[PATCH V11 04/17] locking/qspinlock: Improve xchg_tail for number of cpus >= 16k

[Guo Ren]

On Mon, Sep 11, 2023 at 9:03 PM Waiman Long <longman at redhat.com> wrote:
>
> On 9/10/23 23:09, Guo Ren wrote:
> > On Mon, Sep 11, 2023 at 10:35 AM Waiman Long <longman at redhat.com> wrote:
> >>
> >> On 9/10/23 04:28, guoren at kernel.org wrote:
> >>> From: Guo Ren <guoren at linux.alibaba.com>
> >>>
> >>> The target of xchg_tail is to write the tail to the lock value, so
> >>> adding prefetchw could help the next cmpxchg step, which may
> >>> decrease the cmpxchg retry loops of xchg_tail. Some processors may
> >>> utilize this feature to give a forward guarantee, e.g., RISC-V
> >>> XuanTie processors would block the snoop channel & irq for several
> >>> cycles when prefetch.w instruction (from Zicbop extension) retired,
> >>> which guarantees the next cmpxchg succeeds.
> >>>
> >>> Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
> >>> Signed-off-by: Guo Ren <guoren at kernel.org>
> >>> ---
> >>>    kernel/locking/qspinlock.c | 5 ++++-
> >>>    1 file changed, 4 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> >>> index d3f99060b60f..96b54e2ade86 100644
> >>> --- a/kernel/locking/qspinlock.c
> >>> +++ b/kernel/locking/qspinlock.c
> >>> @@ -223,7 +223,10 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
> >>>     */
> >>>    static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
> >>>    {
> >>> -     u32 old, new, val = atomic_read(&lock->val);
> >>> +     u32 old, new, val;
> >>> +
> >>> +     prefetchw(&lock->val);
> >>> +     val = atomic_read(&lock->val);
> >>>
> >>>        for (;;) {
> >>>                new = (val & _Q_LOCKED_PENDING_MASK) | tail;
> >> That looks a bit weird. You pre-fetch and then immediately read it. How
> >> much performance gain you get by this change alone?
> >>
> >> Maybe you can define an arch specific primitive that default back to
> >> atomic_read() if not defined.
> > Thx for the reply. This is a generic optimization point I would like
> > to talk about with you.
> >
> > First, prefetchw() makes cacheline an exclusive state and serves for
> > the next cmpxchg loop semantic, which writes the idx_tail part of
> > arch_spin_lock. The atomic_read only makes cacheline in the shared
> > state, which couldn't give any guarantee for the next cmpxchg loop
> > semantic. Micro-architecture could utilize prefetchw() to provide a
> > strong forward progress guarantee for the xchg_tail, e.g., the T-HEAD
> > XuanTie processor would hold the exclusive cacheline state until the
> > next cmpxchg write success.
> >
> > In the end, Let's go back to the principle: the xchg_tail is an atomic
> > swap operation that contains write eventually, so giving a prefetchw()
> > at the beginning is acceptable for all architectures..
> > ••••••••••••
>
> I did realize afterward that prefetchw gets the cacheline in exclusive
> state. I will suggest you mention that in your commit log as well as
> adding a comment about its purpose in the code.
Okay, I would do that in v12, thx.

>
> Thanks,
> Longman
>
> >> Cheers,
> >> Longman
> >>
> >
>


-- 
Best Regards
 Guo Ren