On 07/31/20 03:10, Dong, Eric wrote: > Reviewed-by: Eric Dong <eric.d...@intel.com>
Thank you, merged as commit 9001b750df64, via <https://github.com/tianocore/edk2/pull/843>. Laszlo >> -----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 > > > -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#63573): https://edk2.groups.io/g/devel/message/63573 Mute This Topic: https://groups.io/mt/75871294/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
