Re: [PATCH v4 0/5] x86/umwait: Enable user wait instructions

From: Peter Zijlstra
Date: Tue Jun 11 2019 - 05:06:19 EST

On Fri, Jun 07, 2019 at 03:00:32PM -0700, Fenghua Yu wrote:
> Today, if an application needs to wait for a very short duration
> they have to have spinloops. Spinloops consume more power and continue
> to use execution resources that could hurt its thread siblings in a core
> with hyperthreads. New instructions umonitor, umwait and tpause allow
> a low power alternative waiting at the same time could improve the HT
> sibling perform while giving it any power headroom. These instructions
> can be used in both user space and kernel space.
> A new MSR IA32_UMWAIT_CONTROL allows kernel to set a time limit in
> TSC-quanta that prevents user applications from waiting for a long time.
> This allows applications to yield the CPU and the user application
> should consider using other alternatives to wait.

I'm confused on the purpose of this control; what do we win by limiting
this time?

> .../ABI/testing/sysfs-devices-system-cpu | 21 ++
> arch/x86/include/asm/cpufeatures.h | 1 +
> arch/x86/include/asm/msr-index.h | 4 +
> arch/x86/power/Makefile | 1 +
> arch/x86/power/umwait.c | 182 ++++++++++++++++++

You seem to miss the arch/x86/lib/delay.c change to use this fancy new
stuff for udelay(). I'm thinking that's exactly what TPAUSE is good for.