Re: [PATCH 2/2] locking/qspinlock: Limit # of spins in _Q_PENDING_VAL wait loop
From: Will Deacon
Date: Tue Apr 10 2018 - 14:26:32 EST
Hi Waiman,
On Mon, Apr 09, 2018 at 02:08:52PM -0400, Waiman Long wrote:
> A locker in the pending code path is doing an infinite number of spins
> when waiting for the _Q_PENDING_VAL to _Q_LOCK_VAL transition. There
> is a concern that lock starvation can happen concurrent lockers are
> able to take the lock in the cmpxchg loop without queuing and pass it
> around amongst themselves.
>
> To ensure forward progress while still taking advantage of using
> the pending code path without queuing, the code is now modified
> to do a limited number of spins before aborting the effort and
> going to queue itself.
>
> Ideally, the spinning times should be at least a few times the typical
> cacheline load time from memory which I think can be down to 100ns or
> so for each cacheline load with the newest systems or up to several
> hundreds ns for older systems.
>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
> kernel/locking/qspinlock.c | 19 +++++++++++++++++--
> 1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 634a49b..35367cc 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -82,6 +82,15 @@
> #endif
>
> /*
> + * The pending bit spinning loop count.
> + * This parameter can be overridden by another architecture specific
> + * constant. Default is 512.
> + */
> +#ifndef _Q_PENDING_LOOP
> +#define _Q_PENDING_LOOP (1 << 9)
> +#endif
I really dislike heuristics like this because there's never a good number
to choose and it almost certainly varies between systems and workloads
rather than just by architecture. However, I've also not managed to come
up with something better.
If I rewrite your code slightly to look like:
if (val == _Q_PENDING_VAL) {
int cnt = _Q_PENDING_LOOP;
val = atomic_cond_read_relaxed(&lock->val, (VAL != _Q_PENDING_VAL) || !cnt--);
}
then architectures that implement atomic_cond_read_relaxed as something
more interesting than a spinning loop will probably be happy with
_Q_PENDING_LOOP == 1;
I'll post a v2 tomorrow with that change, and I'll add your stat patch to
the series too so that everything is kept together.
Will