Messages in this thread | | | From | Andy Lutomirski <> | Date | Sat, 8 Jun 2019 15:50:32 -0700 | Subject | Re: [PATCH v4 3/5] x86/umwait: Add sysfs interface to control umwait C0.2 state |
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On Fri, Jun 7, 2019 at 3:10 PM Fenghua Yu <fenghua.yu@intel.com> wrote: > > C0.2 state in umwait and tpause instructions can be enabled or disabled > on a processor through IA32_UMWAIT_CONTROL MSR register. > > By default, C0.2 is enabled and the user wait instructions result in > lower power consumption with slower wakeup time. > > But in real time systems which require faster wakeup time although power > savings could be smaller, the administrator needs to disable C0.2 and all > C0.2 requests from user applications revert to C0.1. > > A sysfs interface "/sys/devices/system/cpu/umwait_control/enable_c02" is > created to allow the administrator to control C0.2 state during run time.
This looks better than the previous version. I think the locking is still rather confused. You have a mutex that you hold while changing the value, which is entirely reasonable. But, of the code paths that write the MSR, only one takes the mutex.
I think you should consider making a function that just does:
wrmsr(MSR_IA32_UMWAIT_CONTROL, READ_ONCE(umwait_control_cached), 0);
and using it in all the places that update the MSR. The only thing that should need the lock is the sysfs code to avoid accidentally corrupting the value, but that code should also use WRITE_ONCE to do its update.
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