On Mon, Mar 25, 2019 at 6:41 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Mon, Mar 25, 2019 at 08:47:31AM -0700, Darrick J. Wong wrote:
> > Hmmm.... so it looks like the rw_semaphore behavior has shifted over
> > time, then?
>
> Yes.
>
> > I thought rwsem was supposed to queue read and write waiters in order,
> > at least on x86? Though I suppose that might not matter much since we
> > can only run one writer at a time vs. waking up all the readers at once.
> > Now I'm wondering if there ever was a time when the readers all got
> > batched to the front and starved the writers, but eh I haven't drank
> > enough coffee to remember things like that. :P
> >
> > (I wonder what would happen if rw_semaphore decided to wake up some
> > number of the readers in the rwsem wait_list, not just the ones at the
> > front...)
>
> rwsems currently allow a limited amount of queue-jumping; if a semaphore
> is currently not acquired (it's in transition between two owners), a
> running process can acquire it.
>
> I think it is a bug that we only wake readers at the front of the queue;
> I think we would get better performance if we wake all readers. ie here:
>
> /*
> - * Grant an infinite number of read locks to the readers at the front
> - * of the queue. We know that woken will be at least 1 as we accounted
> + * Grant an infinite number of read locks. We know that woken will
> + * be at least 1 as we accounted
> * for above. Note we increment the 'active part' of the count by the
> * number of readers before waking any processes up.
> */
> list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
> struct task_struct *tsk;
>
> - if (waiter->type == RWSEM_WAITING_FOR_WRITE)
> - break;
>
> Amir, it seems like you have a good test-case for trying this out ...
Sure, but please explain. Why are you waking up the writers?
Thanks,
Amir.
