linux-kernel - Re: [PATCH] sched/deadline: fix the bug in dl_task

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Message-ID: <Zs3NHRmJ-g6zKl5x@jlelli-thinkpadt14gen4.remote.csb>
Date: Tue, 27 Aug 2024 14:57:01 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Huang Shijie <shijie@...amperecomputing.com>
Cc: mingo@...hat.com, patches@...erecomputing.com, peterz@...radead.org,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, vineeth@...byteword.org, bristot@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/deadline: fix the bug in dl_task_of

Hi,

On 26/08/24 10:11, Huang Shijie wrote:
> When we enable the schedstat, we will meet an OS hang like this:
>   --------------------------------------------------------
> 	[  134.104253] kernel BUG at kernel/sched/deadline.c:63!
> 	[  134.132013] ------------[ cut here ]------------
> 	[  134.133441]  x27: 0000000000000001
> 	[  134.138048] kernel BUG at kernel/sched/deadline.c:63!
> 	[  134.146478] x26: 0000000000000001 x25: 0000000000000000 x24: 0000000000000001
> 	[  134.153607] x23: 0000000000000001 x22: 0000000000000000 x21: 0000000000000001
> 	[  134.160734] x20: ffff007dbf1b6d00 x19: ffff007dbf1b7610 x18: 0000000000000014
> 	[  134.162027] ------------[ cut here ]------------
> 	[  134.167861] x17: 000000009deab6cd x16: 00000000527c9a1c x15: 00000000000000dc
> 	[  134.172473] kernel BUG at kernel/sched/deadline.c:63!
> 	[  134.179595] x14: 0000000001200011 x13: 0000000040001000 x12: 0000ffffb6df05bc
> 	[  134.191760] x11: ffff007dbf1b6d00 x10: ffff0001062dd2e8 x9 : ffff8000801215ac
> 	[  134.192036] ------------[ cut here ]------------
> 	[  134.198888] x8 : 0000000000000000 x7 : 0000000000000021 x6 : ffff0001764ed280
> 	[  134.203498] kernel BUG at kernel/sched/deadline.c:63!
> 	[  134.210622] x5 : 0000000000000000 x4 : 0000000000000001 x3 : ffff807d3dd24000
> 	[  134.222787] x2 : 000000028b77a140 x1 : 0000003400000000 x0 : ffff007dbf1b6c80
> 	[  134.229915] Call trace:
> 	[  134.232353]  dl_task_of.part.0+0x0/0x10
> 	[  134.236182]  dl_server_start+0x54/0x158
> 	[  134.240013]  enqueue_task_fair+0x138/0x420
> 	[  134.244100]  enqueue_task+0x44/0xb0
> 	[  134.247584]  wake_up_new_task+0x1c0/0x3a0
> 	[  134.251584]  kernel_clone+0xe8/0x3e8
> 	[  134.252022] ------------[ cut here ]------------
> 	[  134.255156]  __do_sys_clone+0x70/0xa8
> 	[  134.259764] kernel BUG at kernel/sched/deadline.c:63!
> 	[  134.263412]  __arm64_sys_clone+0x28/0x40
> 	[  134.272360]  invoke_syscall+0x50/0x120
> 	[  134.276101]  el0_svc_common+0x44/0xf8
> 	[  134.279753]  do_el0_svc+0x28/0x40
> 	[  134.283058]  el0_svc+0x40/0x150
> 	[  134.286195]  el0t_64_sync_handler+0x100/0x130
> 	[  134.290546]  el0t_64_sync+0x1a4/0x1a8
> 	[  134.294200] Code: 35ffffa2 17ffffe3 d4210000 17ffffb4 (d4210000)
> 	[  134.300283] ---[ end trace 0000000000000000 ]---
> 	[  134.304890] Kernel panic - not syncing: Oops - BUG: Fatal exception
> 	[  134.311147] SMP: stopping secondary CPUs
> 	[  135.365096] SMP: failed to stop secondary CPUs 8-9,16,30,43,86,88,121,149
> 	[  135.371884] Kernel Offset: disabled
> 	[  135.375361] CPU features: 0x00,00100003,80153d29,d75ffea7
> 	[  135.380749] Memory Limit: none
> 	[  135.383793] ---[ end Kernel panic - not syncing: Oops - BUG: Fatal exception ]
>   --------------------------------------------------------
> 
> In dl_server_start(), we set the dl_se->dl_server with 1. When schedstats
> is enabled, in the following:
>    dl_server_start() --> enqueue_dl_entity() --> update_stats_enqueue_dl()
> 	__schedstats_from_dl_se() -->dl_task_of()
> 
> we will meet the BUG_ON.
> 
> So just remove the BUG_ON in dl_task_of to fix the hang.

AFAIU the fair task enqueue (the "real" one) eventually leading to this
BUG has already had its own schedstats tracking dealt with and I don't
think we need/want to track anything for the associated dl_server.

Shouldn't we rather return early if update_stats_{wait_start,wait_end,
enqueue_sleeper}_dl are called for a dl_server dl_se?

Thanks,
Juri