Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 127 insertions, 0 deletions

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+The KVM halt polling system
+===========================
+
+The KVM halt polling system provides a feature within KVM whereby the latency
+of a guest can, under some circumstances, be reduced by polling in the host
+for some time period after the guest has elected to no longer run by cedeing.
+That is, when a guest vcpu has ceded, or in the case of powerpc when all of the
+vcpus of a single vcore have ceded, the host kernel polls for wakeup conditions
+before giving up the cpu to the scheduler in order to let something else run.
+
+Polling provides a latency advantage in cases where the guest can be run again
+very quickly by at least saving us a trip through the scheduler, normally on
+the order of a few micro-seconds, although performance benefits are workload
+dependant. In the event that no wakeup source arrives during the polling
+interval or some other task on the runqueue is runnable the scheduler is
+invoked. Thus halt polling is especially useful on workloads with very short
+wakeup periods where the time spent halt polling is minimised and the time
+savings of not invoking the scheduler are distinguishable.
+
+The generic halt polling code is implemented in:
+
+	virt/kvm/kvm_main.c: kvm_vcpu_block()
+
+The powerpc kvm-hv specific case is implemented in:
+
+	arch/powerpc/kvm/book3s_hv.c: kvmppc_vcore_blocked()
+
+Halt Polling Interval
+=====================
+
+The maximum time for which to poll before invoking the scheduler, referred to
+as the halt polling interval, is increased and decreased based on the perceived
+effectiveness of the polling in an attempt to limit pointless polling.
+This value is stored in either the vcpu struct:
+
+	kvm_vcpu->halt_poll_ns
+
+or in the case of powerpc kvm-hv, in the vcore struct:
+
+	kvmppc_vcore->halt_poll_ns
+
+Thus this is a per vcpu (or vcore) value.
+
+During polling if a wakeup source is received within the halt polling interval,
+the interval is left unchanged. In the event that a wakeup source isn't
+received during the polling interval (and thus schedule is invoked) there are
+two options, either the polling interval and total block time[0] were less than
+the global max polling interval (see module params below), or the total block
+time was greater than the global max polling interval.
+
+In the event that both the polling interval and total block time were less than
+the global max polling interval then the polling interval can be increased in
+the hope that next time during the longer polling interval the wake up source
+will be received while the host is polling and the latency benefits will be
+received. The polling interval is grown in the function grow_halt_poll_ns() and
+is multiplied by the module parameter halt_poll_ns_grow.
+
+In the event that the total block time was greater than the global max polling
+interval then the host will never poll for long enough (limited by the global
+max) to wakeup during the polling interval so it may as well be shrunk in order
+to avoid pointless polling. The polling interval is shrunk in the function
+shrink_halt_poll_ns() and is divided by the module parameter
+halt_poll_ns_shrink, or set to 0 iff halt_poll_ns_shrink == 0.
+
+It is worth noting that this adjustment process attempts to hone in on some
+steady state polling interval but will only really do a good job for wakeups
+which come at an approximately constant rate, otherwise there will be constant
+adjustment of the polling interval.
+
+[0] total block time: the time between when the halt polling function is
+		      invoked and a wakeup source received (irrespective of
+		      whether the scheduler is invoked within that function).
+
+Module Parameters
+=================
+
+The kvm module has 3 tuneable module parameters to adjust the global max
+polling interval as well as the rate at which the polling interval is grown and
+shrunk. These variables are defined in include/linux/kvm_host.h and as module
+parameters in virt/kvm/kvm_main.c, or arch/powerpc/kvm/book3s_hv.c in the
+powerpc kvm-hv case.
+
+Module Parameter    |	     Description	      |	     Default Value
+--------------------------------------------------------------------------------
+halt_poll_ns	    | The global max polling interval | KVM_HALT_POLL_NS_DEFAULT
+		    | which defines the ceiling value |
+		    | of the polling interval for     | (per arch value)
+		    | each vcpu. 		      |
+--------------------------------------------------------------------------------
+halt_poll_ns_grow   | The value by which the halt     |	2
+		    | polling interval is multiplied  |
+		    | in the grow_halt_poll_ns()      |
+		    | function.			      |
+--------------------------------------------------------------------------------
+halt_poll_ns_shrink | The value by which the halt     |	0
+		    | polling interval is divided in  |
+		    | the shrink_halt_poll_ns()	      |
+		    | function.			      |
+--------------------------------------------------------------------------------
+
+These module parameters can be set from the debugfs files in:
+
+	/sys/module/kvm/parameters/
+
+Note: that these module parameters are system wide values and are not able to
+      be tuned on a per vm basis.
+
+Further Notes
+=============
+
+- Care should be taken when setting the halt_poll_ns module parameter as a
+large value has the potential to drive the cpu usage to 100% on a machine which
+would be almost entirely idle otherwise. This is because even if a guest has
+wakeups during which very little work is done and which are quite far apart, if
+the period is shorter than the global max polling interval (halt_poll_ns) then
+the host will always poll for the entire block time and thus cpu utilisation
+will go to 100%.
+
+- Halt polling essentially presents a trade off between power usage and latency
+and the module parameters should be used to tune the affinity for this. Idle
+cpu time is essentially converted to host kernel time with the aim of decreasing
+latency when entering the guest.
+
+- Halt polling will only be conducted by the host when no other tasks are
+runnable on that cpu, otherwise the polling will cease immediately and
+schedule will be invoked to allow that other task to run. Thus this doesn't
+allow a guest to denial of service the cpu.