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

Paolo Bonzini
2021-01-26T22:48:16Z

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