2016-10-14 8:53 GMT+08:00 Suraj Jitindar Singh <sjitindarsingh@xxxxxxxxx>: > There is currently no documentation about the halt polling capabilities > of the kvm module. Add some documentation describing the mechanism as well > as the module parameters to all better understanding of how halt polling > should be used and the effect of tuning the module parameters. How about replace "halt-polling" by "Adaptive halt-polling"? Btw, thanks for your docs. Regards, Wanpeng Li > > Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@xxxxxxxxx> > --- > Documentation/virtual/kvm/00-INDEX | 2 + > Documentation/virtual/kvm/halt-polling.txt | 127 +++++++++++++++++++++++++++++ > 2 files changed, 129 insertions(+) > create mode 100644 Documentation/virtual/kvm/halt-polling.txt > > diff --git a/Documentation/virtual/kvm/00-INDEX b/Documentation/virtual/kvm/00-INDEX > index fee9f2b..69fe1a8 100644 > --- a/Documentation/virtual/kvm/00-INDEX > +++ b/Documentation/virtual/kvm/00-INDEX > @@ -6,6 +6,8 @@ cpuid.txt > - KVM-specific cpuid leaves (x86). > devices/ > - KVM_CAP_DEVICE_CTRL userspace API. > +halt-polling.txt > + - notes on halt-polling > hypercalls.txt > - KVM hypercalls. > locking.txt > diff --git a/Documentation/virtual/kvm/halt-polling.txt b/Documentation/virtual/kvm/halt-polling.txt > new file mode 100644 > index 0000000..4a84183 > --- /dev/null > +++ b/Documentation/virtual/kvm/halt-polling.txt > @@ -0,0 +1,127 @@ > +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. > -- > 2.5.5 > > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
