linux-kernel - Re: [RFC PATCH 0/7] locking/rtqspinlock: Realtime queued spinlocks

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Date:   Wed, 4 Jan 2017 10:55:18 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/7] locking/rtqspinlock: Realtime queued spinlocks

On Wed, 4 Jan 2017 10:25:14 -0500
Waiman Long <longman@...hat.com> wrote:

> I know that in -RT kernel, all the non-raw spinlocks are replaced by
> rtmutex which is a sleeping lock. This can have a real performance
> impact on systems with more than a few cores. The rtmutex isn't fair either.

We do fine on 80+ CPUs. Is that enough cores for you ;-)

Note, it's not a true sleeping lock, because of the adaptive nature.
That is, it spins unless the owner of the lock is sleeping, in which
case, it too will sleep (why spin waiting for a task that isn't
running). But if the owner is running, it will spin too.

We also have tricks to keep normal preemption (like SCHED_OTHER tasks
running out of their time slot) when they have a lock. This keeps
contention down on tasks owning locks while sleeping.

> 
> Do you think it is better to keep the raw spinlocks fair and only have
> the non-raw spinlocks use the RT version?

Yes.

Note, I also want to get rt_mutex into the kernel first for all sleeping
locks. That is, get the logic in before we convert spin_locks to
sleeping locks.

-- Steve