linux-kernel - Re: [RFC PATCH v2 3/5] futex: Throughput-optimized (TO) futexes

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Date:   Thu, 22 Sep 2016 07:41:23 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <waiman.long@....com>,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Jason Low <jason.low2@....com>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH v2 3/5] futex: Throughput-optimized (TO) futexes

On Thu, 22 Sep 2016, Thomas Gleixner wrote:

>On Thu, 22 Sep 2016, Peter Zijlstra wrote:
>> On Wed, Sep 21, 2016 at 07:37:34PM -0400, Waiman Long wrote:
>> > On 09/21/2016 02:59 AM, Mike Galbraith wrote:
>> > >On Tue, 2016-09-20 at 09:42 -0400, Waiman Long wrote:
>> > >>This patch introduces a new futex implementation called
>> > >>throughput-optimized (TO) futexes.
>> > >nit: 'TO' sounds way too much like timeout... TP?  You even use 'to' as
>> > >shorthand for timeout in the next patch.
>> >
>> > I agree. I am not that satisfied with the TO name. So I will change it to TP
>> > in my next revision of the patch. Thanks for the suggestion.
>>
>> I'd leave out the TO part entirely (or only mention it in changelogs).
>>
>> That is, I'd call the futex ops: FUTEX_LOCK and FUTEX_UNLOCK.
>
>That brings me to a different question:
>
>How is user space going to support this, i.e. is this some extra magic for
>code which implements its own locking primitives or is there going to be a
>wide use via e.g. glibc.
>
>Also what's the reason that we can't do probabilistic spinning for
>FUTEX_WAIT and have to add yet another specialized variant of futexes?

Where would this leave the respective FUTEX_WAKE? A nop? Probably have to
differentiate the fact that the queue was empty, but there was a spinning,
instead of straightforward returning 0.

Thanks,
Davidlohr