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Date: Fri, 20 Nov 2020 21:15:28 +1100 From: "Singh, Balbir" <bsingharora@...il.com> To: "Joel Fernandes (Google)" <joel@...lfernandes.org>, Nishanth Aravamudan <naravamudan@...italocean.com>, Julien Desfossez <jdesfossez@...italocean.com>, Peter Zijlstra <peterz@...radead.org>, Tim Chen <tim.c.chen@...ux.intel.com>, Vineeth Pillai <viremana@...ux.microsoft.com>, Aaron Lu <aaron.lwe@...il.com>, Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de, linux-kernel@...r.kernel.org Cc: mingo@...nel.org, torvalds@...ux-foundation.org, fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com, Phil Auld <pauld@...hat.com>, Valentin Schneider <valentin.schneider@....com>, Mel Gorman <mgorman@...hsingularity.net>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org, Chen Yu <yu.c.chen@...el.com>, Christian Brauner <christian.brauner@...ntu.com>, Agata Gruza <agata.gruza@...el.com>, Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>, graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com, pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com, benbjiang@...cent.com, Alexandre Chartre <alexandre.chartre@...cle.com>, James.Bottomley@...senpartnership.com, OWeisse@...ch.edu, Dhaval Giani <dhaval.giani@...cle.com>, Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com, chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>, Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>, Tom Lendacky <thomas.lendacky@....com>, Aubrey Li <aubrey.li@...ux.intel.com>, "Paul E. McKenney" <paulmck@...nel.org>, Tim Chen <tim.c.chen@...el.com> Subject: Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to empty rbtree On 18/11/20 10:19 am, Joel Fernandes (Google) wrote: > From: Peter Zijlstra <peterz@...radead.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? The patch looks functionally correct as in, it passes curr as the reference to pick_next_entity() for left and entity_before comparisons. Balbir Singh
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