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

[Aubrey Li]

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