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

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Message-ID: <a6e6a1fa-1938-ddd5-525d-de1cc991d703@redhat.com>
Date:   Thu, 5 Jan 2017 14:24:36 -0500
From:   Waiman Long <longman@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [RFC PATCH 0/7] locking/rtqspinlock: Realtime queued spinlocks

On 01/05/2017 01:50 PM, Steven Rostedt wrote:
> On Thu, 5 Jan 2017 12:07:21 -0500
> Waiman Long <longman@...hat.com> wrote:
>
>
>> I do make the assumption that spinlock critical sections are behaving
>> well enough. Apparently, that is not a valid assumption. I sent these
>> RFC patches out to see if it was an idea worth pursuing. If not, I can
>> drop these patches. Anyway, thanks for the feedback.
> Yes, the assumption is incorrect. There are places that can hold a spin
> lock for several hundreds of microseconds. If you can't preempt them,
> you'll never get below several hundreds of microseconds in latency.
>
> And it would be hard to pick and choose (we already do this to decide
> what can be a raw_spin_lock), because you need to audit all use cases
> of a spin_lock as well as all the locks taken while holding that
> spin_lock.
>
> -- Steve

Thank for the information.

It has come to my attention that scalability problem may be present in
the -RT kernel because of the longer wait time in the raw_spin_lock side
as the number of CPUs increases. I will look into this some more to see
if my patch set can help under those circumstances.

Cheers,
Longman