On Tue, Nov 17, 2020 at 06:19:40PM -0500, Joel Fernandes (Google) wrote: > From: Peter Zijlstra <pet...@infradead.org> > > The rationale is as follows. In the core-wide pick logic, even if > need_sync == false, we need to go look at other CPUs (non-local CPUs) to > see if they could be running RT. > > Say the RQs in a particular core look like this: > Let CFS1 and CFS2 be 2 tagged CFS tags. Let RT1 be an untagged RT task. > > rq0 rq1 > CFS1 (tagged) RT1 (not tag) > CFS2 (tagged) > > Say schedule() runs on rq0. Now, it will enter the above loop and > pick_task(RT) will return NULL for 'p'. It will enter the above if() block > and see that need_sync == false and will skip RT entirely. > > The end result of the selection will be (say prio(CFS1) > prio(CFS2)): > rq0 rq1 > CFS1 IDLE > > When it should have selected: > rq0 r1 > IDLE RT > > Joel saw this issue on real-world usecases in ChromeOS where an RT task > gets constantly force-idled and breaks RT. Lets cure it. > > NOTE: This problem will be fixed differently in a later patch. It just > kept here for reference purposes about this issue, and to make > applying later patches easier. >
The changelog is hard to read, it refers to above if(), whereas there is no code snippet in the changelog. Also, from what I can see following the series, p->core_cookie is not yet set anywhere (unless I missed it), so fixing it in here did not make sense just reading the series. Balbir
