Messages in this thread Patch in this message | | | From | Pan Xinhui <> | Subject | [PATCH 3/3] locking/osq: Drop the overload of osq_lock() | Date | Mon, 27 Jun 2016 13:41:30 -0400 |
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An over-committed guest with more vCPUs than pCPUs has a heavy overload in osq_lock().
This is because vCPU A hold the osq lock and yield out, vCPU B wait per_cpu node->locked to be set. IOW, vCPU B wait vCPU A to run and unlock the osq lock. Such spinning is meaningless.
So lets use vcpu_is_preempted() to detect if we need stop the spinning
test case: perf record -a perf bench sched messaging -g 400 -p && perf report
before patch: 18.09% sched-messaging [kernel.vmlinux] [k] osq_lock 12.28% sched-messaging [kernel.vmlinux] [k] rwsem_spin_on_owner 5.27% sched-messaging [kernel.vmlinux] [k] mutex_unlock 3.89% sched-messaging [kernel.vmlinux] [k] wait_consider_task 3.64% sched-messaging [kernel.vmlinux] [k] _raw_write_lock_irq 3.41% sched-messaging [kernel.vmlinux] [k] mutex_spin_on_owner.is 2.49% sched-messaging [kernel.vmlinux] [k] system_call
after patch: 20.68% sched-messaging [kernel.vmlinux] [k] mutex_spin_on_owner 8.45% sched-messaging [kernel.vmlinux] [k] mutex_unlock 4.12% sched-messaging [kernel.vmlinux] [k] system_call 3.01% sched-messaging [kernel.vmlinux] [k] system_call_common 2.83% sched-messaging [kernel.vmlinux] [k] copypage_power7 2.64% sched-messaging [kernel.vmlinux] [k] rwsem_spin_on_owner 2.00% sched-messaging [kernel.vmlinux] [k] osq_lock
Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com> --- kernel/locking/osq_lock.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-)
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c index 05a3785..9e86f0b 100644 --- a/kernel/locking/osq_lock.c +++ b/kernel/locking/osq_lock.c @@ -21,6 +21,11 @@ static inline int encode_cpu(int cpu_nr) return cpu_nr + 1; } +static inline int node_cpu(struct optimistic_spin_node *node) +{ + return node->cpu - 1; +} + static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val) { int cpu_nr = encoded_cpu_val - 1; @@ -118,8 +123,17 @@ bool osq_lock(struct optimistic_spin_queue *lock) while (!READ_ONCE(node->locked)) { /* * If we need to reschedule bail... so we can block. + * An over-committed guest with more vCPUs than pCPUs + * might fall in this loop and cause a huge overload. + * This is because vCPU A(prev) hold the osq lock and yield out + * vCPU B(node) wait ->locked to be set, IOW, it wait utill + * vCPU A run and unlock the osq lock. Such spin is meaningless + * use vcpu_is_preempted to detech such case. IF arch does not + * support vcpu preempted check, vcpu_is_preempted is a macro + * defined by false. */ - if (need_resched()) + if (need_resched() || + vcpu_is_preempted(node_cpu(node->prev))) goto unqueue; cpu_relax_lowlatency(); -- 2.4.11
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