linux-kernel - Re: v5.14-rc3-rt1 losing wakeups?

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Message-ID: <edd2f9fd1489e1ff05bf526a3059a1dbb81107df.camel@gmx.de>
Date:   Mon, 02 Aug 2021 09:18:34 +0200
From:   Mike Galbraith <efault@....de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: v5.14-rc3-rt1 losing wakeups?

On Mon, 2021-08-02 at 09:02 +0200, Sebastian Andrzej Siewior wrote:
> On 2021-08-01 17:14:49 [+0200], 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.
>
> Given what you have replied to the locking thread/
> ww_mutex_lock_interruptible() may I assume that the wake_q bits are fine
> and it is just the ww_mutex?

Nope.  Before I even reverted the wake_q bits, I assembled a tree with
the ww_mutex changes completely removed to be absolutely certain that
they were innocent, and it indeed did retain its lost wakeup woes
despite complete loss of newfangled ww_mutex.  5.13-rt acquired those
same wakeup woes by receiving ONLY the wake_q bits, and 5.14-rt was
cured of those woes by ONLY them being reverted. I'm not seeing the
why, but those bits are either the source or the trigger of 5.14-rt
lost wakeup woes... they're toxic in some way shape fashion or form.

	-Mike