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SubjectRe: [PATCH v4 0/5] x86/umwait: Enable user wait instructions
On Tue, Jun 11, 2019 at 11:01:45AM +0200, Peter Zijlstra wrote:
> 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?

In previous patches, there is no time limit (max time is 0 which means no
time limit).

Andy Lutomirski proposed to set the time limit:

"So I propose setting the timeout to either 100 microseconds or 100k
"cycles" by default. In the event someone determines that they save
materially more power or gets materially better performance with a
longer timeout, we can revisit the value."

Does it make sense?

> > .../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.

There may be other places to use the instructions. But I think this
patch set just first enables basic functionalities. We can focus on how to
use the instructions in the future.



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