Re: [RFC PATCH v2 11/17] sched: Basic tracking of matching tasks

[Aubrey Li]

On Thu, May 9, 2019 at 10:14 AM Subhra Mazumdar
<subhra.mazumdar@xxxxxxxxxx> wrote:
>
>
> On 5/8/19 6:38 PM, Aubrey Li wrote:
> > On Thu, May 9, 2019 at 8:29 AM Subhra Mazumdar
> > <subhra.mazumdar@xxxxxxxxxx> wrote:
> >>
> >> On 5/8/19 5:01 PM, Aubrey Li wrote:
> >>> On Thu, May 9, 2019 at 2:41 AM Subhra Mazumdar
> >>> <subhra.mazumdar@xxxxxxxxxx> wrote:
> >>>> On 5/8/19 11:19 AM, Subhra Mazumdar wrote:
> >>>>> On 5/8/19 8:49 AM, Aubrey Li wrote:
> >>>>>>> Pawan ran an experiment setting up 2 VMs, with one VM doing a
> >>>>>>> parallel kernel build and one VM doing sysbench,
> >>>>>>> limiting both VMs to run on 16 cpu threads (8 physical cores), with
> >>>>>>> 8 vcpu for each VM.
> >>>>>>> Making the fix did improve kernel build time by 7%.
> >>>>>> I'm gonna agree with the patch below, but just wonder if the testing
> >>>>>> result is consistent,
> >>>>>> as I didn't see any improvement in my testing environment.
> >>>>>>
> >>>>>> IIUC, from the code behavior, especially for 2 VMs case(only 2
> >>>>>> different cookies), the
> >>>>>> per-rq rb tree unlikely has nodes with different cookies, that is, all
> >>>>>> the nodes on this
> >>>>>> tree should have the same cookie, so:
> >>>>>> - if the parameter cookie is equal to the rb tree cookie, we meet a
> >>>>>> match and go the
> >>>>>> third branch
> >>>>>> - else, no matter we go left or right, we can't find a match, and
> >>>>>> we'll return idle thread
> >>>>>> finally.
> >>>>>>
> >>>>>> Please correct me if I was wrong.
> >>>>>>
> >>>>>> Thanks,
> >>>>>> -Aubrey
> >>>>> This is searching in the per core rb tree (rq->core_tree) which can have
> >>>>> 2 different cookies. But having said that, even I didn't see any
> >>>>> improvement with the patch for my DB test case. But logically it is
> >>>>> correct.
> >>>>>
> >>>> Ah, my bad. It is per rq. But still can have 2 different cookies. Not sure
> >>>> why you think it is unlikely?
> >>> Yeah, I meant 2 different cookies on the system, but unlikely 2
> >>> different cookies
> >>> on one same rq.
> >>>
> >>> If I read the source correctly, for the sched_core_balance path, when try to
> >>> steal cookie from another CPU, sched_core_find() uses dst's cookie to search
> >>> if there is a cookie match in src's rq, and sched_core_find() returns idle or
> >>> matched task, and later put this matched task onto dst's rq (activate_task() in
> >>> sched_core_find()). At this moment, the nodes on the rq's rb tree should have
> >>> same cookies.
> >>>
> >>> Thanks,
> >>> -Aubrey
> >> Yes, but sched_core_find is also called from pick_task to find a local
> >> matching task.
> > Can a local searching introduce a different cookies? Where is it from?
> No. I meant the local search uses the same binary search of sched_core_find
> so it has to be correct.
> >
> >> The enqueue side logic of the scheduler is unchanged with
> >> core scheduling,
> > But only the task with cookies is placed onto this rb tree?
> >
> >> so it is possible tasks with different cookies are
> >> enqueued on the same rq. So while searching for a matching task locally
> >> doing it correctly should matter.
> > May I know how exactly?
> select_task_rq_* seems to be unchanged. So the search logic to find a cpu
> to enqueue when a task becomes runnable is same as before and doesn't do
> any kind of cookie matching.

Okay, that's true in task wakeup path, and also load_balance seems to pull task
without checking cookie too. But my system is not over loaded when I tested this
patch, so there is none or only one task in rq and on the rq's rb
tree, so this patch
does not make a difference.

The question is, should we do cookie checking for task selecting CPU and load
balance CPU pulling task?

Thanks,
-Aubrey