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Message-Id: <65A22475-C7EA-4A5F-A4EC-F92EF8CC17F8@lca.pw>
Date: Tue, 21 Jan 2020 23:16:06 -0500
From: Qian Cai <cai@....pw>
To: "Paul E. McKenney" <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 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.
>
> 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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