linux-kernel - Re: [PATCH v3 1/4] futex: Implement mechanism to wait on any of several futexes

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Date:   Fri, 28 Feb 2020 16:29:05 -0800
From:   "Pierre-Loup A. Griffais" <pgriffais@...vesoftware.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        André Almeida <andrealmeid@...labora.com>
CC:     <linux-kernel@...r.kernel.org>, <kernel@...labora.com>,
        <krisman@...labora.com>, <shuah@...nel.org>,
        <linux-kselftest@...r.kernel.org>, <rostedt@...dmis.org>,
        <ryao@...too.org>, <dvhart@...radead.org>, <mingo@...hat.com>,
        <z.figura12@...il.com>, <steven@...vesoftware.com>,
        <steven@...uorix.net>, <malteskarupke@....de>
Subject: Re: [PATCH v3 1/4] futex: Implement mechanism to wait on any of
 several futexes



On 2/28/20 1:25 PM, Thomas Gleixner wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
>> On Fri, Feb 28, 2020 at 08:07:17PM +0100, Peter Zijlstra wrote:
>>> So I have a problem with this vector layout, it doesn't allow for
>>> per-futex flags, and esp. with that multi-size futex support that
>>> becomes important, but also with the already extand private/shared and
>>> wait_bitset flags this means you cannot have a vector with mixed wait
>>> types.
>>
>> Alternatively, we throw the entire single-syscall futex interface under
>> the bus and design a bunch of new syscalls that are natively vectored or
>> something.
>>
>> Thomas mentioned something like that, the problem is, ofcourse, that we
>> then want to fix a whole bunch of historical ills, and the probmem
>> becomes much bigger.
> 
> We keep piling features on top of an interface and mechanism which is
> fragile as hell and horrible to maintain. Adding vectoring, multi size
> and whatever is not making it any better.
> 
> There is also the long standing issue with NUMA, which we can't address
> with the current pile at all.
> 
> So I'm really advocating that all involved parties sit down ASAP and
> hash out a new and less convoluted mechanism where all the magic new
> features can be addressed in a sane way so that the 'F' in Futex really
> only means Fast and not some other word starting with 'F'.

Are you specifically talking about the interface, or the mechanism 
itself? Would you be OK with a new syscall that calls into the same code 
as this patch? It does seem like that's what we want, so if we rewrote a 
mechanism I'm not convinced it would come out any different. But, the 
interface itself seems fair-game to rewrite, as the current futex 
syscall is turning into an ioctl of sorts.

This solves a real problem with a real usecase; so I'd like to stay 
practical and not go into deeper issues like solving NUMA support for 
all of futex in the interest of users waiting at the other end. Can you 
point us to your preferred approach just for the scope of what we're 
trying to accomplish?

> 
> Thanks,
> 
>          tglx
>