Re: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to entry_SYSENTER_32

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Thu, 2016-02-25 at 09:40 +0100, Ingo Molnar wrote:
> * Mike Galbraith <umgwanakikbuti@gmail.com> wrote:
> 
> > On Thu, 2016-02-25 at 09:14 +0100, Ingo Molnar wrote:
> > 
> > > But but ... 'context tracking' is not really something that a regular distro 
> > > kernel cares about much - it's a nohz-full special AFAICS.
> 
> Let me qualify that: with the timer code maintenance hat on I really love all nohz 
> variants (the deeper the better), but now I have my x86 maintainer hat on, and as 
> such I'm really annoyed at those nohz folks adding overhead to the syscall hot 
> path! ;-)
> 
> > (psst.. distros are shipping it)
> 
> Yeah, indeed, Fedora does - but AFAICS:
> 
>  fomalhaut:~> grep NO_HZ /boot/config-4.1.13-100.fc21.x86_64 
>  CONFIG_NO_HZ_COMMON=y
>  # CONFIG_NO_HZ_IDLE is not set
>  CONFIG_NO_HZ_FULL=y
>  # CONFIG_NO_HZ_FULL_ALL is not set
>  # CONFIG_NO_HZ_FULL_SYSIDLE is not set
>  CONFIG_NO_HZ=y
>  CONFIG_RCU_FAST_NO_HZ=y
> 
> ... which won't result in actual full-nohz CPUs unless you boot it with a special 
> boot parameter, right?

Yeah, you have to manually enable it unless you (in a suicidal moment)
enable CONFIG_NO_HZ_FULL_ALL.

> What is the easiest way to query which/how many CPUs are in nohz-full mode and do 
> context tracking? I somehow thought /proc/timer_* had that info, but that does not 
> appear to be the case.

/sys/devices/system/cpu/nohz_full

	-Mike