monitor/mwait support for idle

[Alfred Perlstein]

Jeff, this is very interesting!

I have a question about your earlier email.  You mentioned that the
IPIs and communication to enter the idle state (hlt, mwait) is expensive.

Perhaps something to track the number of times entering and exiting
the state would be a good idea.

Additionally a tuneable for the number of spins before entering the
state might be a good idea.  Perhaps spinning for "1" or "2" (where
1/2 is one unit of hz) before entering idle might be a good compromise
to avoid hlt enter/exit thrashing.

Let me know your thoughts on this.  It may already be implemented
so just saying "we do that" would be fine too. :)

-Alfred

* Jeff Roberson <jroberson at jroberson.net> [080419 03:56] wrote:
> http://people.freebsd.org/~jeff/mwait.diff
> 
> This patch implements support for the x86/amd64 monitor and mwait 
> instructions in the idle loop.  This also implements idle loop selection 
> via a sysctl string.  The following loops are supported, in 
> decreasing order of performance and power consumption:
> 
> spin      - Simply returns
> mwait     - Always use mwait to sleep.  CPU enters C0 or C1 depending on
>             how busy it is.
> mwait_hlt - Use mwait when busy but fall back to hlt/acpi when not.
> hlt       - pure hlt loop
> acpi      - uses acpi_cpu_idle if available and hlt if not.  This is the 
> default.
> 
> This also introduces a new MD function 'cpu_wake_idle' which allows MD to 
> use a faster mechanism than IPI to wake idle.  In the spin case this is a 
> nop.  For hlt and acpi we resort to an IPI.  If the processor is sleeping 
> in mwait we can simply write to a per-cpu buffer to wake it up.  This 
> saves considerable cpu cycles on the initiator and target.
> 
> The prototype for cpu_idle() changed to accept an integer indication of 
> how busy we are from the scheduler.  If we have been busy MD code may 
> choose to enter a higher power state on idle.  ULE now spins for a short 
> while if we have been very busy regardless of MD settings.
> 
> There seems to be a problem entering C0 on the Xeons I have access to.  It 
> returns from mwait too quickly.  Hopefully intel will respond to my email 
> about that.
> 
> Feedback welcome.
> 
> Thanks,
> Jeff
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-- 
- Alfred Perlstein