linux-kernel - Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall

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Date:   Wed, 31 Oct 2018 15:10:30 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yi Sun <yi.y.sun@...ux.intel.com>
Cc:     Waiman Long <longman@...hat.com>, Juergen Gross <jgross@...e.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        chao.p.peng@...el.com, chao.gao@...el.com,
        isaku.yamahata@...el.com, michael.h.kelley@...rosoft.com,
        tianyu.lan@...rosoft.com, "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall

On Wed, Oct 31, 2018 at 09:54:17AM +0800, Yi Sun wrote:
> On 18-10-23 17:33:28, Yi Sun wrote:
> > On 18-10-23 10:51:27, Peter Zijlstra wrote:

> > > Can you try and explain why vcpu_is_preempted() doesn't work for you?
> > 
> > I thought HvSpinWaitInfo is used to notify hypervisor the spin number
> > which is different with vcpu_is_preempted. So I did not consider
> > vcpu_is_preempted.
> > 
> > But HvSpinWaitInfo is a quite simple function and could be combined
> > with vcpu_is_preempted together. So I think it is OK to use
> > vcpu_is_preempted to make codes clean. I will have a try.
> 
> After checking codes, there is one issue to call vcpu_is_preempted.
> There are two spin loops in qspinlock_paravirt.h. One loop in
> 'pv_wait_node' calls vcpu_is_preempted. But another loop in
> 'pv_wait_head_or_lock' does not call vcpu_is_preempted. It also does
> not call any other ops of 'pv_lock_ops' in the loop. So I am afraid
> we have to add one more ops in 'pv_lock_ops' to do this.

Why? Would not something like the below cure that? Waiman, can you have
a look at this; I always forget how that paravirt crud works.

---
 kernel/locking/qspinlock.c          | 5 +++--
 kernel/locking/qspinlock_paravirt.h | 5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 8a8c3c208c5e..a4ab80f95176 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -286,7 +286,8 @@ static __always_inline void __pv_wait_node(struct mcs_spinlock *node,
 static __always_inline void __pv_kick_node(struct qspinlock *lock,
 					   struct mcs_spinlock *node) { }
 static __always_inline u32  __pv_wait_head_or_lock(struct qspinlock *lock,
-						   struct mcs_spinlock *node)
+						   struct mcs_spinlock *node,
+						   struct mcs_spinlock *prev)
 						   { return 0; }
 
 #define pv_enabled()		false
@@ -500,7 +501,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	 * If PV isn't active, 0 will be returned instead.
 	 *
 	 */
-	if ((val = pv_wait_head_or_lock(lock, node)))
+	if ((val = pv_wait_head_or_lock(lock, node, prev)))
 		goto locked;
 
 	val = atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_PENDING_MASK));
diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index 0130e488ebfe..531dadc955fb 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -399,9 +399,10 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
  * The current value of the lock will be returned for additional processing.
  */
 static u32
-pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
+pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node, struct mcs_spinlock *prev)
 {
 	struct pv_node *pn = (struct pv_node *)node;
+	struct pv_node *pp = (struct pv_node *)prev;
 	struct qspinlock **lp = NULL;
 	int waitcnt = 0;
 	int loop;
@@ -430,7 +431,7 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
 		 * disable lock stealing before attempting to acquire the lock.
 		 */
 		set_pending(lock);
-		for (loop = SPIN_THRESHOLD; loop; loop--) {
+		for (loop = SPIN_THRESHOLD; loop && !vcpu_is_preempted(prev->cpu); loop--) {
 			if (trylock_clear_pending(lock))
 				goto gotlock;
 			cpu_relax();