kvm: add halt_poll_ns module parameter

This patch introduces a new module parameter for the KVM module; when it
is present, KVM attempts a bit of polling on every HLT before scheduling
itself out via kvm_vcpu_block.

This parameter helps a lot for latency-bound workloads---in particular
I tested it with O_DSYNC writes with a battery-backed disk in the host.
In this case, writes are fast (because the data doesn't have to go all
the way to the platters) but they cannot be merged by either the host or
the guest.  KVM's performance here is usually around 30% of bare metal,
or 50% if you use cache=directsync or cache=writethrough (these
parameters avoid that the guest sends pointless flush requests, and
at the same time they are not slow because of the battery-backed cache).
The bad performance happens because on every halt the host CPU decides
to halt itself too.  When the interrupt comes, the vCPU thread is then
migrated to a new physical CPU, and in general the latency is horrible
because the vCPU thread has to be scheduled back in.

With this patch performance reaches 60-65% of bare metal and, more
important, 99% of what you get if you use idle=poll in the guest.  This
means that the tunable gets rid of this particular bottleneck, and more
work can be done to improve performance in the kernel or QEMU.

Of course there is some price to pay; every time an otherwise idle vCPUs
is interrupted by an interrupt, it will poll unnecessarily and thus
impose a little load on the host.  The above results were obtained with
a mostly random value of the parameter (500000), and the load was around
1.5-2.5% CPU usage on one of the host's core for each idle guest vCPU.

The patch also adds a new stat, /sys/kernel/debug/kvm/halt_successful_poll,
that can be used to tune the parameter.  It counts how many HLT
instructions received an interrupt during the polling period; each
successful poll avoids that Linux schedules the VCPU thread out and back
in, and may also avoid a likely trip to C1 and back for the physical CPU.

While the VM is idle, a Linux 4 VCPU VM halts around 10 times per second.
Of these halts, almost all are failed polls.  During the benchmark,
instead, basically all halts end within the polling period, except a more
or less constant stream of 50 per second coming from vCPUs that are not
running the benchmark.  The wasted time is thus very low.  Things may
be slightly different for Windows VMs, which have a ~10 ms timer tick.

The effect is also visible on Marcelo's recently-introduced latency
test for the TSC deadline timer.  Though of course a non-RT kernel has
awful latency bounds, the latency of the timer is around 8000-10000 clock
cycles compared to 20000-120000 without setting halt_poll_ns.  For the TSC
deadline timer, thus, the effect is both a smaller average latency and
a smaller variance.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
index bde494654bccb6a7434ed81f355fbdd426a43b97..6a79314bc1dfcf8055dfb4b790d0d35a2e2bec39 100644 (file)
--- a/arch/arm/include/asm/kvm_host.h
+++ b/arch/arm/include/asm/kvm_host.h
@@ -148,6 +148,7 @@ struct kvm_vm_stat {
 };
 
 struct kvm_vcpu_stat {
+       u32 halt_successful_poll;
        u32 halt_wakeup;
 };
 

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 2c49aa4ac818596540c97694667fb076cbe7c363..8efde89613f26246711a80f5a4f86aa221797a0b 100644 (file)
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -165,6 +165,7 @@ struct kvm_vm_stat {
 };
 
 struct kvm_vcpu_stat {
+       u32 halt_successful_poll;
        u32 halt_wakeup;
 };
 

diff --git a/arch/mips/include/asm/kvm_host.h b/arch/mips/include/asm/kvm_host.h
index f2c249796ea8baa4cb6cc96f79c7efeaa35400d2..ac4fc716062b791003c76f5572d56863bcdcb2cd 100644 (file)
--- a/arch/mips/include/asm/kvm_host.h
+++ b/arch/mips/include/asm/kvm_host.h
@@ -120,6 +120,7 @@ struct kvm_vcpu_stat {
        u32 resvd_inst_exits;
        u32 break_inst_exits;
        u32 flush_dcache_exits;
+       u32 halt_successful_poll;
        u32 halt_wakeup;
 };
 

diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
index e97b907840311d2bffa258443e2d72483605ac61..c9eccf5df912037e2b71bbb4a7dddd2a1d2d866e 100644 (file)
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -49,6 +49,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
        { "resvd_inst",   VCPU_STAT(resvd_inst_exits),   KVM_STAT_VCPU },
        { "break_inst",   VCPU_STAT(break_inst_exits),   KVM_STAT_VCPU },
        { "flush_dcache", VCPU_STAT(flush_dcache_exits), KVM_STAT_VCPU },
+       { "halt_successful_poll", VCPU_STAT(halt_successful_poll), KVM_STAT_VCPU },
        { "halt_wakeup",  VCPU_STAT(halt_wakeup),        KVM_STAT_VCPU },
        {NULL}
 };

diff --cc arch/powerpc/include/asm/kvm_host.h
Simple merge

diff --cc arch/powerpc/kvm/book3s.c
Simple merge

diff --cc arch/powerpc/kvm/booke.c
Simple merge

diff --cc arch/s390/include/asm/kvm_host.h
Simple merge

diff --cc arch/s390/kvm/kvm-s390.c
Simple merge

diff --cc arch/x86/include/asm/kvm_host.h
Simple merge

diff --cc arch/x86/kvm/x86.c
Simple merge

diff --cc include/trace/events/kvm.h
Simple merge

diff --cc virt/kvm/kvm_main.c
Simple merge