Messages in this thread | | | Subject | Re: [RFC] locking/mutex: Fix starvation of sleeping waiters | From | Imre Deak <> | Date | Tue, 19 Jul 2016 19:53:25 +0300 |
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On ma, 2016-07-18 at 10:47 -0700, Jason Low wrote: > 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.
Thanks, with v4 he just sent I couldn't trigger the above problem.
However this only works if mutex spinning is enabled, if it's disabled I still hit the problem due to the other forms of lock stealing. So could we prevent these if mutex spinning is anyway disabled?
--Imre
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