On 20/10/21 13:17, Peter Zijlstra wrote:
On Wed, Oct 20, 2021 at 07:06:38AM -0400, Paolo Bonzini wrote:
In some cases, rcuwait_wake_up can be called even if the actual likelihood
of a wakeup is very low. If CONFIG_PREEMPT_RCU is active, the resulting
rcu_read_lock/rcu_read_unlock pair can be relatively expensive, and in
fact it is unnecessary when there is no w->task to keep alive: the
memory barrier before the read is what matters in order to avoid missed
wakeups.
Therefore, do an early check of w->task right after the barrier, and skip
rcu_read_lock/rcu_read_unlock unless there is someone waiting for a wakeup.
Running kvm-unit-test/vmexit.flat with APICv disabled, most interrupt
injection tests (tscdeadline*, self_ipi*, x2apic_self_ipi*) improve
by around 600 cpu cycles.
*how* ?!?
AFAICT, rcu_read_lock() for PREEMPT_RCU is:
WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1);
barrier();
Paul?
Wanpeng, can you share your full .config?
Paolo
