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Date: Tue, 5 Apr 2022 15:25:36 -0400 From: Joel Fernandes <joel@...lfernandes.org> To: Peter Zijlstra <peterz@...radead.org> Cc: Ingo Molnar <mingo@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Vincent Guittot <vincent.guittot@...aro.org>, LKML <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Dietmar Eggemann <dietmar.eggemann@....com>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, Guenter Roeck <groeck@...omium.org> Subject: Re: [PATCH] sched/core: Fix forceidle balancing On Fri, Apr 1, 2022 at 7:46 AM Peter Zijlstra <peterz@...radead.org> wrote: > > On Thu, Mar 31, 2022 at 03:00:40PM -0400, Joel Fernandes wrote: > > Hi, > > > > By the way, might be slightly related - we still see crashes with > > pick_task_fair() in our kernel even with this change: > > https://lkml.org/lkml/2020/11/17/2137 > > Please as to not use lkml.org. Please use something with a MsgID in like > lore. Yep, will do. > > Is it possible that when doing pick_task_fair() especially on a remote > > CPU, both the "cfs_rq->curr" and the rbtree's "left" be NULL with core > > scheduling? In this case, se will be NULL and can cause crashes right? > > I think the code assumes this can never happen. > > > > +Guenter Roeck kindly debugged pick_task_fair() in a crash as > > follows. Copying some details he mentioned in a bug report: > > > > Assembler/source: > > > > 25: e8 4f 11 00 00 call 0x1179 ; se = > > pick_next_entity(cfs_rq, curr); > > 2a:* 48 8b 98 60 01 00 00 mov 0x160(%rax),%rbx ; trapping > > instruction [cfs_rq = group_cfs_rq(se);] > > 31: 48 85 db test %rbx,%rbx > > 34: 75 d1 jne 0x7 > > 36: 48 89 c7 mov %rax,%rdi > > > > At 2a: RAX = se == NULL after pick_next_entity(). Looking closely into > > pick_next_entity(), it can indeed return NULL if curr is NULL and if > > left in pick_next_entity() is NULL. Per line 7:, curr is in %r14 and > > indeed 0. > > > > Thoughts? > > It is possible for ->curr and ->leftmost to be NULL, but then we should > also be having ->nr_running == 0 and not call pick in the first place. > Because picking a task from no tasks doesn't make much sense. Indeed the code checks for nr_running so it is really bizarre. My guess is this is kernel memory corruption due to an unrelated bug or something, it is also not easy to trigger. Thanks, - Joel
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