Messages in this thread | | | Date | Tue, 16 Feb 2016 20:40:51 -0500 | From | Waiman Long <> | Subject | Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin() |
| |
On 02/16/2016 03:53 AM, Peter Zijlstra wrote: > On Mon, Feb 15, 2016 at 06:22:14PM -0800, Jason Low wrote: >> On Mon, 2016-02-15 at 18:15 -0800, Jason Low wrote: >>> On Fri, 2016-02-12 at 14:14 -0800, Davidlohr Bueso wrote: >>>> On Fri, 12 Feb 2016, Peter Zijlstra wrote: >>>> >>>>> On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote: >>>>>> static bool mutex_optimistic_spin(struct mutex *lock, >>>>>> + struct ww_acquire_ctx *ww_ctx, >>>>>> + const bool use_ww_ctx, int waiter) >>>>>> { >>>>>> struct task_struct *task = current; >>>>>> + bool acquired = false; >>>>>> >>>>>> + if (!waiter) { >>>>>> + if (!mutex_can_spin_on_owner(lock)) >>>>>> + goto done; >>>>> Why doesn't the waiter have to check mutex_can_spin_on_owner() ? >>>> afaict because mutex_can_spin_on_owner() fails immediately when the counter >>>> is -1, which is a nono for the waiters case. >>> mutex_can_spin_on_owner() returns false if the task needs to reschedule >>> or if the lock owner is not on_cpu. In either case, the task will end up >>> not spinning when it enters the spin loop. So it makes sense if the >>> waiter also checks mutex_can_spin_on_owner() so that the optimistic spin >>> queue overhead can be avoided in those cases. >> Actually, since waiters bypass the optimistic spin queue, that means the >> the mutex_can_spin_on_owner() isn't really beneficial. So Waiman is >> right in that it's fine to skip this in the waiter case. > My concern was the 'pointless' divergence between the code-paths. The > less they diverge the easier it is to understand and review. > > If it doesn't hurt, please keep it the same. If it does need to diverge, > include a comment on why.
I will keep the preemption, but will still leave out the mutex_can_spin_on_owner() check for waiter. I will add a comment to document that.
Cheers, Longman
| |