| Message ID | 20240620175703.605111-1-yury.norov@gmail.com |
|---|---|
| Headers | show |
| Series | lib/find: add atomic find_bit() primitives | expand |
On Thu, Jun 20, 2024 at 11:00:38AM -0700, Linus Torvalds wrote: > On Thu, 20 Jun 2024 at 10:57, Yury Norov <yury.norov@gmail.com> wrote: > > > > > > The typical lock-protected bit allocation may look like this: > > If it looks like this, then nobody cares. Clearly the user in question > never actually cared about performance, and you SHOULD NOT then say > "let's optimize this that nobody cares about":. > > Yury, I spend an inordinate amount of time just double-checking your > patches. I ended up having to basically undo one of them just days > ago. Is that in master already? I didn't get any email, and I can't find anything related in the master branch. > New rule: before you send some optimization, you need to have NUMBERS. I tried to underline that it's not a performance optimization at my best. People notice some performance differences, but it's ~3%, no more. > Some kind of "look, this code is visible in profiles, so we actually care". The original motivation comes from a KCSAN report, so it's already visible in profiles. See [1] in cover letter. This series doesn't fix that particular issue, but it adds tooling that allow people to search and acquire bits in bitmaps without firing KCSAN warnings. This series fixes one real bug in the codebase - see #33, and simplifies bitmaps usage in many other places. Many people like it, and acked the patches. Again, this is NOT a performance series. Thanks, Yury > Because without numbers, I'm just not going to pull anything from you. > These insane inlines for things that don't matter need to stop. > > And if they *DO* matter, you need to show that they matter. > > Linus
On Thu, 20 Jun 2024 at 13:20, Yury Norov <yury.norov@gmail.com> wrote: > > FORCE_NR_CPUS helped to generate a better code for me back then. I'll > check again against the current kernel. Of _course_ it generates better code. But when "better code" is a source of bugs, and isn't actually useful in general, it's not better, is it. > The 5d272dd1b343 is wrong. Limiting FORCE_NR_CPUS to UP case makes no > sense because in UP case nr_cpu_ids is already a compile-time macro: Yury, I'm very aware. That was obviously intentional. the whole point of the commit is to just disable the the whole thing as useless and problematic. I could have just ripped it out entirely. I ended up doing a one-liner instead. Linus
--- This v4 moves new API to separate headers, as adding stuff to find.h concerns people, particularly Linus. It also adds few more conversions alongside other cosmetic changes. See full changelog below. --- Add helpers around test_and_{set,clear}_bit() to allow searching for clear or set bits and flipping them atomically. Using atomic search primitives allows to implement lockless bitmap handling where only individual bits are touched by concurrent processes, and where people now have to protect their bitmaps to search for a free or set bit due to the lack of atomic searching routines. The typical lock-protected bit allocation may look like this: unsigned long alloc_bit() { unsigned long bit; spin_lock(bitmap_lock); bit = find_first_zero_bit(bitmap, nbits); if (bit < nbits) __set_bit(bit, bitmap); spin_unlock(bitmap_lock); return bit; } void free_bit(unsigned long bit) { spin_lock(bitmap_lock); __clear_bit(bit, bitmap); spin_unlock(bitmap_lock); } Now with atomic find_and_set_bit(), the above can be implemented lockless, directly by using it and atomic clear_bit(). Patches 36-40 do this in few places in the kernel where the transition is clear. There is likely more candidates for refactoring. The other important case is when people opencode atomic search or atomic traverse on the maps with the patterns looking like: for (idx = 0; idx < nbits; idx++) if (test_and_clear_bit(idx, bitmap)) do_something(idx); Or like this: do { bit = find_first_bit(bitmap, nbits); if (bit >= nbits) return nbits; } while (!test_and_clear_bit(bit, bitmap)); return bit; In both cases, the opencoded loop may be converted to a single function or iterator call. Correspondingly: for_each_test_and_clear_bit(idx, bitmap, nbits) do_something(idx); Or: return find_and_clear_bit(bitmap, nbits); Obviously, the less routine code people have to write themself, the less probability to make a mistake. The patch #33 fixes one such mistake. The new API is not only a handy helpers - it also resolves a non-trivial issue of using non-atomic find_bit() together with atomic test_and_{set,clear)_bit(). The trick is that find_bit() implies that the bitmap is a regular non-volatile piece of memory, and compiler is allowed to use such optimization techniques like re-fetching memory instead of caching it. For example, find_first_bit() is implemented like: for (idx = 0; idx * BITS_PER_LONG < sz; idx++) { val = addr[idx]; if (val) { sz = min(idx * BITS_PER_LONG + __ffs(val), sz); break; } } On register-memory architectures, like x86, compiler may decide to access memory twice - first time to compare against 0, and second time to fetch its value to pass it to __ffs(). When running find_first_bit() on volatile memory, the memory may get changed in-between, and for instance, it may lead to passing 0 to __ffs(), which is an undefined behaviour. This is a potentially dangerous call. find_and_clear_bit() as a wrapper around test_and_clear_bit() naturally treats underlying bitmap as a volatile memory and prevents compiler from such optimizations. Now that KCSAN is catching exactly this type of situations and warns on undercover memory modifications. We can use it to reveal improper usage of find_bit(), and convert it to atomic find_and_*_bit() as appropriate. In some cases concurrent operations with plain find_bit() are acceptable. For example: - two threads running find_*_bit(): safe wrt ffs(0) and returns correct value, because underlying bitmap is unchanged; - find_next_bit() in parallel with set or clear_bit(), when modifying a bit prior to the start bit to search: safe and correct; - find_first_bit() in parallel with set_bit(): safe, but may return wrong bit number; - find_first_zero_bit() in parallel with clear_bit(): same as above. In last 2 cases find_bit() may not return a correct bit number, but it may be OK if caller requires any (not exactly the first) set or clear bit, correspondingly. In such cases, KCSAN may be safely silenced with data_race(). But in most cases where KCSAN detects concurrency we should carefully review the code and likely protect critical sections or switch to atomic find_and_bit(), as appropriate. This patch adds the following atomic primitives: find_and_set_bit(addr, nbits); find_and_set_next_bit(addr, nbits, start); ... Here find_and_{set,clear} part refers to the corresponding test_and_{set,clear}_bit function. Suffixes like _wrap or _lock derive their semantics from corresponding find() or test() functions. For brevity, the naming omits the fact that we search for zero bit in find_and_set, and correspondingly search for set bit in find_and_clear functions. The patch also adds iterators with atomic semantics, like for_each_test_and_set_bit(). Here, the naming rule is to simply prefix corresponding atomic operation with 'for_each'. This series is not aimed on performance, but some performance implications are considered. In [1] Jan reported 2% slowdown in a single-thread search test when switching find_bit() function to treat bitmaps as volatile arrays. On the other hand, kernel robot in the same thread reported +3.7% to the performance of will-it-scale.per_thread_ops test. Assuming that our compilers are sane and generate better code against properly annotated data, the above discrepancy doesn't look weird. When running on non-volatile bitmaps, plain find_bit() outperforms atomic find_and_bit(), and vice-versa. So, all users of find_bit() API, where heavy concurrency is expected, are encouraged to switch to atomic find_and_bit() as appropriate. The 1st patch of this series adds atomic find_and_bit() API, 2nd adds a basic test for new API, and all the following patches spread it over the kernel. [1] https://lore.kernel.org/lkml/634f5fdf-e236-42cf-be8d-48a581c21660@alu.unizg.hr/T/#m3e7341eb3571753f3acf8fe166f3fb5b2c12e615 --- v1: https://lore.kernel.org/netdev/20231118155105.25678-29-yury.norov@gmail.com/T/ v2: https://lore.kernel.org/all/20231204185101.ddmkvsr2xxsmoh2u@quack3/T/ v3: https://lore.kernel.org/linux-pci/ZX4bIisLzpW8c4WM@yury-ThinkPad/T/ v4: - drop patch v3-24: not needed after null_blk refactoring; - add patch 34: "MIPS: sgi-ip27: optimize alloc_level()"; - add patch 35: "uprobes: optimize xol_take_insn_slot()"; - add patches 36-40: get rid of locking scheme around bitmaps; - move new API to separate headers, to not bloat bitmap.h @ Linus; - patch #1: adjust comments to allow returning >= @size; - rebase the series on top of current master. Yury Norov (40): lib/find: add atomic find_bit() primitives lib/find: add test for atomic find_bit() ops lib/sbitmap; optimize __sbitmap_get_word() by using find_and_set_bit() watch_queue: optimize post_one_notification() by using find_and_clear_bit() sched: add cpumask_find_and_set() and use it in __mm_cid_get() mips: sgi-ip30: optimize heart_alloc_int() by using find_and_set_bit() sparc: optimize alloc_msi() by using find_and_set_bit() perf/arm: use atomic find_bit() API drivers/perf: optimize ali_drw_get_counter_idx() by using find_and_set_bit() dmaengine: idxd: optimize perfmon_assign_event() ath10k: optimize ath10k_snoc_napi_poll() wifi: rtw88: optimize the driver by using atomic iterator KVM: x86: hyper-v: optimize and cleanup kvm_hv_process_stimers() PCI: hv: Optimize hv_get_dom_num() by using find_and_set_bit() scsi: core: optimize scsi_evt_emit() by using an atomic iterator scsi: mpi3mr: optimize the driver by using find_and_set_bit() scsi: qedi: optimize qedi_get_task_idx() by using find_and_set_bit() powerpc: optimize arch code by using atomic find_bit() API iommu: optimize subsystem by using atomic find_bit() API media: radio-shark: optimize the driver by using atomic find_bit() API sfc: optimize the driver by using atomic find_bit() API tty: nozomi: optimize interrupt_handler() usb: cdc-acm: optimize acm_softint() RDMA/rtrs: optimize __rtrs_get_permit() by using find_and_set_bit_lock() mISDN: optimize get_free_devid() media: em28xx: cx231xx: optimize drivers by using find_and_set_bit() ethernet: rocker: optimize ofdpa_port_internal_vlan_id_get() bluetooth: optimize cmtp_alloc_block_id() net: smc: optimize smc_wr_tx_get_free_slot_index() ALSA: use atomic find_bit() functions where applicable m68k: optimize get_mmu_context() microblaze: optimize get_mmu_context() sh: mach-x3proto: optimize ilsel_enable() MIPS: sgi-ip27: optimize alloc_level() uprobes: optimize xol_take_insn_slot() scsi: sr: drop locking around SR index bitmap KVM: PPC: Book3s HV: drop locking around kvmppc_uvmem_bitmap wifi: mac80211: drop locking around ntp_fltr_bmap mailbox: bcm-flexrm: simplify locking scheme powerpc/xive: drop locking around IRQ map MAINTAINERS | 2 + arch/m68k/include/asm/mmu_context.h | 12 +- arch/microblaze/include/asm/mmu_context_mm.h | 12 +- arch/mips/sgi-ip27/ip27-irq.c | 13 +- arch/mips/sgi-ip30/ip30-irq.c | 13 +- arch/powerpc/kvm/book3s_hv_uvmem.c | 33 +- arch/powerpc/mm/book3s32/mmu_context.c | 11 +- arch/powerpc/platforms/pasemi/dma_lib.c | 46 +-- arch/powerpc/platforms/powernv/pci-sriov.c | 13 +- arch/powerpc/sysdev/xive/spapr.c | 34 +- arch/sh/boards/mach-x3proto/ilsel.c | 5 +- arch/sparc/kernel/pci_msi.c | 10 +- arch/x86/kvm/hyperv.c | 41 +-- drivers/dma/idxd/perfmon.c | 9 +- drivers/infiniband/ulp/rtrs/rtrs-clt.c | 16 +- drivers/iommu/arm/arm-smmu/arm-smmu.h | 11 +- drivers/iommu/msm_iommu.c | 19 +- drivers/isdn/mISDN/core.c | 10 +- drivers/mailbox/bcm-flexrm-mailbox.c | 21 +- drivers/media/radio/radio-shark.c | 6 +- drivers/media/radio/radio-shark2.c | 6 +- drivers/media/usb/cx231xx/cx231xx-cards.c | 17 +- drivers/media/usb/em28xx/em28xx-cards.c | 38 +-- drivers/net/ethernet/broadcom/bnxt/bnxt.c | 18 +- drivers/net/ethernet/rocker/rocker_ofdpa.c | 12 +- drivers/net/ethernet/sfc/rx_common.c | 5 +- drivers/net/ethernet/sfc/siena/rx_common.c | 5 +- drivers/net/ethernet/sfc/siena/siena_sriov.c | 15 +- drivers/net/wireless/ath/ath10k/snoc.c | 10 +- drivers/net/wireless/realtek/rtw88/pci.c | 6 +- drivers/net/wireless/realtek/rtw89/pci.c | 6 +- drivers/pci/controller/pci-hyperv.c | 8 +- drivers/perf/alibaba_uncore_drw_pmu.c | 11 +- drivers/perf/arm-cci.c | 25 +- drivers/perf/arm-ccn.c | 11 +- drivers/perf/arm_dmc620_pmu.c | 10 +- drivers/perf/arm_pmuv3.c | 9 +- drivers/scsi/mpi3mr/mpi3mr_os.c | 22 +- drivers/scsi/qedi/qedi_main.c | 10 +- drivers/scsi/scsi_lib.c | 8 +- drivers/scsi/sr.c | 15 +- drivers/tty/nozomi.c | 6 +- drivers/usb/class/cdc-acm.c | 6 +- include/linux/cpumask_atomic.h | 20 ++ include/linux/find.h | 4 - include/linux/find_atomic.h | 324 +++++++++++++++++++ kernel/events/uprobes.c | 15 +- kernel/sched/sched.h | 15 +- kernel/watch_queue.c | 7 +- lib/find_bit.c | 86 +++++ lib/sbitmap.c | 47 +-- lib/test_bitmap.c | 62 ++++ net/bluetooth/cmtp/core.c | 11 +- net/smc/smc_wr.c | 11 +- sound/pci/hda/hda_codec.c | 8 +- sound/usb/caiaq/audio.c | 14 +- 56 files changed, 747 insertions(+), 493 deletions(-) create mode 100644 include/linux/cpumask_atomic.h create mode 100644 include/linux/find_atomic.h