Date: Sat, 19 Apr 2008 00:56:44 -1000 (HST) From: Jeff Roberson <jroberson@jroberson.net> To: arch@freebsd.org Subject: monitor/mwait support for idle Message-ID: <20080419004911.R942@desktop>
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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, Jeffhome | help
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