Message-ID: <334e48ebbf34d853777672449cb29d5f06c751b7.camel@gmx.de>
Date: Mon, 09 Jun 2025 07:01:47 +0200
From: Mike Galbraith <efault@....de>
To: Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com, 
 juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com,  rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com,  clm@...a.com
Cc: linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 2/5] sched: Optimize ttwu() / select_task_rq()

Greetings,

This patch gives RT builds terminal heartburn.  This particular boot
survived long/well enough to trigger it with LTP sched tests and still
be able to crash dump the hung box.

(is_migration_disabled() confirmation thingy below gripe)

[   44.379563] WARNING: CPU: 6 PID: 4468 at kernel/sched/core.c:3354 set_task_cpu+0x1c1/0x1d0
[   44.379569] Modules linked in: af_packet nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nft_chain_nat nf_tables ebtable_nat ebtable_broute ip6table_nat ip6table_mangle ip6table_raw ip6table_security iptable_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 iptable_mangle iptable_raw iptable_security bridge stp llc iscsi_ibft iscsi_boot_sysfs rfkill ip_set nfnetlink ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter binfmt_misc intel_rapl_msr intel_rapl_common x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel snd_hda_codec_realtek nls_iso8859_1 nls_cp437 snd_hda_codec_generic snd_hda_codec_hdmi snd_hda_scodec_component snd_hda_intel at24 iTCO_wdt snd_intel_dspcfg r8169 regmap_i2c snd_intel_sdw_acpi intel_pmc_bxt kvm mei_pxp mei_hdcp iTCO_vendor_support realtek snd_hda_codec i2c_i801 mdio_devres snd_hda_core ums_realtek libphy i2c_mux irqbypass pcspkr i2c_smbus snd_hwdep snd_pcm usblp mdio_bus mei_me lpc_ich mfd_core mei snd_timer
[   44.379621]  snd soundcore thermal fan joydev intel_smartconnect tiny_power_button nfsd auth_rpcgss nfs_acl lockd sch_fq_codel grace sunrpc fuse configfs dmi_sysfs ip_tables x_tables uas usb_storage hid_logitech_hidpp hid_logitech_dj hid_generic usbhid nouveau drm_ttm_helper ttm gpu_sched xhci_pci i2c_algo_bit xhci_hcd ahci drm_gpuvm ehci_pci ehci_hcd libahci drm_exec mxm_wmi libata polyval_clmulni usbcore ghash_clmulni_intel drm_display_helper sha512_ssse3 sha1_ssse3 cec rc_core video wmi button sd_mod scsi_dh_emc scsi_dh_rdac scsi_dh_alua sg scsi_mod scsi_common vfat fat ext4 crc16 mbcache jbd2 loop msr efivarfs aesni_intel
[   44.379663] CPU: 6 UID: 0 PID: 4468 Comm: sandbox_ipc_thr Kdump: loaded Not tainted 6.15.0.ge271ed52-master-rt #19 PREEMPT_{RT,(lazy)}  e4f2516a9b85ac19222adb94a538ef0c57343c1c
[   44.379666] Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
[   44.379668] RIP: 0010:set_task_cpu+0x1c1/0x1d0
[   44.379670] Code: 0f 0b e9 8f fe ff ff 80 8b 8c 05 00 00 04 e9 f5 fe ff ff 0f 0b e9 7c fe ff ff 0f 0b 66 83 bb 40 04 00 00 00 0f 84 8b fe ff ff <0f> 0b e9 84 fe ff ff 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90
[   44.379672] RSP: 0018:ffffcd844ef77668 EFLAGS: 00010002
[   44.379673] RAX: 0000000000000200 RBX: ffff896da1e8c700 RCX: 0000000000000000
[   44.379675] RDX: ffff896da1e8cb30 RSI: 0000000000000000 RDI: ffff896da1e8c700
[   44.379676] RBP: 0000000000000000 R08: 0000000000000206 R09: 000000000002361d
[   44.379677] R10: fbfffffffffff79d R11: 0000000000000004 R12: 0000000000000000
[   44.379678] R13: 0000000000000000 R14: 0000000000000028 R15: ffff896da1e8d030
[   44.379679] FS:  0000000000000000(0000) GS:ffff8970f15d8000(0000) knlGS:0000000000000000
[   44.379681] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   44.379682] CR2: 00007f4c297c4688 CR3: 000000011b9de002 CR4: 00000000001726f0
[   44.379683] Call Trace:
[   44.379686]  <TASK>
[   44.379688]  try_to_wake_up+0x245/0x810
[   44.379692]  rt_mutex_slowunlock+0x1d2/0x2d0
[   44.379696]  ? __pfx_lru_activate+0x10/0x10
[   44.379700]  folio_batch_move_lru+0xc7/0x100
[   44.379704]  ? __pfx_lru_activate+0x10/0x10
[   44.379706]  __folio_batch_add_and_move+0xf2/0x110
[   44.379710]  folio_mark_accessed+0x80/0x1b0
[   44.379711]  unmap_page_range+0x176b/0x1a60
[   44.379717]  unmap_vmas+0xae/0x1a0
[   44.379720]  exit_mmap+0xe5/0x3c0
[   44.379725]  mmput+0x6e/0x150
[   44.379729]  do_exit+0x23c/0xa20
[   44.379732]  do_group_exit+0x33/0x90
[   44.379735]  get_signal+0x85d/0x8b0
[   44.379738]  arch_do_signal_or_restart+0x2d/0x240
[   44.379743]  ? place_entity+0x1b/0x130
[   44.379745]  ? __x64_sys_poll+0x47/0x1a0
[   44.379749]  exit_to_user_mode_loop+0x86/0x150
[   44.379753]  do_syscall_64+0x1ba/0x8e0
[   44.379756]  ? wakeup_preempt+0x40/0x70
[   44.379758]  ? ttwu_do_activate+0x84/0x210
[   44.379760]  ? _raw_spin_unlock_irqrestore+0x22/0x40
[   44.379763]  ? try_to_wake_up+0xab/0x810
[   44.379765]  ? preempt_count_add+0x4b/0xa0
[   44.379768]  ? futex_hash_put+0x43/0x90
[   44.379772]  ? futex_wake+0xb2/0x1c0
[   44.379775]  ? do_futex+0x125/0x190
[   44.379776]  ? __x64_sys_futex+0x10b/0x1c0
[   44.379779]  ? do_syscall_64+0x7f/0x8e0
[   44.379781]  ? __do_sys_prctl+0xbe/0xee0
[   44.379783]  ? do_syscall_64+0x7f/0x8e0
[   44.379786]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[   44.379789] RIP: 0033:0x7f4c3571fdef
[   44.379803] Code: Unable to access opcode bytes at 0x7f4c3571fdc5.
[   44.379803] RSP: 002b:00007f4c0c9fe700 EFLAGS: 00000293 ORIG_RAX: 0000000000000007
[   44.379805] RAX: fffffffffffffdfc RBX: 00007f4c0c9fe730 RCX: 00007f4c3571fdef
[   44.379806] RDX: 00000000ffffffff RSI: 0000000000000002 RDI: 00007f4c0c9fe730
[   44.379807] RBP: 00007f4c0c9fe920 R08: 0000000000000000 R09: 0000000000000007
[   44.379808] R10: 00005587485bf1d0 R11: 0000000000000293 R12: 00005587485a7fc0
[   44.379809] R13: 0000000000000000 R14: 0000000000001174 R15: 00007f4c0c1ff000
[   44.379812]  </TASK>

---
 kernel/sched/core.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4313,7 +4313,10 @@ int try_to_wake_up(struct task_struct *p
 		    ttwu_queue_wakelist(p, task_cpu(p), wake_flags))
 			break;
 
-		cpu = select_task_rq(p, p->wake_cpu, &wake_flags);
+		if (is_migration_disabled(p))
+			cpu = -1;
+		else
+			cpu = select_task_rq(p, p->wake_cpu, &wake_flags);
 
 		/*
 		 * If the owning (remote) CPU is still in the middle of schedule() with
@@ -4326,6 +4329,9 @@ int try_to_wake_up(struct task_struct *p
 		 */
 		smp_cond_load_acquire(&p->on_cpu, !VAL);
 
+		if (cpu == -1)
+			cpu = select_task_rq(p, p->wake_cpu, &wake_flags);
+
 		if (task_cpu(p) != cpu) {
 			if (p->in_iowait) {
 				delayacct_blkio_end(p);

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