Messages in this thread | | | Date | Wed, 12 Oct 2022 09:19:53 -0400 | Subject | Re: [PATCH] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath | From | Waiman Long <> |
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On 10/12/22 00:04, Hillf Danton wrote: >> you mean, you want to check and change waiter->handoff_set on every run >> rwsem_try_write_lock(). >> > Yes, with RWSEM_FLAG_HANDOFF set, it is too late for non first waiters to > spin, and with both RWSEM_LOCK_MASK and RWSEM_FLAG_HANDOFF set, the rivals > in the RWSEM_LOCK_MASK have an uphand over the first waiter wrt acquiring > the lock, and it is not a bad option for the first waiter to take a step > back off. > > if (count & RWSEM_LOCK_MASK) { > if (has_handoff || (!rt_task(waiter->task) && > !time_after(jiffies, waiter->timeout))) > return false; > > new |= RWSEM_FLAG_HANDOFF; > } else { > >> But does it break optimistic spinning ? @waiman ? > Waiters spin for acquiring lock instead of lockup and your report shows > spinning too much makes trouble. The key is stop spinning neither too > late nor too early. My proposal is a simple one with as few heuristics > added as possible.
Yes, too much spinning is bad if we have RT tasks in the mix, otherwise it should be fine.
Cheers, Longman
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