On Wed, Jan 22, 2020 at 12:20:02AM -0500, Qian Cai wrote: > > > > On Jan 22, 2020, at 12:07 AM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > On Tue, Jan 21, 2020 at 11:16:06PM -0500, Qian Cai wrote: > >> > >> > >>> On Jan 21, 2020, at 3:46 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > >>> > >>> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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? > >> > >> Yes, it works fine. > > > > Very good, thank you! May I apply your Tested-by? > > Just one thing minor, > > Applying: Boot warning at rcu_check_gp_start_stall() > .git/rebase-apply/patch:66: space before tab in indent. > READ_ONCE(rnp_root->gp_seq_needed)) || > warning: 1 line adds whitespace errors. Good catch, fixed! > Otherwise, > > Tested-by: Qian Cai <cai@xxxxxx> Applied, thank you! Thanx, Paul > >>> ------------------------------------------------------------------------ > >>> > >>> commit fb21277f8f1c5cc40a8d41da2db4b0c499459821 > >>> Author: Paul E. McKenney <paulmck@xxxxxxxxxx> > >>> 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@xxxxxx> > >>> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > >>> > >>> 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) || >
