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Date:   Thu, 7 Jul 2022 12:04:12 -0700
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Juri Lelli <juri.lelli@redhat.com>,
        Mike Stowell <mstowell@redhat.com>
Subject: Re: [PATCH v3] locking/rtmutex: Limit # of lock stealing for non-RT
 waiters
Message-ID: <YscuLP2PuMEnMh35@boqun-archlinux>
References: <20220706135916.980580-1-longman@redhat.com>
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On Thu, Jul 07, 2022 at 02:45:10PM -0400, Waiman Long wrote:
> On 7/7/22 14:22, Boqun Feng wrote:
> > On Wed, Jul 06, 2022 at 10:03:10AM -0400, Waiman Long wrote:
> > > On 7/6/22 09:59, Waiman Long wrote:
> > > > Commit 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock
> > > > stealing") allows unlimited number of lock stealing's for non-RT
> > > > tasks. That can lead to lock starvation of non-RT top waiter tasks if
> > > > there is a constant incoming stream of non-RT lockers. This can cause
> > > > rcu_preempt self-detected stall or even task lockup in PREEMPT_RT kernel.
> > > > For example,
> > > > 
> > > > [77107.424943] rcu: INFO: rcu_preempt self-detected stall on CPU
> > > > [ 1249.921363] INFO: task systemd:2178 blocked for more than 622 seconds.
> > > > 
> > > > Avoiding this problem and ensuring forward progress by limiting the
> > > > number of times that a lock can be stolen from each waiter. This patch
> > > > sets a threshold of 32. That number is arbitrary and can be changed
> > > > if needed.
> > > > 
> > > > Fixes: 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock stealing")
> > > > Signed-off-by: Waiman Long <longman@redhat.com>
> > > > ---
> > > >    kernel/locking/rtmutex.c        | 9 ++++++---
> > > >    kernel/locking/rtmutex_common.h | 8 ++++++++
> > > >    2 files changed, 14 insertions(+), 3 deletions(-)
> > > > 
> > > >    [v3: Increase threshold to 32 and add rcu_preempt self-detected stall]
> > > Note that I decided to increase the threshold to 32 from 10 to reduce the
> > > potential performance impact of this change, if any. We also found out that
> > > this patch can fix some of the rcu_preempt self-detected stall problems that
> > > we saw with the PREEMPT_RT kernel. So I added that information in the patch
> > > description.
> > > 
> > Have you considered (and tested) whether we can set the threshold
> > directly proportional to nr_cpu_ids? Because IIUC, the favorable case
> > for lock stealing is that every CPU gets a chance to steal once. If one
> > CPU can steal twice, 1) either there is a context switch between two
> > tasks, which costs similarly as waking up the waiter, or 2) a task drops
> > and re-graps a lock, which means the task wants to yield to other
> > waiters of the lock.
> 
> There is no inherent restriction on not allowing the same cpu stealing the
> lock twice or more. With rtmutex, the top waiter may be sleeping and the

Well, I'm not saying we need to restrict the same cpu to steal a lock
twice or more. Think about this, when there is a task running on CPU 1
already steals a lock once, for example:

	<lock release>
	{task C is the top waiter}

	CPU 1
	=====
	<now task A running>
	lock(); // steal the lock
	...
	unlock():
	  // set owner to NULL
	  <switch task B> // similar cost to wake up A
	  lock(); // steal the lock

, which means if a CPU steals a lock twice or more, it's almost certain
that a context happened between two steals ("almost" because there could
be a case where task A lock()+unlock() twice, but as I said, it
means that task A is willing to yield.).

Therefore if there are @nr_cpu_ids lock steals, it means either there is
a context switch somewhere or a task has been willing to yield. And I
think it's a reasonable signal to stop lock stealing.

Thoughts?

Regards,
Boqun

> wakeup latency can be considerable. By allowing another ready lock waiter to
> steal the lock for productive use, it can improve system throughput. There
> is no fairness in lock stealing and I don't believe it is a worthwhile goal
> to allow each cpu to steal the lock once. It will just complicate the code.
> 
> On the other hand, unlimited lock stealing is bad and we have a put a limit
> somehow to ensure forward progress.
> 
> Cheers,
> Longman
>