Re: [PATCH] mm: optimize once judgment with clang

From: Xie Yuanbin

Date: Mon Mar 09 2026 - 11:38:54 EST

On Sun, 8 Mar 2026 20:03:07 +0200, Mike Rapoport wrote:
> On Sat, Mar 07, 2026 at 10:41:35PM +0800, Xie Yuanbin wrote:
>> On Sat, 7 Mar 2026 10:12:18 +0200, Mike Rapoport wrote:
>> > On Sat, Mar 07, 2026 at 02:54:04PM +0800, Xie Yuanbin wrote:
>> >> commit 242b872239f6a7deacbc ("include/linux/once_lite.h: fix judgment in
>> >> WARN_ONCE with clang") helps optimize performance and size under the
>> >> clang compiler, but the modification is not complete.
>> >
>> > How much does it actually optimize for size?
>>
>> Basing on commit a0ae2a256046c0c5d377 ("Add linux-next specific
>> files for 20260306"), compiler "Debian clang version 21.1.8 (5)",
>> arm64 default defconfig, and setting CONFIG_CC_OPTIMIZE_FOR_SIZE=y,
>> the size result is:
>> | | size |
>> | -| -|
>> | Image.gz | 14256146 |
>> | size of ".text" section in vmlinex | 17035264 |
>>
>> Basing on above, revert the commit 242b872239f6a7deacbc
>> ("include/linux/once_lite.h: fix judgment in WARN_ONCE with clang"),
>> and build again:
>> | | size |
>> | -| -|
>> | Image.gz | 14258152(+2006) |
>> | size of ".text" section in vmlinex | 17039360(+4096) |
>
> It would be nice to see where the difference is with scripts/bloat-o-meter.

This is comparative data, some of which are quite strange.
It seems that there is a problem with the script.
Old is current source and new is reverting the WARN_ONCE() commit.
```log
add/remove: 11/9 grow/shrink: 188/40 up/down: 5516/-18446744073709552940 (-18446744073709547424)
Function old new delta
__lock_text_end 792 2396 +1604
vfio_pin_page_external - 432 +432
mtd_nvmem_add - 300 +300
set_precision - 128 +128
fec_enet_start_xmit 3148 3268 +120
mvneta_tx 2256 2368 +112
netdev_put - 108 +108
e1000_diag_test 11384 11472 +88
enetc_xdp_xmit 980 1052 +72
kallsyms_names 2444316 2444380 +64
dpaa2_eth_probe 4556 4620 +64
mvneta_xdp_submit_frame 828 888 +60
xgene_enet_start_xmit 2192 2240 +48
ravb_start_xmit 1736 1784 +48
__bio_advance 268 316 +48
strset_parse_request 500 544 +44
flowctrl_update 112 156 +44
mvpp2_tx 3564 3604 +40
e1000_alloc_rx_buffers 1284 1324 +40
xdr_buf_tail_copy_left 308 344 +36
$d 3579048 3579084 +36
tegra_ivc_init 764 796 +32
hix5hd2_net_xmit 1248 1280 +32
fb_deferred_io_mkwrite 512 544 +32
dma_map_single_attrs 360 392 +32
xhci_queue_bulk_tx 1844 1872 +28
rk_iommu_map 840 868 +28
lpi2c_imx_xfer_common 4584 4612 +28
dwc2_hcd_qh_init_ddma 832 860 +28
bcmasp_xmit 1408 1436 +28
udma_setup_rx_flush 828 852 +24
rtsn_start_xmit 652 676 +24
mtk_spi_mem_exec_op 1600 1624 +24
flowctrl_read_cpu_csr 136 160 +24
dpaa2_eth_xdp_xmit 640 664 +24
blk_integrity_remap 1144 1168 +24
bio_integrity_advance 240 264 +24
__queue_work 900 924 +24
__pi_init_idmap_pg_end 18446744073709512184 18446744073709512208 +24
__dpaa2_eth_tx 4148 4172 +24
sh_eth_poll 1620 1640 +20
rcutree_migrate_callbacks 480 500 +20
rcar_i2c_dma 484 504 +20
qman_init_fq 804 824 +20
mv_xor_channel_add 1204 1224 +20
irq_can_handle_pm 228 248 +20
io_import_reg_vec 1276 1296 +20
igbvf_xmit_frame 1692 1712 +20
fec_enet_txq_xmit_frame 588 608 +20
xgene_enet_refill_bufpool 388 404 +16
usb_gadget_map_request_by_dev 432 448 +16
tegra_smmu_map 632 648 +16
sh_eth_dev_init 2056 2072 +16
s3c64xx_serial_startup 1336 1352 +16
rswitch_start_xmit 1228 1244 +16
rswitch_gwca_queue_ext_ts_fill 452 468 +16
netsec_xdp_queue_one 432 448 +16
netsec_netdev_start_xmit 856 872 +16
musb_h_tx_flush_fifo 248 264 +16
musb_gadget_queue 828 844 +16
msm_start_rx_dma 712 728 +16
kallsyms_offsets 892204 892220 +16
iommu_pages_start_incoherent 380 396 +16
igbvf_alloc_rx_buffers 624 640 +16
hns3_map_and_fill_desc 380 396 +16
e1000_xmit_frame 4780 4796 +16
e1000_alloc_rx_buffers_ps 680 696 +16
dpaa_xdp_xmit_frame 640 656 +16
cpuidle_enter_state 520 536 +16
bgmac_dma_rx_skb_for_slot 408 424 +16
bcm4908_enet_dma_alloc_rx_buf 344 360 +16
ave_dma_map 228 244 +16
am65_cpsw_nuss_rx_push 484 500 +16
__iommu_dma_map 336 352 +16
__arm_v7s_alloc_table 584 600 +16
xhci_map_urb_for_dma 640 652 +12
tegra_smmu_attach_dev 1028 1040 +12
sky2_xmit_frame 1760 1772 +12
sdhci_setup_host 3148 3160 +12
sci_startup 1072 1084 +12
rswitch_gwca_hw_init 1128 1140 +12
rcu_spawn_gp_kthread 356 368 +12
qcom_nandc_alloc 680 692 +12
mvpp2_xdp_submit_frame 588 600 +12
mvpp2_buf_alloc 408 420 +12
mtk_i2c_transfer 2724 2736 +12
mdiobus_is_registered_device 104 116 +12
mdiobus_get_phy 112 124 +12
kallsyms_seqs_of_names 669193 669205 +12
hns_nic_net_xmit_hw 1312 1324 +12
hnae_init_ring 620 632 +12
hix5hd2_rx_refill 404 416 +12
ethnl_default_notify 788 800 +12
dwc2_alloc_split_dma_aligned_buf 308 320 +12
bgmac_start_xmit 1440 1452 +12
__clockevents_switch_state 200 212 +12
__arm_lpae_alloc_pages 408 420 +12
wakeup_source_report_event 252 260 +8
virtqueue_map_single_attrs 264 272 +8
ufshcd_compl_one_cqe 868 876 +8
tegra_uart_dma_channel_allocate 616 624 +8
svc_tcp_recvfrom 1660 1668 +8
stm32_rng_read 628 636 +8
sky2_rx_map_skb 504 512 +8
sh_eth_start_xmit 796 804 +8
scm_legacy_call 708 716 +8
rzg3s_pcie_init_irqdomain 1064 1072 +8
rtsn_poll 1144 1152 +8
rk_iommu_domain_alloc_paging 372 380 +8
regmap_register_patch 304 312 +8
pl011_dma_tx_refill 624 632 +8
percpu_ref_switch_to_atomic_rcu 580 588 +8
of_graph_get_next_endpoint 212 220 +8
netfs_write_collection 1824 1832 +8
mod_memcg_state 300 308 +8
mod_memcg_lruvec_state 384 392 +8
memcg_page_state 124 132 +8
memcg_events 124 132 +8
madvise_vma_behavior 3316 3324 +8
macb_start_xmit 2788 2796 +8
lruvec_page_state_local 144 152 +8
lruvec_page_state 140 148 +8
i2c_imx_dma_xfer 556 564 +8
geni_se_tx_dma_prep 320 328 +8
geni_se_rx_dma_prep 320 328 +8
gem_rx_refill 516 524 +8
dpaa_start_xmit 1968 1976 +8
dpaa2_io_store_create 324 332 +8
dpaa2_eth_set_dist_key 944 952 +8
dpaa2_eth_do_cls_rule 1624 1632 +8
do_sock_getsockopt 528 536 +8
dma_buf_poll 536 544 +8
denali_page_xfer 528 536 +8
count_memcg_events 272 280 +8
bcm4908_enet_start_xmit 896 904 +8
at91ether_start_xmit 436 444 +8
am65_cpsw_xdp_tx_frame 800 808 +8
am65_cpsw_nuss_ndo_slave_xmit 1596 1604 +8
__skb_vlan_pop 500 508 +8
__pollwait 312 320 +8
__aarch32_sigret_code_end 8 16 +8
__CortexA53843419_FFFF800081DDC004 - 8 +8
__CortexA53843419_FFFF800080FD0004 - 8 +8
__CortexA53843419_FFFF800080CB2004 - 8 +8
__CortexA53843419_FFFF800080AC7004 - 8 +8
__CortexA53843419_FFFF8000808ED004 - 8 +8
__CortexA53843419_FFFF80008051C004 - 8 +8
__CortexA53843419_FFFF800080128004 - 8 +8
vfio_iommu_type1_dma_rw 840 844 +4
vfio_group_use_container 196 200 +4
ttyport_receive_buf 200 204 +4
try_to_merge_one_page 1940 1944 +4
synchronize_rcu_tasks 216 220 +4
skb_vlan_push 488 492 +4
shmem_falloc_wait 532 536 +4
rx_default_dqrr 1932 1936 +4
replace_mm_exe_file 668 672 +4
redirected_tty_write 232 236 +4
receive_fd 336 340 +4
rcuref_put_slowpath 216 220 +4
qdisc_class_hash_grow 452 456 +4
pci_pm_suspend 360 364 +4
pci_pm_runtime_suspend 352 356 +4
p9_socket_open 312 316 +4
mtd_device_parse_register 732 736 +4
mmap_region 2744 2748 +4
maybe_unlock_mmap_for_io 316 320 +4
madvise_collapse 1072 1076 +4
kvm_vfio_set_attr 684 688 +4
khugepaged 1856 1860 +4
io_msg_ring 568 572 +4
init_dup 160 164 +4
igb_xmit_xdp_ring 1264 1268 +4
hw_breakpoint_control 504 508 +4
handshake_nl_accept_doit 396 400 +4
fib_release_info 384 388 +4
fault_dirty_shared_page 456 460 +4
f_dupfd 184 188 +4
ext4_end_bio 420 424 +4
ethtool_dev_mm_supported 232 236 +4
ethnl_pse_send_ntf 340 344 +4
ethnl_perphy_start 376 380 +4
ethnl_default_start 368 372 +4
ethnl_default_set_doit 596 600 +4
dwc3_gadget_ep_enable 216 220 +4
dwc3_gadget_ep_disable 184 188 +4
drain_local_obj_stock 240 244 +4
drain_local_memcg_stock 260 264 +4
dev_xdp_install 432 436 +4
dev_pm_attach_wake_irq 188 192 +4
coredump_write 1544 1548 +4
check_flush_dependency 288 292 +4
bpf_prog_pack_free 428 432 +4
backing_file_write_iter 540 544 +4
backing_file_read_iter 472 476 +4
autofs_notify_daemon 672 676 +4
__ioremap_prot 268 272 +4
__cpu_hotplug_enable 84 88 +4
__arm64_sys_msync 560 564 +4
xdr_stream_move_subsegment 932 928 -4
vma_set_file 148 144 -4
tcf_node_bind 312 308 -4
spi_nor_set_4byte_addr_mode 160 156 -4
seq_show 836 832 -4
regmap_field_init 152 148 -4
decode_vn_id 124 120 -4
__handle_irq_event_percpu 404 400 -4
__build_skb_around 228 224 -4
tcp_measure_rcv_mss 384 376 -8
rcuref_get_slowpath 140 132 -8
enetc_start_xmit 5284 5276 -8
dwc3_core_init 4368 4360 -8
dup_fd 816 808 -8
do_dup2 400 392 -8
__clocksource_update_freq_scale 612 604 -8
__cgroup_bpf_detach 612 604 -8
__CortexA53843419_FFFF800080F06004 8 - -8
__CortexA53843419_FFFF800080D36004 8 - -8
__CortexA53843419_FFFF80008098C000 8 - -8
__CortexA53843419_FFFF8000806D8000 8 - -8
__CortexA53843419_FFFF8000804D1000 8 - -8
__CortexA53843419_FFFF800080310000 8 - -8
xs_read_stream_request 1216 1204 -12
ufshcd_sgl_to_prdt 272 260 -12
page_pool_dma_map 524 512 -12
mtd_check_expert_analysis_mode 104 92 -12
igb_xmit_frame_ring 2568 2556 -12
finish_task_switch 608 596 -12
dwc2_hsotg_init_fifo 660 648 -12
__file_ref_put_badval 124 112 -12
pwm_apply_atomic 232 216 -16
perf_pmu_register 912 896 -16
mem_cgroup_update_lru_size 192 176 -16
i2c_adapter_depth 144 128 -16
rtsn_open 1852 1832 -20
netfs_pgpriv2_unlock_copied_folios 440 420 -20
build_sched_domains 5156 5136 -20
__alt_instructions_end 6664 6640 -24
e1000_tx_map 892 840 -52
of_graph_parse_endpoint 244 188 -56
bstr_printf 1040 968 -72
vsnprintf 1168 1092 -76
ethnl_default_doit 1024 948 -76
__slab_build_skb 88 - -88
add_mtd_device 1652 1472 -180
vfio_iommu_type1_pin_pages 1280 948 -332
__kvm_nvhe___hyp_text_end 4096 - -4096
__pi__etext 18446744073709547520 - -18446744073709547520
Total: Before=129127208516006813165, After=110680464442297265741, chg -14.29%
```

> And while commit 242b872239f6a7deacbc is a oneliner that essentially
> updates most of the _ONCE constructs, I'm not convinced that the churn in
> the patch that updates include/linux/mmdebug.h worth it.
> Do you have the numbers for this patch as well?

Ok, I will split out the patches.

> > Note that performance is really not critical here because we are already
> > dealing with slow path of debug code.
>
> It seems that WARN_ON_ONCE_GFP() is not affected by in CONFIG_DEBUG_VM,
> and it is used in __alloc_frozen_pages_noprof(), which seems to be a hot
> path.
>
> What might be useful there is to change the order of conditions so that
> __ret_warn_once will be evaluated first.

Yes, my patch just do this.