Reviewed-by: Eric Dong <eric.d...@intel.com> > -----Original Message----- > From: Laszlo Ersek <ler...@redhat.com> > Sent: Thursday, July 30, 2020 2:52 AM > To: edk2-devel-groups-io <devel@edk2.groups.io> > Cc: Dong, Eric <eric.d...@intel.com>; Philippe Mathieu-Daudé > <phi...@redhat.com>; Kumar, Rahul1 <rahul1.ku...@intel.com>; Ni, Ray > <ray...@intel.com> > Subject: [PATCH] UefiCpuPkg/PiSmmCpuDxeSmm: pause in > WaitForSemaphore() before re-fetch > > Most busy waits (spinlocks) in > "UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c" > already call CpuPause() in their loop bodies; see SmmWaitForApArrival(), > APHandler(), and SmiRendezvous(). However, the "main wait" within > APHandler(): > > > // > > // Wait for something to happen > > // > > WaitForSemaphore (mSmmMpSyncData->CpuData[CpuIndex].Run); > > doesn't do so, as WaitForSemaphore() keeps trying to acquire the > semaphore without pausing. > > The performance impact is especially notable in QEMU/KVM + OVMF > virtualization with CPU overcommit (that is, when the guest has significantly > more VCPUs than the host has physical CPUs). The guest BSP is working > heavily in: > > BSPHandler() [MpService.c] > PerformRemainingTasks() [PiSmmCpuDxeSmm.c] > SetUefiMemMapAttributes() [SmmCpuMemoryManagement.c] > > while the many guest APs are spinning in the "Wait for something to happen" > semaphore acquisition, in APHandler(). The guest APs are generating useless > memory traffic and saturating host CPUs, hindering the guest BSP's progress > in SetUefiMemMapAttributes(). > > Rework the loop in WaitForSemaphore(): call CpuPause() in every iteration > after the first check fails. Due to Pause Loop Exiting (known as Pause Filter > on > AMD), the host scheduler can favor the guest BSP over the guest APs. > > Running a 16 GB RAM + 512 VCPU guest on a 448 PCPU host, this patch > reduces OVMF boot time (counted until reaching grub) from 20-30 minutes > to less than 4 minutes. > > The patch should benefit physical machines as well -- according to the Intel > SDM, PAUSE "Improves the performance of spin-wait loops". Adding PAUSE > to the generic WaitForSemaphore() function is considered a general > improvement. > > Cc: Eric Dong <eric.d...@intel.com> > Cc: Philippe Mathieu-Daudé <phi...@redhat.com> > Cc: Rahul Kumar <rahul1.ku...@intel.com> > Cc: Ray Ni <ray...@intel.com> > Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1861718 > Signed-off-by: Laszlo Ersek <ler...@redhat.com> > --- > > Notes: > Repo: https://pagure.io/lersek/edk2.git > Branch: sem_wait_pause_rhbz1861718 > > UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c | 18 +++++++++++------- > 1 file changed, 11 insertions(+), 7 deletions(-) > > diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c > b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c > index 57e788c01b1f..4bcd217917d7 100644 > --- a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c > +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c > @@ -40,14 +40,18 @@ WaitForSemaphore ( > { > UINT32 Value; > > - do { > + for (;;) { > Value = *Sem; > - } while (Value == 0 || > - InterlockedCompareExchange32 ( > - (UINT32*)Sem, > - Value, > - Value - 1 > - ) != Value); > + if (Value != 0 && > + InterlockedCompareExchange32 ( > + (UINT32*)Sem, > + Value, > + Value - 1 > + ) == Value) { > + break; > + } > + CpuPause (); > + } > return Value - 1; > } > > -- > 2.19.1.3.g30247aa5d201
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