Messages in this thread | | | Date | Fri, 10 Sep 2021 18:37:04 +0200 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH] locking: Remove rt_rwlock_is_contended() |
| |
On 2021-09-10 18:16:14 [+0200], Peter Zijlstra wrote: > On Tue, Sep 07, 2021 at 12:34:58PM +0200, Sebastian Andrzej Siewior wrote: > > rt_rwlock_is_contended() has not users. It makes no sense to use it as > > rwlock_is_contended() because it is a sleeping lock on RT and preemption > > is possible. It reports always != 0 if used by a writer and even if > > there is a waiter then the lock might not be handed over if the > > current owner has the highest priority. > > I'm confused now... so first you have two patches that wire up > {spin,rwlock}_is_contended() and how you're arguing we shouldn't do > that?
Yes. I got arguments against it after sleeping :)
> AFAICT the _is_contended() can still use useful even with preemption, > the typicla use case is a long lock-holder deciding to drop the lock in > order to let someone else in. That still works with preemptible locks, > no?
Sure. We can do that. Then we should look into: - fixing rwsem_is_contended() for the writer. The writer always observes true even with no waiter around.
- checking the top waiter list vs priority of the lock owner/current. If the current lock owner has the highest priority then the unlock+lock is probably pointless as he regains the lock. For the spin_lock() case, if the owner is SCHED_OTHER and the waiter is SCHED_OTHER then unlock+lock will give the lock to the previous owner due to rt_mutex_steal() working in his favour. Unless there is a preemption.
- reader checking for contention is probably pointless. It works with a pending writer and one reader since a second reader will hold-off the writer from acquiring the lock. Also if the reader does unlock+lock then writer might not be quick enough.
Sebastian
| |