From: Vladislav Levenetz <vlevenetz@mm-sol.com>
To: Petr Mladek <pmladek@suse.com>, Viresh Kumar <viresh.kumar@linaro.org>
Cc: Jan Kara <jack@suse.cz>,
Andrew Morton <akpm@linux-foundation.org>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Jan Kara <jack@suse.com>, Tejun Heo <tj@kernel.org>,
Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Byungchul Park <byungchul.park@lge.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async
Date: Mon, 15 Aug 2016 17:26:50 +0300 [thread overview]
Message-ID: <57B1D12A.6030106@mm-sol.com> (raw)
In-Reply-To: <20160812094447.GD7339@pathway.suse.cz>
On 08/12/2016 12:44 PM, Petr Mladek wrote:
> On Wed 2016-08-10 14:17:55, Viresh Kumar wrote:
>> +Vladi/Greg,
>>
>> On Wed, Apr 6, 2016 at 1:27 AM, Jan Kara <jack@suse.cz> wrote:
>>> On Mon 04-04-16 15:51:49, Andrew Morton wrote:
>>>>> +static int __init init_printk_kthread(void)
>>>>> +{
>>>>> + struct task_struct *thread;
>>>>> +
>>>>> + if (printk_sync)
>>>>> + return 0;
>>>>> +
>>>>> + thread = kthread_run(printk_kthread_func, NULL, "printk");
>>>> This gets normal scheduling policy, so a spinning userspace SCHED_FIFO
>>>> task will block printk for ever. This seems bad.
>>> I have to research this a bit but won't the SCHED_FIFO task that has
>>> potentially unbounded amount of work lockup the CPU even though it does
>>> occasional cond_resched()?
>> We are facing complete hogs because of the printk thread being a SCHED_FIFO
>> task and have this patch to fix it up for now.
>>
>> Author: Vladislav Levenetz <vblagoev@mm-sol.com>
>> Date: Wed Aug 10 13:58:00 2016 -0700
>>
>> SW-7786: printk: Lower the priority of printk thread
>>
>> Flooding the console (with a test module) in a tight loop indefinitely
>> makes android user interface very sluggish. Opening YouTube app and the
>> device hangs and becomes even more unresponsive to the point it
>> completely hangs.
>>
>> The asynchronous printk thread is a SCHED FIFO thread with priority
>> MAX_RT_PRIO - 1. If we create it as a simple thread (i.e. no SCHED FIFO)
>> instead, we observe much better performance using the same printk flood
>> test. We don't even notice any kind of sluggishness during device usage.
>> We can play a YouTube clip smoothly and use the device normally in
>> general. The kernel log looks fine as well, as the flood of messages
>> continue normally.
>>
>> Signed-off-by: Vladislav Levenetz <vblagoev@mm-sol.com>
>> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
>> ---
>> kernel/printk/printk.c | 4 ----
>> 1 file changed, 4 deletions(-)
>>
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>> index c32872872cb6..ad5b30e5e6d9 100644
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -2856,9 +2856,6 @@ static int printk_kthread_func(void *data)
>> static int __init_printk_kthread(void)
>> {
>> struct task_struct *thread;
>> - struct sched_param param = {
>> - .sched_priority = MAX_RT_PRIO - 1,
>> - };
>>
>> if (!printk_kthread_can_run || printk_sync || printk_kthread)
>> return 0;
>> @@ -2870,7 +2867,6 @@ static int __init_printk_kthread(void)
>> return PTR_ERR(thread);
>> }
>>
>> - sched_setscheduler(thread, SCHED_FIFO, ¶m);
>> printk_kthread = thread;
>> return 0;
>> }
> IMHO, this is fine. We force the synchronous mode in critical
> situations anyway.
>
> But I was curious if we could hit a printk from the wake_up_process().
> The change above causes using the fair scheduler and there is
> the following call chain [*]
>
> vprintk_emit()
> -> wake_up_process()
> -> try_to_wake_up()
> -> ttwu_queue()
> -> ttwu_do_activate()
> -> ttwu_activate()
> -> activate_task()
> -> enqueue_task()
> -> enqueue_task_fair() via p->sched_class->enqueue_task
> -> cfs_rq_of()
> -> task_of()
> -> WARN_ON_ONCE(!entity_is_task(se))
>
> We should never trigger this because printk_kthread is a task.
> But what if the date gets inconsistent?
>
> Then there is the following chain:
>
> vprintk_emit()
> -> wake_up_process()
> -> try_to_wake_up()
> -> ttwu_queue()
> -> ttwu_do_activate()
> -> ttwu_activate()
> -> activate_task()
> -> enqueue_task()
> -> enqueue_task_fair() via p->sched_class->enqueue_task
> ->hrtick_update()
> -> hrtick_start_fair()
> -> WARN_ON(task_rq(p) != rq)
>
> This looks like another paranoid consistency check that might be
> triggered when the scheduler gets messed.
>
> I see few possible solutions:
>
> 1. Replace the WARN_ONs by printk_deferred().
>
> This is the usual solution but it would make debugging less convenient.
>
>
> 2. Force synchronous printk inside WARN()/BUG() macros.
>
> This would make sense even from other reasons. These are printed
> when the system is in a strange state. There is no guarantee that
> the printk_kthread will get scheduled.
>
>
> 3. Force printk_deferred() inside WARN()/BUG() macros via the per-CPU
> printk_func.
>
> It might be elegant. But we do not want this outside the scheduler
> code. Therefore we would need special variants of WARN_*_SCHED()
> BUG_*_SCHED() macros.
>
>
> I personally prefer the 2nd solution. What do you think about it,
> please?
>
>
> Best Regards,
> Petr
Hi Petr,
Sorry with for the late reply.
Hitting a WARN()/BUG() from wake_up calls will lead to a deadlock if
only a single CPU is running.
We already had such a situation with system suspend. During a specific
test on our device sometimes we hit a WARN from the time keeping core.
(Cannot recall which one exactly. Viresh have it) from a printk wake_up
path during system suspend and with already only one CPU running.
So we were forced to make printing synchronous in the suspend path prior
disabling all non-boot cpu's.
Your solution number 2) sounds reasonable to me.
I'm wondering if we could hit a WARN()/BUG() somewhere from the fair
scheduler like the example you made for the RT sched?
Thanks.
next prev parent reply other threads:[~2016-08-15 14:36 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-04 16:57 [PATCH v10 0/2] printk: Make printk() completely async Sergey Senozhatsky
2016-04-04 16:57 ` [PATCH v10 1/2] " Sergey Senozhatsky
2016-04-04 22:51 ` Andrew Morton
2016-04-05 5:17 ` Sergey Senozhatsky
2016-04-05 7:39 ` Sergey Senozhatsky
2016-04-06 0:19 ` Sergey Senozhatsky
2016-04-06 8:27 ` Jan Kara
2016-04-07 9:48 ` Sergey Senozhatsky
2016-04-07 12:08 ` Sergey Senozhatsky
2016-04-07 13:15 ` Jan Kara
2016-08-10 21:17 ` Viresh Kumar
2016-08-12 9:44 ` Petr Mladek
2016-08-15 14:26 ` Vladislav Levenetz [this message]
2016-08-16 9:04 ` Petr Mladek
2016-08-18 2:27 ` Sergey Senozhatsky
2016-08-18 9:33 ` Petr Mladek
2016-08-18 9:51 ` Sergey Senozhatsky
2016-08-18 10:56 ` Petr Mladek
2016-08-19 6:32 ` Sergey Senozhatsky
2016-08-19 9:54 ` Petr Mladek
2016-08-19 19:00 ` Jan Kara
2016-08-20 5:24 ` Sergey Senozhatsky
2016-08-22 4:15 ` Sergey Senozhatsky
2016-08-23 12:19 ` Petr Mladek
2016-08-24 1:33 ` Sergey Senozhatsky
2016-08-25 21:10 ` Petr Mladek
2016-08-26 1:56 ` Sergey Senozhatsky
2016-08-26 8:20 ` Sergey Senozhatsky
2016-08-30 9:29 ` Petr Mladek
2016-08-31 2:31 ` Sergey Senozhatsky
2016-08-31 9:38 ` Petr Mladek
2016-08-31 12:52 ` Sergey Senozhatsky
2016-09-01 8:58 ` Petr Mladek
2016-09-02 7:58 ` Sergey Senozhatsky
2016-09-02 15:15 ` Petr Mladek
2016-09-06 7:16 ` Sergey Senozhatsky
2016-08-23 13:03 ` Petr Mladek
2016-08-23 13:48 ` Petr Mladek
2016-04-04 16:57 ` [PATCH v10 2/2] printk: Make wake_up_klogd_work_func() async Sergey Senozhatsky
2016-08-23 3:32 [PATCH v10 1/2] printk: Make printk() completely async Andreas Mohr
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