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

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Message-ID: <20170928105814.cfzsyi42asybctfo@hirez.programming.kicks-ass.net>
Date:   Thu, 28 Sep 2017 12:58:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Gerd Gerats <gerd.gerats.lkml@....de>
Cc:     tglx@...utronix.de, mingo@...hat.com, dvhart@...radead.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] futex: hashbucket as list of futex instead of waiters

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..