Messages in this thread | | | Subject | Re: [RFC] locking/mutex: Fix starvation of sleeping waiters | From | Jason Low <> | Date | Mon, 18 Jul 2016 10:47:49 -0700 |
| |
On Mon, 2016-07-18 at 19:15 +0200, Peter Zijlstra wrote: > On Mon, Jul 18, 2016 at 07:16:47PM +0300, Imre Deak wrote: > > Currently a thread sleeping on a mutex wait queue can be delayed > > indefinitely by other threads managing to steal the lock, that is > > acquiring the lock out-of-order before the sleepers. I noticed this via > > a testcase (see the Reference: below) where one CPU was unlocking / > > relocking a mutex in a tight loop while another CPU was delayed > > indefinitely trying to wake up and get the lock but losing out to the > > first CPU and going back to sleep: > > > > CPU0: CPU1: > > mutex_lock->acquire > > mutex_lock->sleep > > mutex_unlock->wake CPU1 > > wakeup > > mutex_lock->acquire > > trylock fail->sleep > > mutex_unlock->wake CPU1 > > wakeup > > mutex_lock->acquire > > trylock fail->sleep > > ... ... > > > > To fix this we can make sure that CPU1 makes progress by avoiding the > > fastpath locking, optimistic spinning and trylocking if there is any > > waiter on the list. The corresponding check can be done without holding > > wait_lock, since the goal is only to make sure sleepers make progress > > and not to guarantee that the locking will happen in FIFO order. > > I think we went over this before, that will also completely destroy > performance under a number of workloads.
Yup, once a thread becomes a waiter, all other threads will need to follow suit, so this change would effectively disable optimistic spinning in some workloads.
A few months ago, we worked on patches that allow the waiter to return to optimistic spinning to help reduce starvation. Longman sent out a version 3 patch set, and it sounded like we were fine with the concept.
Jason
| |