LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Thomas Gleixner <tglx@linutronix.de>
To: Mike Galbraith <efault@gmx.de>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	linux-rt-users@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: v5.14-rc3-rt1 losing wakeups?
Date: Mon, 02 Aug 2021 11:12:12 +0200	[thread overview]
Message-ID: <87k0l42osz.ffs@tglx> (raw)
In-Reply-To: <ed1d5f9ec17a5b8d758c234562dad47cfc872ed8.camel@gmx.de>

Mike,

On Sun, Aug 01 2021 at 17:14, Mike Galbraith wrote:

> On Sun, 2021-08-01 at 05:36 +0200, Mike Galbraith wrote:
>> On Fri, 2021-07-30 at 22:49 +0200, Thomas Gleixner wrote:
>> > >
>> > > First symptom is KDE/Plasma's task manager going comatose.  Notice soon
>> >
>> > KDE/Plasma points at the new fangled rtmutex based ww_mutex from
>> > Peter.
>>
>> Seems not.  When booting KVM box with nomodeset, there's exactly one
>> early boot ww_mutex lock/unlock, ancient history at the failure point.
>
> As you've probably already surmised given it isn't the ww_mutex bits,
> it's the wake_q bits.  Apply the below, 5.14-rt ceases to fail.  Take
> perfectly healthy 5.13-rt, apply those bits, and it instantly begins
> failing as 5.14-rt had been.

now staring at it makes it pretty obvious. When I picked up Peter's
patch I thought about it briefly and then ignored my doubts :(

>  /* RT mutex specific wake_q wrappers */
> -static __always_inline void rt_mutex_wake_q_add(struct rt_wake_q_head *wqh,
> +static __always_inline void rt_mutex_wake_q_add(struct rt_mutex_wake_q_head *wqh,
>  						struct rt_mutex_waiter *w)
>  {
>  	if (IS_ENABLED(CONFIG_PREEMPT_RT) && w->wake_state != TASK_NORMAL) {
> -		wake_q_add(&wqh->rt_head, w->task);
> +		get_task_struct(w->task);
> +		wqh->rtlock_task = w->task;

This is the key. With the original asymmetric version the wake_q_add for
wake_state != TASK_NORMAL is storing the task unconditionally in
wqh->rtlock_task.

With that wake_q_add() we end up with the following situation:

Some code, e.g. futex does:

     wake_q_add(..., task)

which links task->wake_q->next is !NULL. Ergo the wake_q_add() in the
rtmutex code bails out. Same the other way round if the rtmutex side
queues first then the second - regular wakeup - will not be queued.

There's two ways to fix that:

  1) Go back to my original version

  2) Add another wake_q head to task_struct

#2 is overkill IMO simply because the rtlock wait is not subject to
multiple wakeups.

Thanks a lot Mike for tracking this down!

Thanks,

        tglx

      parent reply	other threads:[~2021-08-02  9:12 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-30 11:07 [ANNOUNCE] v5.14-rc3-rt1 Sebastian Andrzej Siewior
2021-07-30 15:21 ` v5.14-rc3-rt1 losing wakeups? Mike Galbraith
2021-07-30 20:49   ` Thomas Gleixner
2021-07-31  1:03     ` Mike Galbraith
2021-07-31  3:33       ` Mike Galbraith
2021-07-31  8:50         ` Mike Galbraith
2021-08-01  3:36     ` Mike Galbraith
2021-08-01 15:14       ` Mike Galbraith
2021-08-02  7:02         ` Sebastian Andrzej Siewior
2021-08-02  7:18           ` Mike Galbraith
2021-08-02  8:25             ` Sebastian Andrzej Siewior
2021-08-02  8:40               ` Mike Galbraith
2021-08-02  9:12         ` Thomas Gleixner [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=87k0l42osz.ffs@tglx \
    --to=tglx@linutronix.de \
    --cc=bigeasy@linutronix.de \
    --cc=efault@gmx.de \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-rt-users@vger.kernel.org \
    --cc=peterz@infradead.org \
    --cc=rostedt@goodmis.org \
    --subject='Re: v5.14-rc3-rt1 losing wakeups?' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).