intel_idle will, for the bare metal case, usually have one or more deep
power states that have the CPUIDLE_FLAG_TLB_FLUSHED flag set. When
a state with this flag is selected by the cpuidle framework, it will also
flush the TLBs as part of entering this state. The benefit of doing this is
that the kernel does not need to wake the cpu out of this deep power state
just to flush the TLBs... for which the latency can be very high due to
the exit latency of deep power states.
In a VM guest currently, this benefit of avoiding the wakeup does not exist,
while the problem (long exit latency) is even more severe. Linux will need
to wake up a vCPU (causing the host to either come out of a deep C state,
or the VMM to have to deschedule something else to schedule the vCPU) which
can take a very long time.. adding a lot of latency to tlb flush operations
(including munmap and others).
To solve this, add a "Long HLT" C state to the state table for the VM guest
case that has the CPUIDLE_FLAG_TLB_FLUSHED flag set. The result of that is
that for long idle periods (where the VMM is likely to do things that cause
large latency) the cpuidle framework will flush the TLBs (and avoid the
wakeups), while for short/quick idle durations, the existing behavior is
retained.
Now, there is still only "hlt" available in the guest, but for long idle,
the host can go to a deeper state (say C6). There is a reasonable debate
one can have to what to set for the exit_latency and break even point for
this "Long HLT" state. The good news is that intel_idle has these values
available for the underlying CPU (even when mwait is not exposed). The
solution thus is to just use the latency and break even of the deepest state
from the bare metal CPU. This is under the assumption that this is a pretty
reasonable estimate of what the VMM would do to cause latency.
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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