Messages in this thread | | | Subject | Re: [PATCH 15/24] kvm: mmu: Wrap mmu_lock cond_resched and needbreak | From | Paolo Bonzini <> | Date | Tue, 26 Jan 2021 21:47:50 +0100 |
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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
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