On Wed, Feb 08, 2017 at 12:05:40PM +0800, Boqun Feng wrote: > On Wed, Feb 08, 2017 at 11:39:10AM +0800, Xinhui Pan wrote: > > 2016-12-26 4:26 GMT+08:00 Waiman Long <[email protected]>: > > > > > A number of cmpxchg calls in qspinlock_paravirt.h were replaced by more > > > relaxed versions to improve performance on architectures that use LL/SC. > > > > > > All the locking related cmpxchg's are replaced with the _acquire > > > variants: > > > - pv_queued_spin_steal_lock() > > > - trylock_clear_pending() > > > > > > The cmpxchg's related to hashing are replaced by either by the _release > > > or the _relaxed variants. See the inline comment for details. > > > > > > Signed-off-by: Waiman Long <[email protected]> > > > > > > v1->v2: > > > - Add comments in changelog and code for the rationale of the change. > > > > > > --- > > > kernel/locking/qspinlock_paravirt.h | 50 ++++++++++++++++++++++++------ > > > ------- > > > 1 file changed, 33 insertions(+), 17 deletions(-) > > > > > > > > > @@ -323,8 +329,14 @@ static void pv_wait_node(struct mcs_spinlock *node, > > > struct mcs_spinlock *prev) > > > * If pv_kick_node() changed us to vcpu_hashed, retain > > > that > > > * value so that pv_wait_head_or_lock() knows to not also > > > try > > > * to hash this lock. > > > + * > > > + * The smp_store_mb() and control dependency above will > > > ensure > > > + * that state change won't happen before that. > > > Synchronizing > > > + * with pv_kick_node() wrt hashing by this waiter or by > > > the > > > + * lock holder is done solely by the state variable. There > > > is > > > + * no other ordering requirement. > > > */ > > > - cmpxchg(&pn->state, vcpu_halted, vcpu_running); > > > + cmpxchg_relaxed(&pn->state, vcpu_halted, vcpu_running); > > > > > > /* > > > * If the locked flag is still not set after wakeup, it is > > > a > > > @@ -360,9 +372,12 @@ static void pv_kick_node(struct qspinlock *lock, > > > struct mcs_spinlock *node) > > > * pv_wait_node(). If OTOH this fails, the vCPU was running and > > > will > > > * observe its next->locked value and advance itself. > > > * > > > - * Matches with smp_store_mb() and cmpxchg() in pv_wait_node() > > > + * Matches with smp_store_mb() and cmpxchg_relaxed() in > > > pv_wait_node(). > > > + * A release barrier is used here to ensure that node->locked is > > > + * always set before changing the state. See comment in > > > pv_wait_node(). > > > */ > > > - if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted) > > > + if (cmpxchg_release(&pn->state, vcpu_halted, vcpu_hashed) > > > + != vcpu_halted) > > > return; > > > > > > hi, Waiman > > We can't use _release here, a full barrier is needed. > > > > There is pv_kick_node vs pv_wait_head_or_lock > > > > [w] l->locked = _Q_SLOW_VAL //reordered here > > > > if (READ_ONCE(pn->state) == vcpu_hashed) //False. > > > > lp = (struct qspinlock **)1; > > > > [STORE] pn->state = vcpu_hashed lp = pv_hash(lock, > > pn); > > pv_hash() if > > (xchg(&l->locked, _Q_SLOW_VAL) == 0) // fasle, not unhashed. > > > > This analysis is correct, but.. >
Hmm.. look at this again, I don't think this analysis is meaningful,
let's say the reordering didn't happen, we still got(similar to your
case):
if (READ_ONCE(pn->state) ==
vcpu_hashed) // false.
lp = (struct qspinlock **)1;
cmpxchg(pn->state, vcpu_halted, vcpu_hashed);
if(!lp) {
lp = pv_hash(lock, pn);
WRITE_ONCE(l->locked, _Q_SLOW_VAL);
pv_hash();
if (xchg(&l->locked,
_Q_SLOW_VAL) == 0) // fasle, not unhashed.
, right?
Actually, I think this or your case could not happen because we have
cmpxchg(pn->state, vcpu_halted, vcpu_running);
in pv_wait_node(), which makes us either observe vcpu_hashed or set
pn->state to vcpu_running before pv_kick_node() trying to do the hash.
I may miss something subtle, but does switching back to cmpxchg() could
fix the RCU stall you observed?
Regards,
Boqun
> > Then the same lock has hashed twice but only unhashed once. So at last as
> > the hash table grows big, we hit RCU stall.
> >
> > I hit RCU stall when I run netperf benchmark
> >
>
> how will a big hash table hit RCU stall? Do you have the call trace for
> your RCU stall?
>
> Regards,
> Boqun
>
> > thanks
> > xinhui
> >
> >
> > > --
> > > 1.8.3.1
> > >
> > >
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