On 05/24/2016 09:57 AM, Wanpeng Li wrote: > From: Wanpeng Li <[email protected]> > > If an emulated lapic timer will fire soon(in the scope of 10us the > base of dynamic halt-polling, lower-end of message passing workload > latency TCP_RR's poll time < 10us) we can treat it as a short halt, > and poll to wait it fire, the fire callback apic_timer_fn() will set > KVM_REQ_PENDING_TIMER, and this flag will be check during busy poll. > This can avoid context switch overhead and the latency which we wake > up vCPU. > > This feature is slightly different from current advance expiration > way. Advance expiration rely on the vCPU is running(do polling before > vmentry). But in some cases, the timer interrupt may be blocked by > other thread(i.e., IF bit is clear) and vCPU cannot be scheduled to > run immediately. So even advance the timer early, vCPU may still see > the latency. But polling is different, it ensures the vCPU to aware > the timer expiration before schedule out. > > echo HRTICK > /sys/kernel/debug/sched_features in dynticks guests. > > Context switching - times in microseconds - smaller is better > ------------------------------------------------------------------------- > Host OS 2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K > ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw > --------- ------------- ------ ------ ------ ------ ------ ------- ------- > kernel Linux 4.6.0+ 7.9800 11.0 10.8 14.6 9.4300 13.0 10.2 > vanilla > kernel Linux 4.6.0+ 15.3 13.6 10.7 12.5 9.0000 12.8 7.38000 > poll > > Cc: Paolo Bonzini <[email protected]> > Cc: Radim Krčmář <[email protected]> > Cc: David Matlack <[email protected]> > Cc: Christian Borntraeger <[email protected]> > Cc: Yang Zhang <[email protected]> > Signed-off-by: Wanpeng Li <[email protected]> > --- > v3 -> v4: > * add module parameter halt_poll_ns_timer > * rename patch subject since lapic maybe just for x86. > v2 -> v3: > * add Yang's statement to patch description > v1 -> v2: > * add return statement to non-x86 archs > * capture never expire case for x86 (hrtimer is not started) > > arch/arm/include/asm/kvm_host.h | 4 ++++ > arch/arm64/include/asm/kvm_host.h | 4 ++++ > arch/mips/include/asm/kvm_host.h | 4 ++++ > arch/powerpc/include/asm/kvm_host.h | 4 ++++ > arch/s390/include/asm/kvm_host.h | 4 ++++ > arch/x86/kvm/lapic.c | 11 +++++++++++ > arch/x86/kvm/lapic.h | 1 + > arch/x86/kvm/x86.c | 5 +++++ > include/linux/kvm_host.h | 1 + > virt/kvm/kvm_main.c | 15 +++++++++++---- > 10 files changed, 49 insertions(+), 4 deletions(-) [...]
@@ -1966,7 +1970,7 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu) > grow = READ_ONCE(halt_poll_ns_grow); > /* 10us base */ > if (val == 0 && grow) > - val = 10000; > + val = halt_poll_ns_timer; Drop this hunk and leave this at 10000, so that a user can disable the timer logic, but keep the old polling? > else > val *= grow; > >
