Protect vcpu->pid with a rwlock instead of RCU, so that running a vCPU
with a different task doesn't require a full RCU synchronization, which
can introduce a non-trivial amount of jitter, especially on large systems.

I've had this mini-series sitting around for ~2 years, pretty much as-is.
I could have sworn past me thought there was a flaw in using a rwlock, and
so I never posted it, but for the life of me I can't think of any issues.

Extra eyeballs would be much appreciated.

Sean Christopherson (2):
  KVM: Return '0' directly when there's no task to yield to
  KVM: Protect vCPU's "last run PID" with rwlock, not RCU

 arch/arm64/include/asm/kvm_host.h |  2 +-
 include/linux/kvm_host.h          |  3 ++-
 virt/kvm/kvm_main.c               | 36 +++++++++++++++++--------------
 3 files changed, 23 insertions(+), 18 deletions(-)


base-commit: 332d2c1d713e232e163386c35a3ba0c1b90df83f
-- 
2.46.0.rc2.264.g509ed76dc8-goog

On Fri, 02 Aug 2024 13:01:34 -0700, Sean Christopherson wrote:
> Protect vcpu->pid with a rwlock instead of RCU, so that running a vCPU
> with a different task doesn't require a full RCU synchronization, which
> can introduce a non-trivial amount of jitter, especially on large systems.
> 
> I've had this mini-series sitting around for ~2 years, pretty much as-is.
> I could have sworn past me thought there was a flaw in using a rwlock, and
> so I never posted it, but for the life of me I can't think of any issues.
> 
> [...]

Applied to kvm-x86 generic, with a comment explaining the vcpu->mutex is the
true protector of vcpu->pid.

Oliver, I didn't add the requested lockdep notification, I really want to make
that a separate discussion:

https://lore.kernel.org/all/20241009150455.1057573-7-seanjc@google.com

[1/2] KVM: Return '0' directly when there's no task to yield to
      https://github.com/kvm-x86/linux/commit/6cf9ef23d942
[2/2] KVM: Protect vCPU's "last run PID" with rwlock, not RCU
      https://github.com/kvm-x86/linux/commit/3e7f43188ee2

--
https://github.com/kvm-x86/linux/tree/next

On Fri, Aug 02, 2024 at 01:01:34PM -0700, Sean Christopherson wrote:
> Protect vcpu->pid with a rwlock instead of RCU, so that running a vCPU
> with a different task doesn't require a full RCU synchronization, which
> can introduce a non-trivial amount of jitter, especially on large systems.
> 
> I've had this mini-series sitting around for ~2 years, pretty much as-is.
> I could have sworn past me thought there was a flaw in using a rwlock, and
> so I never posted it, but for the life of me I can't think of any issues.
> 
> Extra eyeballs would be much appreciated.

Besides the nitpicks:

Acked-by: Oliver Upton <oliver.upton@linux.dev>

-- 
Thanks,
Oliver