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
