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SubjectRe: [(RT RFC) PATCH v2 5/9] adaptive real-time lock support
>>> On Mon, Feb 25, 2008 at  5:03 PM, in message
<20080225220313.GG2659@elf.ucw.cz>, Pavel Machek <pavel@ucw.cz> wrote:
> Hi!
>
>> +/*
>> + * Adaptive-rtlocks will busywait when possible, and sleep only if
>> + * necessary. Note that the busyloop looks racy, and it is....but we do
>> + * not care. If we lose any races it simply means that we spin one more
>> + * time before seeing that we need to break-out on the next iteration.
>> + *
>> + * We realize this is a relatively large function to inline, but note that
>> + * it is only instantiated 1 or 2 times max, and it makes a measurable
>> + * performance different to avoid the call.
>> + *
>> + * Returns 1 if we should sleep
>> + *
>> + */
>> +static inline int
>> +adaptive_wait(struct rt_mutex *lock, struct rt_mutex_waiter *waiter,
>> + struct adaptive_waiter *adaptive)
>> +{
>> + int sleep = 0;
>> +
>> + for (;;) {
>> + /*
>> + * If the task was re-awoken, break out completely so we can
>> + * reloop through the lock-acquisition code.
>> + */
>> + if (!waiter->task)
>> + break;
>> +
>> + /*
>> + * We need to break if the owner changed so we can reloop
>> + * and safely acquire the owner-pointer again with the
>> + * wait_lock held.
>> + */
>> + if (adaptive->owner != rt_mutex_owner(lock))
>> + break;
>> +
>> + /*
>> + * If we got here, presumably the lock ownership is still
>> + * current. We will use it to our advantage to be able to
>> + * spin without disabling preemption...
>> + */
>> +
>> + /*
>> + * .. sleep if the owner is not running..
>> + */
>> + if (!adaptive->owner->se.on_rq) {
>> + sleep = 1;
>> + break;
>> + }
>> +
>> + /*
>> + * .. or is running on our own cpu (to prevent deadlock)
>> + */
>> + if (task_cpu(adaptive->owner) == task_cpu(current)) {
>> + sleep = 1;
>> + break;
>> + }
>> +
>> + cpu_relax();
>> + }
>> +
>> + put_task_struct(adaptive->owner);
>> +
>> + return sleep;
>> +}
>> +
>
> You want to inline this?

Yes. As the comment indicates, there are 1-2 users tops, and it has a significant impact on throughput (> 5%) to take the hit with a call. I don't think its actually much code anyway...its all comments.

>
>> +static inline void
>> +prepare_adaptive_wait(struct rt_mutex *lock, struct adaptive_waiter
> *adaptive)
> ...
>> +#define prepare_adaptive_wait(lock, busy) {}
>
> This is evil. Use empty inline function instead (same for the other
> function, there you can maybe get away with it).

Ok.


> Pavel





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