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? 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) ||