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To avoid jitter on KVM_RUN due to synchronize_rcu(), use a rwlock instead of RCU to protect vcpu->pid, a.k.a. the pid of the task last used to a vCPU. When userspace is doing M:N scheduling of tasks to vCPUs, e.g. to run SEV migration helper vCPUs during post-copy, the synchronize_rcu() needed to change the PID associated with the vCPU can stall for hundreds of milliseconds, which is problematic for latency sensitive post-copy operations. In the directed yield path, do not acquire the lock if it's contended, i.e. if the associated PID is changing, as that means the vCPU's task is already running. Reported-by: Steve Rutherford <srutherford@google.com> Reviewed-by: Steve Rutherford <srutherford@google.com> Acked-by: Oliver Upton <oliver.upton@linux.dev> Link: https://lore.kernel.org/r/20240802200136.329973-3-seanjc@google.com Signed-off-by: Sean Christopherson <seanjc@google.com> |
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