On Thu, May 9, 2019 at 10:14 AM Subhra Mazumdar <subhra.mazum...@oracle.com> wrote: > > > On 5/8/19 6:38 PM, Aubrey Li wrote: > > On Thu, May 9, 2019 at 8:29 AM Subhra Mazumdar > > <subhra.mazum...@oracle.com> wrote: > >> > >> On 5/8/19 5:01 PM, Aubrey Li wrote: > >>> On Thu, May 9, 2019 at 2:41 AM Subhra Mazumdar > >>> <subhra.mazum...@oracle.com> 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