Messages in this thread | | | Date | Tue, 24 Nov 2020 09:31:02 +1100 | From | Balbir Singh <> | Subject | Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to empty rbtree |
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On Fri, Nov 20, 2020 at 01:11:06PM -0500, Vineeth Pillai wrote: > Hi Balbir, > > On 11/20/20 5:15 AM, Singh, Balbir wrote: > > On 18/11/20 10:19 am, Joel Fernandes (Google) wrote: > > > From: Peter Zijlstra <peterz@infradead.org> > > > > > > pick_next_entity() is passed curr == NULL during core-scheduling. Due to > > > this, if the rbtree is empty, the 'left' variable is set to NULL within > > > the function. This can cause crashes within the function. > > > > > > This is not an issue if put_prev_task() is invoked on the currently > > > running task before calling pick_next_entity(). However, in core > > > scheduling, it is possible that a sibling CPU picks for another RQ in > > > the core, via pick_task_fair(). This remote sibling would not get any > > > opportunities to do a put_prev_task(). > > > > > > Fix it by refactoring pick_task_fair() such that pick_next_entity() is > > > called with the cfs_rq->curr. This will prevent pick_next_entity() from > > > crashing if its rbtree is empty. > > > > > > Also this fixes another possible bug where update_curr() would not be > > > called on the cfs_rq hierarchy if the rbtree is empty. This could effect > > > cross-cpu comparison of vruntime. > > > > > It is not clear from the changelog as to what does put_prev_task() do to prevent > > the crash from occuring? Why did we pass NULL as curr in the first place to > > pick_next_entity? > A little more context on this crash in v8 is here: > https://lwn.net/ml/linux-kernel/8230ada7-839f-2335-9a55-b09f6a813e91@linux.microsoft.com/ > > The issue here arises from the fact that, we try to pick task for a > sibling while sibling is running a task. Running tasks are not in the > cfs_rq and pick_next_entity can return NULL if there is only one cfs > task in the cfs_rq. This would not happen normally because > put_prev_task is called before pick_task and put_prev_task adds the > task back to cfs_rq. But for coresched, pick_task is called on a > remote sibling's cfs_rq without calling put_prev_task and this can > lead to pick_next_entity returning NULL. > > The initial logic of passing NULL would work fine as long as we call > put_prev_task before calling pick_task_fair. But for coresched, we > call pick_task_fair on siblings while the task is running and would > not be able to call put_prev_task. So this refactor of the code fixes > the crash by explicitly passing curr. > > Hope this clarifies.. >
Yes, it does!
Thanks, Balbir Singh.
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