Re: [PATCH v2 00/12] Convert barrier pairs to acquire/release for better performance

[Kuniyuki Iwashima]

On Mon, Aug 31, 2026 at 7:42 PM Jinjie Ruan <ruanjinjie@xxxxxxxxxx> wrote:
>
> Hi,
>
> This series converts some existing smp_wmb()/smp_rmb() barrier pairs to
> smp_store_release()/smp_load_acquire() across various subsystems.
>
> Background
> ==========
>
> Many architectures support load acquire and store release instructions
> which can replace explicit memory barriers and save cycles. As noted
> in the ARM architecture reference [1]:
>
>   "Weaker ordering requirements that are imposed by Load-Acquire and
>    Store-Release instructions allow for micro-architectural
>    optimizations, which could reduce some of the performance impacts
>    that are otherwise imposed by an explicit memory barrier.
>
>    If the ordering requirement is satisfied using either a Load-Acquire
>    or Store-Release, then it would be preferable to use these
>    instructions instead of a DMB."
>
> On arm64, a typical seqcount [2] read loop requires 13 cycles with DMB
> barriers. Replacing the read barrier with smp_load_acquire() reduces
> this to 8 cycles on an Ampere Altra.
>
> We also observed significant barrier overhead while profiling Unxibench
> syscall test on arm64: a single getuid() call is ~8ns slower than on
> a comparable x86 system, with the dominant cost in map_id_up()'s smp_rmb(),
> which is a DMB ISHLD on arm64. Converting it to smp_load_acquire() allows
> the use of LDAR, eliminating the measurable overhead.
>
> This motivated a broader search for existing barrier pairs that can
> be converted to the lighter acquire/release semantics.
>
> Changes
> =======
>
> Each patch in this series targets a specific barrier pair where the
> publish/subscribe pattern is already present:
>
> - Writers populate data, then publish a flag/count/pointer via
>   smp_store_release()
>
> - Readers load the flag/count/pointer via smp_load_acquire(), then
>   consume the data
>
> This preserves the existing memory ordering guarantees while allowing
> architectures with native acquire/release instructions (e.g. arm64's
> STLR/LDAR) to avoid the cost of full one-way barriers (DMB ISHST/ISHLD).
> On architectures without native support, the generated code is
> generally no worse than the explicit barrier pair.
>
> The conversions are mechanical and no functional change is intended.
>
> Testing (arm64 Kunpeng HIP09 server)
> ================
>
> 1. UNIXBENCH syscall
>         Baseline: 715.27
>         Patched:  718.83
>         Improvement: +0.50%
>
> 2. fs/aio (fio + null_blk, 4 jobs):
>         Baseline: 1441k IOPS, 86.46us
>         Patched:  1452k IOPS, 85.80us
>         Improvement: ~0.8%
>
> 3. soreuseport (wrk, 8 servers):
>         Baseline: 162.6k req/s, 452.5us
>         Patched:  164.2k req/s, 449.4us
>         Improvement: ~1.0%
>
> Both improvements are consistent across runs and align with the
> expected savings from replacing DMB with LDAR/STLR on arm64.
>
> [1]: https://support.arm.com/documentation/102336/0100/Load-Acquire-and-Store-Release-instructions
> [2]: https://github.com/torvalds/linux/commit/d0dd066a0fa26d55c19ace9e89dedd9504c5bcba
>
> Changes in v2:
> - Fix pre-existing issue for ext4 and 8021q [3].
> - Fix missing copy_mnt_idmap() udapte [3].
> - Drop nacked isotp patch.
> - Add test data.
> - Add Reviewed-by and update fs patch as Jan suggested.
>
> [3]: https://sashiko.dev/#/patchset/20260825095422.3166067-1-ruanjinjie%40huawei.com
>
> Jinjie Ruan (12):
>   user_namespace: Use acquire/release for nr_extents synchronization
>   lib/vsprintf: Use acquire/release for ptr_key publication
>   fs: aio: Use acquire/release for ring->tail publication
>   fs: Use acquire/release for fdtable resize synchronization
>   pidfs: Use test_bit_acquire() for attr flag tests
>   super: Use acquire for SB_BORN check in super_cache_count()
>   ext4: Fix out-of-bounds read in ext4_get_group_info()
>   ext4: Convert group-count barrier protocol to acquire/release
>   soreuseport: publish num_socks with acquire/release
>   net: sched: act_gact: use acquire/release for tcfg_ptype
>   8021q: Fix data race when publishing vlan net_device pointers
>   8021q: publish vlan_devices_arrays entries with acquire/release

Please post networking patches separately with the target tree specified:

  Subject: [PATCH vX net-next] soreuseport: ...

8021q changes can be posted a series.