Messages in this thread Patch in this message | | | Date | Thu, 13 May 2021 10:23:44 +0300 | From | Alexey Dobriyan <> | Subject | Re: [PATCH 1/4] sched: make nr_running() return 32-bit |
| |
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;
| |