use simple waitqueue for kvm vcpu waitqueue (v5)
Status from this message: Applied, thanks.
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Patches in v1 Applied
| # | Subject and files | Size | Replies | Applied |
|---|---|---|---|---|
| 1/2 | KVM: use simple waitqueue for vcpu->wqarch/ arch/ arch/ +7 more | +36 | 0 replies | |
| 2/2 | KVM: lapic: mark LAPIC timer handler as irqsafearch/ | +39 | 0 replies |
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Cover letter [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v5)
Sebastian,
rebased against v3.18.7-rt2 as requested.
The problem:
On -RT, an emulated LAPIC timer instance has the following path:
1) hard interrupt
2) ksoftirqd is scheduled
3) ksoftirqd wakes up vcpu thread
4) vcpu thread is scheduled
This extra context switch introduces unnecessary latency in the LAPIC path for a KVM guest.
The solution:
Allow waking up vcpu thread from hardirq context, thus avoiding the need for ksoftirqd to be scheduled.
Normal waitqueues make use of spinlocks, which on -RT are sleepable locks. Therefore, waking up a waitqueue waiter involves locking a sleeping lock, which is not allowed from hard interrupt context.
cyclictest command line:
# cyclictest -m -n -q -p99 -l 1000000 -h60 -D 1m
This patch reduces the average latency in my tests from 14us to 11us.
v2: improve changelog (Rik van Riel)
v3: limit (once) guest triggered printk and WARN_ON (Paolo Bonzini)
v4: fix typo (Steven Rostedt)
v5: rebase against v3.18.7-rt2.