Re: [PATCH 15/24] kvm: mmu: Wrap mmu_lock cond_resched and needbreak

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 26/01/21 19:11, Ben Gardon wrote:
When I did a strict replacement I found ~10% worse memory population
performance.
Running dirty_log_perf_test -v 96 -b 3g -i 5 with the TDP MMU
disabled, I got 119 sec to populate memory as the baseline and 134 sec
with an earlier version of this series which just replaced the
spinlock with an rwlock. I believe this difference is statistically
significant, but didn't run multiple trials.
I didn't take notes when profiling, but I'm pretty sure the rwlock
slowpath showed up a lot. This was a very high contention scenario, so
it's probably not indicative of real-world performance.
In the slow path, the rwlock is certainly slower than a spin lock.

If the real impact doesn't seem too large, I'd be very happy to just
replace the spinlock.

Ok, so let's use the union idea and add a "#define KVM_HAVE_MMU_RWLOCK" to x86. The virt/kvm/kvm_main.c MMU notifiers functions can use the #define to pick between write_lock and spin_lock.

For x86 I want to switch to tdp_mmu=1 by default as soon as parallel page faults are in, so we can use the rwlock unconditionally and drop the wrappers, except possibly for some kind of kvm_mmu_lock/unlock_root that choose between read_lock for TDP MMU and write_lock for shadow MMU.

Thanks!

Paolo




[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux