Messages in this thread | | | Subject | Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to empty rbtree | From | Vineeth Pillai <> | Date | Fri, 20 Nov 2020 13:11:06 -0500 |
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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..
Thanks, Vineeth
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