From: Alexey Dobriyan <adobriyan@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: mingo@redhat.com, peterz@infradead.org,
linux-kernel@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH 1/4] sched: make nr_running() return 32-bit
Date: Thu, 13 May 2021 10:23:44 +0300 [thread overview]
Message-ID: <YJzUAHQwFj1x2HCH@localhost.localdomain> (raw)
In-Reply-To: <87fsyr5wtj.ffs@nanos.tec.linutronix.de>
On Thu, May 13, 2021 at 01:58:16AM +0200, Thomas Gleixner wrote:
> 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?
The long term goal is to use 32-bit data more. People will see it in
core kernel and copy everywhere elase.
> 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.
I collected numbers initially but then stopped because noone cared and
they can be config and arch dependent.
> Aside of the obvious useless churn,
oh... Sometimes I think churn is the whole point.
> REX prefixes are universaly true for
> all architectures, right? There is a world outside x86 ...
In general, 32-bitness is preferred for code generation.
32-bit RISCs naturally prefers 32-bit.
64-bit RISCs don't care because they remember 32-bit roots and
have necessary 32-bit fixed width(!) instructions.
x86_64 is the only arch where going 64-bit generally adds more bytes
to the instruction stream.
Effects can be smudged by compilers of course, in this case, percpu
stuff. That "unsigned int i" is a mistake. Proper diff looks like this:
-ffffffff811115fa: 8b 44 18 04 mov eax,DWORD PTR [rax+rbx*1+0x4]
-ffffffff811115fe: 49 01 c4 add r12,rax
+ffffffff811115fa: 44 03 64 18 04 add r12d,DWORD PTR [rax+rbx*1+0x4]
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4348,9 +4348,10 @@ context_switch(struct rq *rq, struct task_struct *prev,
* externally visible scheduler statistics: current number of runnable
* threads, total number of context switches performed since bootup.
*/
-unsigned long nr_running(void)
+unsigned int nr_running(void)
{
- unsigned long i, sum = 0;
+ unsigned int sum = 0;
+ unsigned long i;
for_each_online_cpu(i)
sum += cpu_rq(i)->nr_running;
next prev parent reply other threads:[~2021-05-13 7:23 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-22 20:02 [PATCH 1/4] sched: make nr_running() return 32-bit Alexey Dobriyan
2021-04-22 20:02 ` [PATCH 2/4] sched: make nr_iowait() return 32-bit value Alexey Dobriyan
2021-05-12 20:01 ` [tip: sched/core] sched: Make " tip-bot2 for Alexey Dobriyan
2021-04-22 20:02 ` [PATCH 3/4] sched: make nr_iowait_cpu() return 32-bit Alexey Dobriyan
2021-05-12 20:01 ` [tip: sched/core] sched: Make nr_iowait_cpu() return 32-bit value tip-bot2 for Alexey Dobriyan
2021-04-22 20:02 ` [PATCH 4/4] sched: make multiple runqueue task counters 32-bit Alexey Dobriyan
2021-05-12 19:36 ` Ingo Molnar
2021-05-12 20:01 ` [tip: sched/core] sched: Make " tip-bot2 for Alexey Dobriyan
2021-05-12 20:01 ` [tip: sched/core] sched: Make nr_running() return 32-bit value tip-bot2 for Alexey Dobriyan
2021-05-12 23:58 ` [PATCH 1/4] sched: make nr_running() return 32-bit Thomas Gleixner
2021-05-13 7:23 ` Alexey Dobriyan [this message]
2021-05-13 9:58 ` Ingo Molnar
2021-05-13 21:22 ` Alexey Dobriyan
2021-05-14 12:52 ` Thomas Gleixner
2021-05-14 18:18 ` Thomas Gleixner
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