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

[Subhra Mazumdar]

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.
> Thanks,
> -Aubrey