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Date: Mon, 20 Feb 2017 12:58:39 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Andrea Parri <parri.andrea@...il.com>
Cc: Waiman Long <longman@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v3] locking/pvqspinlock: Relax cmpxchg's to improve
performance on some archs
(Really add Will this time ...)
On Mon, Feb 20, 2017 at 12:53:58PM +0800, Boqun Feng wrote:
> On Mon, Feb 20, 2017 at 05:20:52AM +0100, Andrea Parri wrote:
> > On Fri, Feb 17, 2017 at 03:43:40PM -0500, Waiman Long wrote:
> > > All the locking related cmpxchg's in the following functions are
> > > replaced with the _acquire variants:
> > > - pv_queued_spin_steal_lock()
> > > - trylock_clear_pending()
> > >
> > > This change should help performance on architectures that use LL/SC.
> > >
> > > On a 2-core 16-thread Power8 system with pvqspinlock explicitly
> > > enabled, the performance of a locking microbenchmark with and without
> > > this patch on a 4.10-rc8 kernel with Xinhui's PPC qspinlock patch
> > > were as follows:
> > >
> > > # of thread w/o patch with patch % Change
> > > ----------- --------- ---------- --------
> > > 4 4053.3 Mop/s 4223.7 Mop/s +4.2%
> > > 8 3310.4 Mop/s 3406.0 Mop/s +2.9%
> > > 12 2576.4 Mop/s 2674.6 Mop/s +3.8%
> > >
> > > Signed-off-by: Waiman Long <longman@...hat.com>
> > > ---
> > >
> > > v2->v3:
> > > - Reduce scope by relaxing cmpxchg's in fast path only.
> > >
> > > v1->v2:
> > > - Add comments in changelog and code for the rationale of the change.
> > >
> > > kernel/locking/qspinlock_paravirt.h | 15 +++++++++------
> > > 1 file changed, 9 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> > > index e6b2f7a..a59dc05 100644
> > > --- a/kernel/locking/qspinlock_paravirt.h
> > > +++ b/kernel/locking/qspinlock_paravirt.h
> > > @@ -72,7 +72,7 @@ static inline bool pv_queued_spin_steal_lock(struct qspinlock *lock)
> > > struct __qspinlock *l = (void *)lock;
> > >
> > > if (!(atomic_read(&lock->val) & _Q_LOCKED_PENDING_MASK) &&
> > > - (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
> > > + (cmpxchg_acquire(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
> > > qstat_inc(qstat_pv_lock_stealing, true);
> > > return true;
> > > }
> > > @@ -101,16 +101,16 @@ static __always_inline void clear_pending(struct qspinlock *lock)
> > >
> > > /*
> > > * The pending bit check in pv_queued_spin_steal_lock() isn't a memory
> > > - * barrier. Therefore, an atomic cmpxchg() is used to acquire the lock
> > > - * just to be sure that it will get it.
> > > + * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the
> > > + * lock just to be sure that it will get it.
> > > */
> > > static __always_inline int trylock_clear_pending(struct qspinlock *lock)
> > > {
> > > struct __qspinlock *l = (void *)lock;
> > >
> > > return !READ_ONCE(l->locked) &&
> > > - (cmpxchg(&l->locked_pending, _Q_PENDING_VAL, _Q_LOCKED_VAL)
> > > - == _Q_PENDING_VAL);
> > > + (cmpxchg_acquire(&l->locked_pending, _Q_PENDING_VAL,
> > > + _Q_LOCKED_VAL) == _Q_PENDING_VAL);
> > > }
> > > #else /* _Q_PENDING_BITS == 8 */
> > > static __always_inline void set_pending(struct qspinlock *lock)
> > > @@ -138,7 +138,7 @@ static __always_inline int trylock_clear_pending(struct qspinlock *lock)
> > > */
> > > old = val;
> > > new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
> > > - val = atomic_cmpxchg(&lock->val, old, new);
> > > + val = atomic_cmpxchg_acquire(&lock->val, old, new);
> > >
> > > if (val == old)
> > > return 1;
> > > @@ -361,6 +361,9 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
> > > * observe its next->locked value and advance itself.
> > > *
> > > * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
> > > + *
> > > + * We can't used relaxed form of cmpxchg here as the loading of
> > > + * pn->state can happen before setting next->locked in some archs.
> > > */
> > > if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted)
> >
> > Hi Waiman.
> >
> > cmpxchg() does not guarantee the (here implied) smp_mb(), in general; c.f.,
> > e.g., arch/arm64/include/asm/atomic_ll_sc.h, where cmpxchg() get "compiled"
> > to something like:
> >
> > _loop: ldxr; eor; cbnz _exit; stlxr; cbnz _loop; dmb ish; _exit:
> >
>
> Yes, sorry for be late for this one.
>
> So Waiman, the fact is that in this case, we want the following code
> sequence:
>
> CPU 0 CPU 1
> ================= ====================
> {pn->state = vcpu_running, node->locked = 0}
>
> smp_store_smb(&pn->state, vcpu_halted):
> WRITE_ONCE(pn->state, vcpu_halted);
> smp_mb();
> r1 = READ_ONCE(node->locked);
> arch_mcs_spin_unlock_contented();
> WRITE_ONCE(node->locked, 1)
>
> cmpxchg(&pn->state, vcpu_halted, vcpu_hashed);
>
> never ends up in:
>
> r1 == 0 && cmpxchg fail(i.e. the read part of cmpxchg reads the
> value vcpu_running).
>
> We can have such a guarantee if cmpxchg has a smp_mb() before its load
> part, which is true for PPC. But semantically, cmpxchg() doesn't provide
> any order guarantee if it fails, which is true on ARM64, IIUC. (Add Will
> in Cc for his insight ;-)).
>
> So a possible "fix"(in case ARM64 will use qspinlock some day), would be
> replace cmpxchg() with smp_mb() + cmpxchg_relaxed().
>
> Regards,
> Boqun
>
> > Andrea
> >
> >
> > > return;
> > > --
> > > 1.8.3.1
> > >
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