linux-kernel - Re: [RFC PATCH v2 00/13] Add futex2 syscall

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Message-ID: <bc54423b-753f-44be-4e4f-4535e27ad35c@collabora.com>
Date:   Thu, 4 Mar 2021 15:58:07 -0300
From:   André Almeida <andrealmeid@...labora.com>
To:     Peter Oskolkov <posk@...k.io>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        kernel@...labora.com, krisman@...labora.com,
        pgriffais@...vesoftware.com, z.figura12@...il.com,
        joel@...lfernandes.org, malteskarupke@...tmail.fm,
        linux-api@...r.kernel.org, fweimer@...hat.com,
        libc-alpha@...rceware.org, linux-kselftest@...r.kernel.org,
        shuah@...nel.org, acme@...nel.org, Jonathan Corbet <corbet@....net>
Subject: Re: [RFC PATCH v2 00/13] Add futex2 syscall

Hi Peter,

Às 02:44 de 04/03/21, Peter Oskolkov escreveu:
> On Wed, Mar 3, 2021 at 5:22 PM André Almeida <andrealmeid@...labora.com> wrote:
>>
>> Hi,
>>
>> This patch series introduces the futex2 syscalls.
>>
>> * FAQ
>>
>>   ** "And what's about FUTEX_64?"
>>
>>   By supporting 64 bit futexes, the kernel structure for futex would
>>   need to have a 64 bit field for the value, and that could defeat one of
>>   the purposes of having different sized futexes in the first place:
>>   supporting smaller ones to decrease memory usage. This might be
>>   something that could be disabled for 32bit archs (and even for
>>   CONFIG_BASE_SMALL).
>>
>>   Which use case would benefit for FUTEX_64? Does it worth the trade-offs?
> 
> The ability to store a pointer value on 64bit platforms is an
> important use case.
> Imagine a simple producer/consumer scenario, with the producer updating
> some shared memory data and waking the consumer. Storing the pointer
> in the futex makes it so that only one shared memory location needs to be
> accessed "atomically", etc. With two atomics synchronization becomes
> more involved (= slower).
> 

So the idea is to, instead of doing this:

T1:
atomic_set(&shm_addr, buffer_addr);
atomic_set(&futex, 0);
futex_wake(&futex, 1);

T2:
consume(shm_addr);

To do that:

T1:
atomic_set(&futex, buffer_addr);
futex_wake(&futex, 1);

T2:
consume(futex);

Right?

I'll try to write a small test to see how the perf numbers looks like.