linux-kernel - Re: [RFC PATCH 2/3] futex: Add basic infrastructure for local task local hash.

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Message-ID: <87y128335h.ffs@tglx>
Date: Mon, 28 Oct 2024 11:58:18 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, André Almeida
 <andrealmeid@...lia.com>,
 Darren Hart <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>,
 Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, Peter
 Zijlstra <peterz@...radead.org>, Valentin Schneider <vschneid@...hat.com>,
 Waiman Long <longman@...hat.com>
Subject: Re: [RFC PATCH 2/3] futex: Add basic infrastructure for local task
 local hash.

On Mon, Oct 28 2024 at 11:30, Sebastian Andrzej Siewior wrote:
> On 2024-10-27 13:19:54 [+0100], Thomas Gleixner wrote:
>> So T1 and T2 create their local hash and the subsequent usage will fail
>> because they operate on different hashs. You have the same problem
>> vs. your allocation scheme when two threads do prctl(ALLOC). We really
>> want to make this as simple as possible.
>
> So I moved this to struct signal_struct and limited allocation to the
> group leader.
>
> You want automated creation of this? For everyone or with a hint? This
> is 64 bytes per slot due to the cache alignment but event without this
> struct takes 56 bytes on PREEMPT_RT and 24 bytes on non-RT. So the four
> slots are 256 bytes. Assuming 2.5k tasks it takes 625 KiB. Maybe not
> that much.
>
> Let me post v2 the signal_struct and then think about auto ON.

It only affects actual futex users. A lot of executables never use
them. For ease of testing, can you please make this automatic so there
is no need to modify a test case?

Aside of that for RT we really want it automatically enabled and as
Linus suggested back then probably for NUMA too.

Stick a trace point or a debugfs counter into the allocation so you can
observe how many of those are actually allocated and used concurrently.

Thanks,

        tglx