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Date: Fri, 7 Jun 2019 16:36:56 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Vineeth Remanan Pillai <vpillai@...italocean.com>
Cc: Nishanth Aravamudan <naravamudan@...italocean.com>,
Julien Desfossez <jdesfossez@...italocean.com>,
Peter Zijlstra <peterz@...radead.org>,
Tim Chen <tim.c.chen@...ux.intel.com>, mingo@...nel.org,
tglx@...utronix.de, pjt@...gle.com, torvalds@...ux-foundation.org,
linux-kernel@...r.kernel.org, subhra.mazumdar@...cle.com,
fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
Phil Auld <pauld@...hat.com>, Aaron Lu <aaron.lwe@...il.com>,
Aubrey Li <aubrey.intel@...il.com>,
Valentin Schneider <valentin.schneider@....com>,
Mel Gorman <mgorman@...hsingularity.net>,
Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v3 13/16] sched: Add core wide task selection and
scheduling.
On Wed, May 29, 2019 at 08:36:49PM +0000, Vineeth Remanan Pillai wrote:
> From: Peter Zijlstra <peterz@...radead.org>
>
> Instead of only selecting a local task, select a task for all SMT
> siblings for every reschedule on the core (irrespective which logical
> CPU does the reschedule).
>
> NOTE: there is still potential for siblings rivalry.
> NOTE: this is far too complicated; but thus far I've failed to
> simplify it further.
Looks like there are still some race conditions while bringing cpu
online/offline. I am seeing an easy to reproduce panic when turning SMT on/off
in a loop with core scheduling ON. I dont see the panic with core scheduling
OFF.
Steps to reproduce:
mkdir /sys/fs/cgroup/cpu/group1
mkdir /sys/fs/cgroup/cpu/group2
echo 1 > /sys/fs/cgroup/cpu/group1/cpu.tag
echo 1 > /sys/fs/cgroup/cpu/group2/cpu.tag
echo $$ > /sys/fs/cgroup/cpu/group1/tasks
while [ 1 ]; do
echo on > /sys/devices/system/cpu/smt/control
echo off > /sys/devices/system/cpu/smt/control
done
Panic logs:
[ 274.629437] BUG: unable to handle kernel NULL pointer dereference at
0000000000000024
[ 274.630366] #PF error: [normal kernel read fault]
[ 274.630933] PGD 800000003e52c067 P4D 800000003e52c067 PUD 0
[ 274.631613] Oops: 0000 [#1] SMP PTI
[ 274.632016] CPU: 0 PID: 1470 Comm: bash Tainted: G W
5.1.4+ #33
[ 274.632854] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS ?-20180724_192412-buildhw-07.phx2.fedoraproject.org-1.fc29
04/01/2014
[ 274.634248] RIP: 0010:__schedule+0x9d4/0x1350
[ 274.634699] Code: da 0f 83 21 04 00 00 48 8b 35 70 f3 ab 00 48 c7 c7
51 1c a8 81 e8 4c 4e 6b ff 49 8b 85 b8 0b 00 00 48 85 c0 0f 84 2f 09 00
01
[ 274.636648] RSP: 0018:ffffc900008f3ca8 EFLAGS: 00010046
[ 274.637197] RAX: 0000000000000000 RBX: 0000000000000001 RCX:
0000000000000040
[ 274.637941] RDX: 0000000000000000 RSI: 0000000000000000 RDI:
ffffffff82544890
[ 274.638691] RBP: ffffc900008f3d40 R08: 00000000000004c7 R09:
0000000000000030
[ 274.639449] R10: 0000000000000001 R11: ffffc900008f3b28 R12:
ffff88803d2d0e80
[ 274.640172] R13: ffff88803eaa0a40 R14: ffff88803ea20a40 R15:
ffff88803d2d0e80
[ 274.640915] FS: 0000000000000000(0000) GS:ffff88803ea00000(0063)
knlGS:00000000f7f8b780
[ 274.641755] CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
[ 274.642355] CR2: 0000000000000024 CR3: 000000003c01a005 CR4:
0000000000360ef0
[ 274.643135] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[ 274.643995] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7:
0000000000000400
[ 274.645023] Call Trace:
[ 274.645336] schedule+0x28/0x70
[ 274.645621] native_cpu_up+0x271/0x6d0
[ 274.645959] ? cpus_read_trylock+0x40/0x40
[ 274.646324] bringup_cpu+0x2d/0xe0
[ 274.646631] cpuhp_invoke_callback+0x94/0x550
[ 274.647032] ? ring_buffer_record_is_set_on+0x10/0x10
[ 274.647478] _cpu_up+0xa9/0x140
[ 274.647763] store_smt_control+0x1cb/0x260
[ 274.648132] kernfs_fop_write+0x108/0x190
[ 274.648498] vfs_write+0xa5/0x1a0
[ 274.648794] ksys_write+0x57/0xd0
[ 274.649100] do_fast_syscall_32+0x92/0x220
[ 274.649468] entry_SYSENTER_compat+0x7c/0x8e
NULL pointer exception is triggered when sibling is offline during core task
pick in pick_next_task() leaving rq_i->core_pick = NULL and if sibling comes
online before the "Reschedule siblings" block in the same function it causes
panic in is_idle_task(rq_i->core_pick).
Traces for the scenario:
[ 274.599567] bash-1470 0d... 273921815us : __schedule: cpu(0) is online during core_pick
[ 274.600339] bash-1470 0d... 273921816us : __schedule: cpu(1) is offline during core_pick
[ 274.601106] bash-1470 0d... 273921816us : __schedule: picked: bash/1470 ffff88803cb9c000
[ 274.602106] bash-1470 0d... 273921816us : __schedule: cpu(0) is online.. during Reschedule siblings
[ 274.603219] bash-1470 0d... 273921816us : __schedule: cpu(1) is online.. during Reschedule siblings
[ 274.604333] <idle>-0 1d... 273921816us : start_secondary: cpu(1) is online now
[ 274.605239] bash-1470 0d... 273922148us : __schedule: rq_i->core_pick on cpu(1) is NULL
I am not able to reproduce the panic after the below change. Not sure if this
is the right fix. Maybe we don't have to allow cpus to go online/offline while
pick_next_task() is executing.
-------------- 8< ---------------
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 90655c9ad937..b230b095772a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3874,7 +3874,7 @@ next_class:;
for_each_cpu(i, smt_mask) {
struct rq *rq_i = cpu_rq(i);
- if (cpu_is_offline(i))
+ if (cpu_is_offline(i) || !rq_i->core_pick)
continue;
WARN_ON_ONCE(!rq_i->core_pick);
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