I cannot understand the following function called from load_balance()
It is called with this_rq->lock held, from load_balance() (et. al) to make sure
that we hold both runque->lock's before move_tasks().
static void double_lock_balance(runqueue_t *this_rq, runqueue_t *busiest)
{
if (unlikely(!spin_trylock(&busiest->lock))) {
if (busiest < this_rq) {
spin_unlock(&this_rq->lock);
spin_lock(&busiest->lock);
spin_lock(&this_rq->lock);
} else
spin_lock(&busiest->lock);
}
}
What I understand is that, the branch unlikely() is very unlikely to be taken.
=> !spin_trylock(&busiest->lock) is almost always false (0)
=> spin_trylock(&busieset->lock) is almost always nonzero
=> failure to lock, i.e, already locked by some other thread.
If it fails, then this function does not do anything, it simply returns.
And to complicate matters, move_tasks() assumes that it is called with both
busiest->lock and this_rq->lock held. (presumably by the calling thread)
What is wrong with my understanding? or is it a bug in kernel ;-) ?
Regards,
Om.
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