Re: [PATCH RFC 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Aug 02, 2013 at 01:53:22AM +0530, Raghavendra K T wrote:

You need to learn to trim your replies.. I already stopped reading that
paravirt thread because of it. Soon I'll introduce you to my /dev/null
mail reader.

> On 08/01/2013 08:07 AM, Waiman Long wrote:
> >+static __always_inline void queue_spin_lock(struct qspinlock *lock)
> >+{
> >+	if (likely(queue_spin_trylock(lock)))
> >+		return;
> >+	queue_spin_lock_slowpath(lock);
> >+}
> 
> quickly falling into slowpath may hurt performance in some cases. no?
> 
> Instead, I tried something like this:
> 
> #define SPIN_THRESHOLD 64
> 
> static __always_inline void queue_spin_lock(struct qspinlock *lock)
> {
>         unsigned count = SPIN_THRESHOLD;
>         do {
>                 if (likely(queue_spin_trylock(lock)))
>                         return;
>                 cpu_relax();
>         } while (count--);
>         queue_spin_lock_slowpath(lock);
> }
> 
> Though I could see some gains in overcommit, but it hurted undercommit
> in some workloads :(.

This would break the FIFO nature of the lock.