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Message-Id: <FD18F879-048F-40E9-B04D-3C189C59181B@lca.pw>
Date: Wed, 22 Jan 2020 00:20:02 -0500
From: Qian Cai <cai@....pw>
To: paulmck@...nel.org
Cc: rcu@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Boot warning at rcu_check_gp_start_stall()
> On Jan 22, 2020, at 12:07 AM, Paul E. McKenney <paulmck@...nel.org> 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@...nel.org> 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@...nel.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@...nel.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?
>>
>> 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.
Otherwise,
Tested-by: Qian Cai <cai@....pw>
>
> Thanx, Paul
>
>>> ------------------------------------------------------------------------
>>>
>>> commit fb21277f8f1c5cc40a8d41da2db4b0c499459821
>>> Author: Paul E. McKenney <paulmck@...nel.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@....pw>
>>> Signed-off-by: Paul E. McKenney <paulmck@...nel.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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