On Fri, 2016-10-14 at 09:16 +0800, Wanpeng Li wrote: > 2016-10-14 8:53 GMT+08:00 Suraj Jitindar Singh <sjitindarsingh@gmail. > com>: > > > > 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, Yeah that's slightly more descriptive I guess > 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
