Re: [Xen-devel] [PATCH 03/11] qspinlock: Add pending bit

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Jun 17, 2014 at 04:51:57PM -0400, Waiman Long wrote:
> On 06/17/2014 04:36 PM, Konrad Rzeszutek Wilk wrote:
> >On Sun, Jun 15, 2014 at 02:47:00PM +0200, Peter Zijlstra wrote:
> >>Because the qspinlock needs to touch a second cacheline; add a pending
> >>bit and allow a single in-word spinner before we punt to the second
> >>cacheline.
> >Could you add this in the description please:
> >
> >And by second cacheline we mean the local 'node'. That is the:
> >mcs_nodes[0] and mcs_nodes[idx]
> >
> >Perhaps it might be better then to split this in the header file
> >as this is trying to not be a slowpath code - but rather - a
> >pre-slow-path-lets-try-if-we can do another cmpxchg in case
> >the unlocker has just unlocked itself.
> >
> >So something like:
> >
> >diff --git a/include/asm-generic/qspinlock.h 
> >b/include/asm-generic/qspinlock.h
> >index e8a7ae8..29cc9c7 100644
> >--- a/include/asm-generic/qspinlock.h
> >+++ b/include/asm-generic/qspinlock.h
> >@@ -75,11 +75,21 @@ extern void queue_spin_lock_slowpath(struct qspinlock 
> >*lock, u32 val);
> >   */
> >  static __always_inline void queue_spin_lock(struct qspinlock *lock)
> >  {
> >-    u32 val;
> >+    u32 val, new;
> >
> >     val = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
> >     if (likely(val == 0))
> >             return;
> >+
> >+    /* One more attempt - but if we fail mark it as pending. */
> >+    if (val == _Q_LOCKED_VAL) {
> >+            new = Q_LOCKED_VAL |_Q_PENDING_VAL;
> >+
> >+            old = atomic_cmpxchg(&lock->val, val, new);
> >+            if (old == _Q_LOCKED_VAL) /* YEEY! */
> >+                    return;
> 
> No, it can leave like that. The unlock path will not clear the pending bit.

Err, you are right. It needs to go back in the slowpath.

> We are trying to make the fastpath as simple as possible as it may be
> inlined. The complexity of the queue spinlock is in the slowpath.

Sure, but then it shouldn't be called slowpath anymore as it is not
slow. It is a combination of fast path (the potential chance of
grabbing the lock and setting the pending lock) and the real slow
path (the queuing). Perhaps it should be called 'queue_spinlock_complex' ?

> 
> Moreover, an cmpxchg followed immediately followed by another cmpxchg will
> just increase the level of memory contention when a lock is fairly
> contended. The chance of second cmpxchg() succeeding will be pretty low.

Then why even do the pending bit - which is what the slowpath does
for the first time. And if it grabs it (And sets the pending bit) it
immediately exits. Why not perculate that piece of code in-to this header.

And the leave all that slow code (queing, mcs_lock access, etc) in the slowpath.

> 
> -Longman
> 
> 

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