[tip:locking/core] locking/osq: Break out of spin-wait busy waiting loop for a preempted vCPU in osq_lock()

From: tip-bot for Pan Xinhui
Date: Tue Nov 22 2016 - 07:37:43 EST

Commit-ID: 5aff60a191e579ae00ae5ca6ce16c13b687bc8a3
Gitweb: http://git.kernel.org/tip/5aff60a191e579ae00ae5ca6ce16c13b687bc8a3
Author: Pan Xinhui <xinhui.pan@xxxxxxxxxxxxxxxxxx>
AuthorDate: Wed, 2 Nov 2016 05:08:29 -0400
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 22 Nov 2016 12:48:10 +0100

locking/osq: Break out of spin-wait busy waiting loop for a preempted vCPU in osq_lock()

An over-committed guest with more vCPUs than pCPUs has a heavy overload
in osq_lock().

This is because if vCPU-A holds the osq lock and yields out, vCPU-B ends
up waiting for per_cpu node->locked to be set. IOW, vCPU-B waits for
vCPU-A to run and unlock the osq lock.

Use the new vcpu_is_preempted(cpu) interface to detect if a vCPU is
currently running or not, and break out of the spin-loop if so.

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

Suggested-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Tested-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Pan Xinhui <xinhui.pan@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: David.Laight@xxxxxxxxxx
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: benh@xxxxxxxxxxxxxxxxxxx
Cc: bsingharora@xxxxxxxxx
Cc: dave@xxxxxxxxxxxx
Cc: kernellwp@xxxxxxxxx
Cc: konrad.wilk@xxxxxxxxxx
Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
Cc: mpe@xxxxxxxxxxxxxx
Cc: paulmck@xxxxxxxxxxxxxxxxxx
Cc: paulus@xxxxxxxxx
Cc: rkrcmar@xxxxxxxxxx
Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: will.deacon@xxxxxxx
Cc: xen-devel-request@xxxxxxxxxxxxxxxxxxxx
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1478077718-37424-3-git-send-email-xinhui.pan@xxxxxxxxxxxxxxxxxx
[ Translated to English. ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/locking/osq_lock.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 4ea2710..a316794 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 +123,10 @@ bool osq_lock(struct optimistic_spin_queue *lock)
while (!READ_ONCE(node->locked)) {
* If we need to reschedule bail... so we can block.
+ * Use vcpu_is_preempted() to avoid waiting for a preempted
+ * lock holder:
- if (need_resched())
+ if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
goto unqueue;