> On Tue, Oct 07, 2008 at 02:09:12PM -0700, H. Peter Anvin wrote:
>> Venki Pallipadi wrote:
>>> For Intel 7400 series CPUs, the recommendation is to use a clflush on the
>>> monitored address just before monitor and mwait pair [1]. This clflush makes
>>> sure that there are no false wakeups from mwait when the monitored address
>>> was recently written to.
>>>
>>> [1] "MONITOR/MWAIT Recommendations for Intel Xeon Processor 7400 series"
>>> section in specification update document of 7400 series
>>>
http://download.intel.com/design/xeon/specupdt/32033601.pdf
>>>
>>> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
>> This seems very expensive. It really makes me wonder if it wouldn't
>> just be better to either declare monitor/mwait non-functional on this
>> chip, or make sure that mwaits can handle false wakeups.
>
> mwait can handle false wakeups. Today we wake backup all the way and find
> out there is nothing to do and go back to idle. And second time around this
> false wakeup does not happen as we do not write to monitored address in the
> interim. The problem we saw was the places where we try to look at how
> long each idle period was and take power management decision for the next idle.
> Such algorithms get confused with false wakeups.
