Messages in this thread | | | From | Uladzislau Rezki <> | Date | Fri, 25 Aug 2017 00:19:53 +0200 | Subject | Re: [RFC][PATCH]: sched/fair: search a task from the tail of the queue |
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On Thu, Aug 24, 2017 at 7:46 PM, Peter Zijlstra <peterz@infradead.org> wrote: > > On Thu, Aug 24, 2017 at 12:44:45PM +0200, Uladzislau Rezki (Sony) wrote: > > From: Uladzislau Rezki <urezki@gmail.com> > > > > When a task is enqueued back from a physical CPU to the running > > list it is placed in the beginning of the queue. Thus, the cfs_tasks > > list is more or less sorted (except woken tasks) starting from recently > > given CPU time tasks toward tasks with max wait time in a run-queue. > > Hurm... that is only true for short running tasks, the moment you get > things like involuntary preemption that's completely off. > > Imagine starting 3 busy-spinning tasks, lets call then A, B and C. > > So our cfs_tasks list is ordered: C B A, since C is the last task we > started. > > At this point, C might also be the leftmost task, since it has ran > least. But the moment we let A run its full quantum it will become the > rightmost and we'll pick C. Let C run its full quantum and that becomes > the rightmost. > > So now we have, in our tree: B A C, while our list is still C B A. No > relation left what so ever. > I totally agree with your point and explanation.
> How, hackbench will be very short running tasks, so the list tends to be > better ordered vs the tree. > > That said, functionally it really doesn't matter what way around the > list we iterate for migration, so if this is a win for some, that's > nice. But it would be nice to get more benchmarks ran to see if there is > cases where it hurts.
> > Another thing you could play with is making pick_next_task_fair() move > the selected task to the front of the list. That way the list becomes a > MRU and picking from the tail always makes sense. > Apparently, briefly looking at account_entity_enqueue function and not paying much attention, i thought that the list is updated each time when a task is moved from/to rb tree, but that is not true. Thank you for your point!
I have uploaded a new patch set here: https://lkml.org/lkml/2017/8/24/860
-- Vlad Rezki
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