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Message-ID: <CANn89iJVQe=wedLheJmjZjOTJsWHijT0jZs=iRxKssJZbjAxHw@mail.gmail.com> Date: Mon, 19 Jan 2026 11:25:52 +0100 From: Eric Dumazet <edumazet@...gle.com> To: David Laight <david.laight.linux@...il.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <eric.dumazet@...il.com>, Paolo Abeni <pabeni@...hat.com>, Nicolas Pitre <npitre@...libre.com> Subject: Re: [PATCH] compiler_types: Introduce inline_for_performance On Mon, Jan 19, 2026 at 10:33 AM David Laight <david.laight.linux@...il.com> wrote: > > On Sun, 18 Jan 2026 16:01:25 -0800 > Andrew Morton <akpm@...ux-foundation.org> wrote: > > > On Sun, 18 Jan 2026 22:58:02 +0000 David Laight <david.laight.linux@...il.com> wrote: > > > > > > mm/ alone has 74 __always_inlines, none are documented, I don't know > > > > why they're present, many are probably wrong. > > > > > > > > Shit, uninlining only __get_user_pages_locked does this: > > > > > > > > text data bss dec hex filename > > > > 115703 14018 64 129785 1faf9 mm/gup.o > > > > 103866 13058 64 116988 1c8fc mm/gup.o-after > > > > > > The next questions are does anything actually run faster (either way), > > > and should anything at all be marked 'inline' rather than 'always_inline'. > > > > > > After all, if you call a function twice (not in a loop) you may > > > want a real function in order to avoid I-cache misses. > > > > yup > > I had two adjacent strlen() calls in a bit of code, the first was an > array (in a structure) and gcc inlined the 'word at a time' code, the > second was a pointer and it called the library function. > That had to be sub-optimal... > > > > But I'm sure there is a lot of code that is 'inline_for_bloat' :-) > > > > ooh, can we please have that? > > Or 'inline_to_speed_up_benchmark' and the associated 'unroll this loop > because that must make it faster'. > > > I do think that every always_inline should be justified and commented, > > but I haven't been energetic about asking for that. > > Apart from the 4-line functions where it is clearly obvious. > Especially since the compiler can still decide to not-inline them > if they are only 'inline'. > > > A fun little project would be go through each one, figure out whether > > were good reasons and if not, just remove them and see if anyone > > explains why that was incorrect. > > It's not just always_inline, a lot of the inline are dubious. > Probably why the networking code doesn't like it. Many __always_inline came because of clang's reluctance to inline small things, even if the resulting code size is bigger and slower. It is a bit unclear, this seems to happen when callers are 'big enough'. noinstr (callers) functions are also a problem. Let's take the list_add() call from dev_gro_receive() : clang does not inline it, for some reason. After adding __always_inline to list_add() and __list_add() we have smaller and more efficient code, for real workloads, not only benchmarks. $ scripts/bloat-o-meter -t net/core/gro.o.old net/core/gro.o.new add/remove: 2/4 grow/shrink: 2/1 up/down: 86/-130 (-44) Function old new delta dev_gro_receive 1795 1845 +50 .Ltmp93 - 16 +16 .Ltmp89 - 16 +16 napi_gro_frags 968 972 +4 .Ltmp94 16 - -16 .Ltmp90 16 - -16 .Ltmp83 16 - -16 .Ltmp0 8396 8364 -32 list_add 50 - -50 Over the whole kernel (and also using __always_inline for list_add_tail(), __list_del(), list_del()) we have a similar outcome: $ size vmlinux.old vmlinux.new text data bss dec hex filename 39037635 23688605 4254712 66980952 3fe0c58 vmlinux.old 39035644 23688605 4254712 66978961 3fe0491 vmlinux.new $ scripts/bloat-o-meter -t vmlinux.old vmlinux.new add/remove: 2/6 grow/shrink: 103/52 up/down: 6179/-6473 (-294) Function old new delta __list_del_entry - 672 +672 __do_semtimedop 1819 2204 +385 __pfx___list_del_entry - 256 +256 tracer_alloc_buffers 760 920 +160 ext4_orphan_add 1122 1254 +132 madvise_cold_or_pageout_pte_range 2202 2328 +126 iommu_sva_bind_device 744 865 +121 copy_page_range 12353 12473 +120 psi_trigger_create 767 886 +119 power_supply_register_extension 540 658 +118 srcu_gp_start_if_needed 1440 1548 +108 fanout_add 996 1097 +101 kvm_dev_ioctl 1507 1599 +92 relay_open 681 761 +80 lru_lazyfree 784 863 +79 input_register_device 1602 1680 +78 seccomp_notify_ioctl 1882 1959 +77 __iommu_probe_device 1203 1279 +76 spi_register_controller 1748 1823 +75 copy_process 4112 4186 +74 newseg 825 898 +73 optimize_kprobe 210 282 +72 do_msgsnd 1290 1362 +72 __bmc_get_device_id 3332 3404 +72 iscsi_queuecommand 948 1019 +71 hci_register_dev 667 737 +70 tcf_register_action 518 586 +68 memcg_write_event_control 1083 1147 +64 handle_one_recv_msg 3307 3371 +64 btf_module_notify 1669 1733 +64 tcf_mirred_init 1229 1292 +63 register_stat_tracer 340 403 +63 qdisc_get_stab 596 658 +62 io_submit_one 2056 2117 +61 fuse_dev_do_read 1186 1247 +61 bpf_map_offload_map_alloc 622 683 +61 zswap_setup 584 644 +60 register_acpi_bus_type 109 169 +60 ipv6_add_addr 1007 1067 +60 mraid_mm_register_adp 1379 1438 +59 mlock_folio_batch 3137 3196 +59 register_ife_op 230 288 +58 perf_event_alloc 2591 2649 +58 intel_iommu_domain_alloc_nested 498 556 +58 lru_deactivate_file 1280 1336 +56 fib_create_info 2567 2623 +56 iscsi_register_transport 533 588 +55 ext4_register_li_request 448 503 +55 do_msgrcv 1536 1589 +53 net_devmem_bind_dmabuf 1036 1088 +52 init_one_iommu 309 360 +51 shmem_get_folio_gfp 1379 1429 +50 dev_gro_receive 1795 1845 +50 .Ltmp68 160 208 +48 .Ltmp337 48 96 +48 .Ltmp204 48 96 +48 dm_get_device 501 548 +47 vfs_move_mount 509 555 +46 intel_nested_attach_dev 391 435 +44 fw_devlink_dev_sync_state 212 255 +43 bond_ipsec_add_sa 406 447 +41 bd_link_disk_holder 459 499 +40 bcm_tx_setup 1414 1454 +40 devl_linecard_create 401 440 +39 copy_tree 677 716 +39 fscrypt_setup_encryption_info 1479 1517 +38 region_del 631 668 +37 devlink_nl_rate_new_doit 501 536 +35 .Ltmp57 48 80 +32 .Ltmp126 48 80 +32 netdev_run_todo 1341 1372 +31 ipmi_create_user 411 442 +31 pci_register_host_bridge 1675 1705 +30 p9_read_work 961 990 +29 acpi_device_add 859 888 +29 mntput_no_expire_slowpath 603 631 +28 vmemmap_remap_pte 499 524 +25 move_cluster 160 184 +24 handle_userfault 2035 2059 +24 rdtgroup_mkdir 1500 1522 +22 clk_notifier_register 474 489 +15 bpf_event_notify 311 326 +15 dm_register_path_selector 245 256 +11 bpf_crypto_register_type 209 220 +11 usb_driver_set_configuration 232 242 +10 parse_gate_list 912 921 +9 devl_rate_leaf_create 266 274 +8 bt_accept_enqueue 465 473 +8 __flush_workqueue 1233 1240 +7 dev_forward_change 772 778 +6 thermal_add_hwmon_sysfs 861 865 +4 set_node_memory_tier 1027 1031 +4 napi_gro_frags 968 972 +4 mei_cl_notify_request 1067 1071 +4 mb_cache_shrink 447 451 +4 kfence_guarded_free 764 768 +4 blk_mq_try_issue_directly 606 610 +4 __neigh_update 2452 2456 +4 __folio_freeze_and_split_unmapped 3198 3202 +4 rwsem_down_write_slowpath 1595 1598 +3 eventfs_create_dir 477 480 +3 alloc_vmap_area 1967 1970 +3 unix_add_edges 618 620 +2 hid_connect 1529 1530 +1 dwc_prep_dma_memcpy 637 638 +1 scsi_eh_test_devices 700 699 -1 acpi_extract_power_resources 559 558 -1 deactivate_slab 758 756 -2 __team_options_change_check 220 218 -2 sock_map_update_common 524 521 -3 rcu_nocb_gp_kthread 2710 2707 -3 memsw_cgroup_usage_register_event 19 16 -3 kthread 564 561 -3 st_add_path 457 453 -4 flow_block_cb_setup_simple 543 539 -4 ep_try_send_events 875 871 -4 elv_register 524 520 -4 __mptcp_move_skbs_from_subflow 1318 1314 -4 __check_limbo 460 456 -4 __bpf_list_add 277 273 -4 trim_marked 375 370 -5 megaraid_mbox_runpendq 345 340 -5 ipmi_timeout_work 1813 1808 -5 handle_new_recv_msgs 419 414 -5 mei_cl_send_disconnect 186 180 -6 mei_cl_send_connect 186 180 -6 mptcp_sendmsg 1776 1769 -7 deferred_split_folio 561 554 -7 pcibios_allocate_resources 879 871 -8 find_css_set 1690 1682 -8 af_alg_sendmsg 2384 2376 -8 isolate_migratepages_block 4113 4104 -9 posixtimer_send_sigqueue 950 939 -11 mei_irq_write_handler 1350 1338 -12 dpm_prepare 1173 1161 -12 dpll_xa_ref_pin_add 679 667 -12 css_set_move_task 513 501 -12 cache_mark 583 571 -12 scsi_queue_rq 3449 3434 -15 mtd_queue_rq 1068 1053 -15 link_css_set 323 308 -15 key_garbage_collector 1119 1104 -15 do_dma_probe 1664 1649 -15 configfs_new_dirent 306 290 -16 .Ltmp69 208 192 -16 xfrm_state_walk 677 660 -17 __dpll_pin_register 761 742 -19 i2c_do_add_adapter 1018 998 -20 complete_io 421 401 -20 fsnotify_insert_event 390 368 -22 scsi_eh_ready_devs 3026 2997 -29 .Ltmp71 128 96 -32 .Ltmp58 128 96 -32 .Ltmp127 80 48 -32 migrate_pages_batch 4623 4584 -39 .Ltmp338 96 48 -48 .Ltmp205 96 48 -48 __pfx_list_del 256 - -256 __pfx_list_add_tail 384 - -384 __pfx_list_add 656 - -656 list_del 944 - -944 list_add_tail 1360 - -1360 list_add 2212 - -2212 Total: Before=25509319, After=25509025, chg -0.00%
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