linux-kernel - Re: [PATCH 1/4] sched: make nr

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Message-ID: <87fsyr5wtj.ffs@nanos.tec.linutronix.de>
Date:   Thu, 13 May 2021 01:58:16 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexey Dobriyan <adobriyan@...il.com>, mingo@...hat.com,
        peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org,
        Alexey Dobriyan <adobriyan@...il.com>, x86@...nel.org
Subject: Re: [PATCH 1/4] sched: make nr_running() return 32-bit

Alexey,

On Thu, Apr 22 2021 at 23:02, Alexey Dobriyan wrote:
> Creating 2**32 tasks is impossible due to futex pid limits and wasteful
> anyway. Nobody has done it.
>

this whole pile lacks useful numbers. What's the actual benefit of that
churn?

Just with the default config for one of my reference machines:

   text		data	bss	dec	 hex	 filename
16679864	6627950	1671296	24979110 17d26a6 ../build/vmlinux-before
16679894	6627950	1671296	24979140 17d26c4 ../build/vmlinux-after
------------------------------------------------------------------------
     +30

I'm truly impressed by the massive savings of this change and I'm even
more impressed by the justification:

> Bring nr_running() into 32-bit world to save on REX prefixes.

Aside of the obvious useless churn, REX prefixes are universaly true for
all architectures, right? There is a world outside x86 ...

Thanks,

        tglx