Message-ID: <9b5eb88c-83b3-23f4-fc31-1bd1b9e3dc87@efficios.com>
Date:   Wed, 12 Apr 2023 16:57:50 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Aaron Lu <aaron.lu@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Olivier Dion <odion@...icios.com>,
        michael.christie@...cle.com
Subject: Re: [RFC PATCH v4] sched: Fix performance regression introduced by
 mm_cid

On 2023-04-12 00:27, Aaron Lu wrote:
> Just noticed below warning in dmesg on v4, the warning is triggered by
> WARN_ON_ONCE((int) mm_cid < 0); in rseq_update_cpu_node_id().

I think I know what triggers this in v4.

See sched_mm_cid_migrate_from() (near the end):

+       /*
+        * The src_cid is unused, so it can be unset.
+        */
+       if (cmpxchg(src_pcpu_cid, mm_cid_set_lazy_put(src_cid), MM_CID_UNSET) !=
+           mm_cid_set_lazy_put(src_cid))
+               return;
+       __mm_cid_put(mm, src_cid);

There is a short window here between successful cmpxchg and clear mask bit
where the thread has ownership of the cid without rq lock held. This can
lead to rare over-allocation of cids beyond nr_possible_cpus if the src cpu
runqueue reallocates a cid concurrently, which causes __mm_cid_get() to
return -1.

Because we want to avoid taking the src rq lock in migrate-from, I plan to
fix this by doing the following:

- disable interrupts around this cmpxchg and __mm_cid_put(),
- modify __mm_cid_get so it retries if cpumask_first_zero() returns -1.

This should take care of this kind of extremely rare over-allocation
scenario through retry, which will be bounded by the duration of the
instruction sequence between cmpxchg and clearing the bit in the bitmask.

I have not reproduced the warning on my end, only figured this out from
code review.

Thoughts ?

Thanks,

Mathieu

> 
> [ 1819.649803] ------------[ cut here ]------------
> [ 1819.649813] WARNING: CPU: 188 PID: 29881 at kernel/rseq.c:95 __rseq_handle_notify_resume+0x49b/0x590
> [ 1819.649823] Modules linked in: veth tls xt_conntrack nft_chain_nat xt_MASQUERADE nf_nat nf_conntrack_netlink nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 xfrm_user xfrm_algo xt_addrtype nft_compat nf_tables nfnetlink br_netfilter bridge stp llc overlay intel_rapl_msr intel_rapl_common intel_uncore_frequency intel_uncore_frequency_common i10nm_edac nfit x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel binfmt_misc kvm nls_iso8859_1 rapl intel_cstate mei_me isst_if_mbox_pci isst_if_mmio idxd input_leds joydev isst_if_common idxd_bus mei ipmi_ssif acpi_ipmi ipmi_si ipmi_devintf ipmi_msghandler acpi_power_meter acpi_pad mac_hid sch_fq_codel dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua msr ramoops pstore_blk reed_solomon pstore_zone efi_pstore ip_tables x_tables autofs4 btrfs blake2b_generic raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear hid_generic usbhid hid ast i2c_algo_bit drm_shmem_helper drm_kms_helper syscopyarea sysfillrect
> [ 1819.649903]  sysimgblt crct10dif_pclmul crc32_pclmul ghash_clmulni_intel sha512_ssse3 aesni_intel dax_hmem cxl_acpi crypto_simd nvme cryptd cxl_core nvme_core i2c_i801 xhci_pci i40e drm i2c_smbus xhci_pci_renesas i2c_ismt wmi pinctrl_emmitsburg
> [ 1819.649924] CPU: 188 PID: 29881 Comm: hackbench Not tainted 6.3.0-rc6-00002-g1acfd6ae9afc #2
> [ 1819.649927] Hardware name: Intel Corporation D50DNP1SBB/D50DNP1SBB, BIOS SE5C7411.86B.8901.D03.2210131232 10/13/2022
> [ 1819.649929] RIP: 0010:__rseq_handle_notify_resume+0x49b/0x590
> [ 1819.649934] Code: f0 ff ff ff ff ff 00 e9 00 fe ff ff 48 ba 00 f0 ff ff ff ff ff 00 e9 3a fe ff ff 48 ba 00 f0 ff ffff ff ff 00 e9 66 fe ff ff <0f> 0b e9 7d fc ff ff 0f 01 ca e9 a6 fc ff ff 48 8b 4c 24 30 48 8b
> [ 1819.649936] RSP: 0018:ffa0000018b0fe60 EFLAGS: 00010286
> [ 1819.649939] RAX: ff11007f73500000 RBX: 00007f81a7226fe0 RCX: 0000000000000000
> [ 1819.649941] RDX: 00000000ffffffff RSI: 0000000000000001 RDI: ffffffff828f9477
> [ 1819.649943] RBP: ffa0000018b0fee8 R08: ff110040c192cc28 R09: ff1100408e2a3980
> [ 1819.649944] R10: 0000000000000001 R11: 0000000000000000 R12: ff110040caa64000
> [ 1819.649946] R13: 000000000002fa40 R14: 0000000000000000 R15: 00000000000000bc
> [ 1819.649947] FS:  00007f81a7226640(0000) GS:ff11007f73500000(0000) knlGS:0000000000000000
> [ 1819.649950] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 1819.649951] CR2: 00007fd8b00d5000 CR3: 00000040cf060001 CR4: 0000000000f71ee0
> [ 1819.649953] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 1819.649955] DR3: 0000000000000000 DR6: 00000000fffe07f0 DR7: 0000000000000400
> [ 1819.649957] PKRU: 55555554
> [ 1819.649958] Call Trace:
> [ 1819.649960]  <TASK>
> [ 1819.649964]  exit_to_user_mode_prepare+0x13b/0x1a0
> [ 1819.649970]  syscall_exit_to_user_mode+0x2a/0x50
> [ 1819.649976]  ? __x64_sys_read+0x1d/0x30
> [ 1819.649980]  do_syscall_64+0x6d/0x90
> [ 1819.649984]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
> [ 1819.649990] RIP: 0033:0x7f81a77149cc
> [ 1819.649996] Code: ec 28 48 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 b9 c0 f7 ff 48 8b 54 24 18 48 8b 74 24 10 41 89 c0 8b 7c 24 08 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 34 44 89 c7 48 89 44 24 08 e8 ff c0 f7 ff 48
> [ 1819.649998] RSP: 002b:00007f81a7225d70 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> [ 1819.650004] RAX: 0000000000000064 RBX: 00007f81a7225da0 RCX: 00007f81a77149cc
> [ 1819.650005] RDX: 0000000000000064 RSI: 00007f81a7225da0 RDI: 0000000000000135
> [ 1819.650007] RBP: 00007f81a7225e50 R08: 0000000000000000 R09: 0000000000000000
> [ 1819.650008] R10: 0000000000000008 R11: 0000000000000246 R12: 00000000004a3c12
> [ 1819.650009] R13: 00007f81a7225e10 R14: 0000000000000000 R15: 0000558df7af6d00
> [ 1819.650012]  </TASK>
> [ 1819.650013] ---[ end trace 0000000000000000 ]---

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

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