Re: [RFC] futex: hashbucket as list of futex instead of waiters

[Peter Zijlstra]

On Sun, Sep 10, 2017 at 11:41:58PM +0200, Gerd Gerats wrote:
> When using futex as a condition variable, for example: to manage a
> threadpool, there may be a lot of threads inside the futex_wait to sleep on
> this futex. The futex_hash_bucket consists therefore of many struct futex_q
> for the same futex.
> 
> On bad luck another futex, used as mutex, hashed into the same bucket.
> Every futex_wake on this mutex, has to scan the whole chain of above waiter
> to find the struct futex_q for this mutex. For non-unusual threadpool sizes
> of more than 20, this should be a considerable effort.
> 
> I therefore suggest to include in the hash-bucketchain only one struct
> futex_q per futex and to queue additional waiter in an extrachain at the
> 'top' futex_q entry. Thus different futex are isolated from each other, the
> cost of a hash collision is reduced.

So I don't dislike that idea.. however

> To show the idea, I added a sample patch. Here, the plist is exchanged for
> a futex-specific implementation. kernel/pring.h is certainly not not the
> right place.

So I suppose the purpose of that plist in futex is to enable waking up
the highest prio waiter, but with the advent of SCHED_DEADLINE that no
longer works.

I think Thomas resisted going the RB-tree route earlier..