcu_get_root()->gp_seq_needed),
@@ -633,7 +633,8 @@ static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,

if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
- READ_ONCE(rnp_root->gp_seq_needed)))
+ READ_ONCE(rnp_root->gp_seq_needed)) ||
+ !smp_load_acquire(&rcu_state.gp_kthread))
return;
j = jiffies; /* Expensive access, and in common case don't get here. */
if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
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