On 2016/2/3 5:19, Davidlohr Bueso wrote: > On Mon, 01 Feb 2016, Peter Zijlstra wrote: > >> Subject: locking/mutex: Avoid spinner vs waiter starvation >> From: Peter Zijlstra <pet...@infradead.org> >> Date: Fri, 22 Jan 2016 12:06:53 +0100 >> >> Ding Tianhong reported that under his load the optimistic spinners >> would totally starve a task that ended up on the wait list. >> >> Fix this by ensuring the top waiter also partakes in the optimistic >> spin queue. >> >> There are a few subtle differences between the assumed state of >> regular optimistic spinners and those already on the wait list, which >> result in the @acquired complication of the acquire path. >> >> Most notable are: >> >> - waiters are on the wait list and need to be taken off >> - mutex_optimistic_spin() sets the lock->count to 0 on acquire >> even though there might be more tasks on the wait list. > > Right, the main impact I see with these complications are that the > window of when a waiter takes the lock via spinning and then acquires > the wait_lock to remove itself from the list, will allow an unlock > thread to set the lock as available in the fastpath which could in > turn allow a third thread the steal the lock. With high contention, > this window will be come obviously larger as we contend for the > wait_lock. > > CPU-0 CPU-1 CPU-3 > __mutex_lock_common mutex_optimistic_spin > (->count now 0) > __mutex_fastpath_unlock > (->count now 1) __mutex_fastpath_lock > (stolen) > > spin_lock_mutex(&lock->wait_lock, flags); > > But we've always been bad when it comes to counter and waiters. >
Agree, but this patch is going to help the waiter in the wait list to get the lock, your scene probability looks more too low and I don't think it is a problem. Thanks Ding > Thanks, > Davidlohr > > . >
