Re: futex performance regression from "futex: Allow automatic allocation of process wide futex hash"

[Chris Mason]

On 6/6/25 3:06 AM, Sebastian Andrzej Siewior wrote:
> On 2025-06-05 20:55:27 [-0400], Chris Mason wrote:
>>>> We've got large systems that are basically dedicated to single
>>>> workloads, and those will probably miss the larger global hash table,
>>>> regressing like schbench did.  Then we have large systems spread over
>>>> multiple big workloads that will love the private tables.
>>>>
>>>> In either case, I think growing the hash table as a multiple of thread
>>>> count instead of cpu count will probably better reflect the crazy things
>>>> multi-threaded applications do?  At any rate, I don't think we want
>>>> applications to need prctl to get back to the performance they had on
>>>> older kernels.
>>>
>>> This is only an issue if all you CPUs spend their time in the kernel
>>> using the hash buckets at the same time.
>>> This was the case in every benchmark I've seen so far. Your thing might
>>> be closer to an actual workload.
>>>
>>
>> I didn't spend a ton of time looking at the perf profiles of the slower
>> kernel, was the bottleneck in the hash chain length or in contention for
>> the buckets?
> 
> Every futex operation does a rcuref_get() (which is an atomic inc) on
> the private hash. This is before anything else happens. If you have two
> threads, on two CPUs, which simultaneously do a futex() operation then
> both do this rcuref_get(). That atomic inc ensures that the cacheline
> bounces from one CPU to the other. On the exit of the syscall there is a
> matching rcuref_put().
> 
>>>> For people that want to avoid that memory overhead, I'm assuming they
>>>> want the CONFIG_FUTEX_PRIVATE_HASH off, so the Kconfig help text should
>>>> make that more clear.
>>>>
>>>>> Then there the possibility of 
>>> …
>>>>> 256 cores, 2xNUMA:
>>>>> | average rps: 1 701 947.02 Futex HBs: 0 immutable: 1
>>>>> | average rps:   785 446.07 Futex HBs: 1024 immutable: 0
>>>>> | average rps: 1 586 755.62 Futex HBs: 1024 immutable: 1> | average
>>>> rps:   736 769.77 Futex HBs: 2048 immutable: 0
>>>>> | average rps: 1 555 182.52 Futex HBs: 2048 immutable: 1
>>>>
>>>>
>>>> How long are these runs?  That's a huge benefit from being immutable
>>>> (1.5M vs 736K?) but the hash table churn should be confined to early in
>>>> the schbench run right?
>>>
>>> I think 30 secs or so. I used your command line. 
>>
>> Ah ok, my command line is 60 seconds.  It feels like something is
>> strange for the immutable flag to make it that much faster?  schbench
>> starts all the threads up front, so it should hit steady state pretty
>> quickly.  More on NUMA below, but I'll benchmark with the immutable flag
>> on the turin box in the morning to see if it is the extra atomics.
> 
> That immutable flag makes this rcuref_get()/ put() go away. The price is
> that you can't change the size of the private hash anymore. So if your
> workload works best with a hash size of X and you don't intend to change
> it during the runtime of the program, set the immutable flag.

ok, for the benchmarks, I hard coded the number of buckets at 1024 and
the only thing I flipped was the immutable flag.  I was running on top
of c0c9379f235df33a12ceae94370ad80c5278324d (just today's Linus).

schbench -L -m 4 -M auto -t 256 -n 0 -r 60 -s 0

1024 bucket without immutable:

RPS 2 297 856

1024 with immutable:

RPS 3 665 920

This is pretty similar to what 6.15 obtains, so I'm hoping we can find a
way to get the 6.15 performance levels without the applications needing
to call prctl manually.

I took some profiles on the 1024 + non-immutable

    16.92%  schbench         [kernel.kallsyms]  [k] futex_hash_put
            |
            |--8.93%--fpost
            |
            |--6.99%--xlist_wake_all
            |
             --0.58%--entry_SYSCALL_64
                       syscall

    13.27%  schbench         libc.so.6          [.] syscall
            |
            |--8.90%--xlist_wake_all
            |
             --3.98%--entry_SYSCALL_64
                       syscall

    12.36%  schbench         [kernel.kallsyms]  [k] entry_SYSCALL_64
            |
            |--11.04%--__futex_hash
            |          futex_hash
            |          futex_wake
            |          do_futex
            |          __x64_sys_futex
            |          do_syscall_64
            |          entry_SYSCALL_64_after_hwframe
            |          syscall
            |
             --1.23%--futex_hash
                       futex_wake
                       do_futex
                       __x64_sys_futex
                       do_syscall_64
                       entry_SYSCALL_64_after_hwframe
                       syscall

Percent │    0xffffffff813f2f80 <futex_hash_put>:
   0.05 │    → callq __fentry__
   0.06 │      pushq %rbx
   0.08 │      movq  0x18(%rdi),%rbx
   0.05 │      testq %rbx,%rbx
   0.03 │    ↓ je    15
  12.49 │      cmpb  $0x0,0x21(%rbx)
  12.58 │    ↓ je    17
        │15:   popq  %rbx
        │    ← retq
   0.24 │17:   movl  $0xffffffff,%esi
  12.76 │      lock
        │      xaddl %esi,(%rbx)
   7.80 │   ┌──subl  $0x1,%esi
  22.96 │   ├──js    27
  23.05 │   │  popq  %rbx
   7.85 │   │← retq
        │27:└─→movq  %rbx,%rdi
        │    → callq rcuref_put_slowpath
        │      testb %al,%al
        │    ↑ je    15
        │      movq  0x18(%rbx),%rdi
        │      popq  %rbx
        │    → jmp   wake_up_var

If you like profiles with line numbers:
12830 samples (10.24%) Comms: schbench
                 futex_hash_put @ /root/linux-6.15/kernel/futex/core.c:179:2
                  futex_private_hash_put @ /root/linux-6.15/kernel/futex/core.c:148:6 [inlined]
                  futex_private_hash_put @ /root/linux-6.15/kernel/futex/core.c:153:6 [inlined]
                  rcuref_put @ /root/linux-6.15/./include/linux/rcuref.h:173:13 [inlined]
                  __rcuref_put @ /root/linux-6.15/./include/linux/rcuref.h:110:5 [inlined]
                 futex_wake @ /root/linux-6.15/kernel/futex/waitwake.c:200:1
                 do_futex @ /root/linux-6.15/kernel/futex/syscalls.c:107:10
                 __x64_sys_futex @ /root/linux-6.15/kernel/futex/syscalls.c:160:1
                  __se_sys_futex @ /root/linux-6.15/kernel/futex/syscalls.c:160:1 [inlined]
                  __do_sys_futex @ /root/linux-6.15/kernel/futex/syscalls.c:179:9 [inlined]
                 do_syscall_64 @ /root/linux-6.15/arch/x86/entry/syscall_64.c:94:7
                  do_syscall_x64 @ /root/linux-6.15/arch/x86/entry/syscall_64.c:63:12 [inlined]
                 entry_SYSCALL_64_after_hwframe
13795 samples (11.01%) Comms: schbench
                 futex_hash @ /root/linux-6.15/kernel/futex/core.c:311:15
                  futex_private_hash_get @ /root/linux-6.15/kernel/futex/core.c:143:5 [inlined]
                 futex_wake @ /root/linux-6.15/kernel/futex/waitwake.c:172:2
                  class_hb_constructor @ /root/linux-6.15/kernel/futex/futex.h:242:1 [inlined]
                 do_futex @ /root/linux-6.15/kernel/futex/syscalls.c:107:10
                 __x64_sys_futex @ /root/linux-6.15/kernel/futex/syscalls.c:160:1
                  __se_sys_futex @ /root/linux-6.15/kernel/futex/syscalls.c:160:1 [inlined]
                  __do_sys_futex @ /root/linux-6.15/kernel/futex/syscalls.c:179:9 [inlined]
                 do_syscall_64 @ /root/linux-6.15/arch/x86/entry/syscall_64.c:94:7
                  do_syscall_x64 @ /root/linux-6.15/arch/x86/entry/syscall_64.c:63:12 [inlined]
                 entry_SYSCALL_64_after_hwframe
14364 samples (11.46%) Comms: schbench
                 entry_SYSCALL_64
17612 samples (14.05%) Comms: schbench
                 futex_hash @ /root/linux-6.15/kernel/futex/core.c:311:15
                  futex_private_hash_get @ /root/linux-6.15/kernel/futex/core.c:145:9 [inlined]
                  rcuref_get @ /root/linux-6.15/./include/linux/rcuref.h:87:5 [inlined]
                 futex_wake @ /root/linux-6.15/kernel/futex/waitwake.c:172:2
                  class_hb_constructor @ /root/linux-6.15/kernel/futex/futex.h:242:1 [inlined]
                 do_futex @ /root/linux-6.15/kernel/futex/syscalls.c:107:10
                 __x64_sys_futex @ /root/linux-6.15/kernel/futex/syscalls.c:160:1
                  __se_sys_futex @ /root/linux-6.15/kernel/futex/syscalls.c:160:1 [inlined]
                  __do_sys_futex @ /root/linux-6.15/kernel/futex/syscalls.c:179:9 [inlined]
                 do_syscall_64 @ /root/linux-6.15/arch/x86/entry/syscall_64.c:94:7
                  do_syscall_x64 @ /root/linux-6.15/arch/x86/entry/syscall_64.c:63:12 [inlined]
                 entry_SYSCALL_64_after_hwframe

With immutable, futexes are not in the top 10.

-chris