Messages in this thread | | | Date | Fri, 29 Sep 2017 00:39:32 +0200 | From | Gerd Gerats <> | Subject | Re: [RFC] futex: hashbucket as list of futex instead of waiters |
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On Thu, Sep 28, 2017 at 12:58:14PM +0200, Peter Zijlstra wrote: > 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 do not understand, plist is the original data structure. I assume, it works together with SCHED_DEADLINE. Or is there something on the way not pressent in 4.13.
pring replaces this data structure, retaining prio sorting rules, by a head-less ring. This ring contains all waiter for one futex_key, ordered the same way as by plist.
Thanx, Gerd
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