Re: [RFC/RFT PATCH 0/6] Improve get_random_u8() for use in randomize kstack

Ryan Roberts
2025-11-27T14:18:22Z

On 27/11/2025 12:28, Ard Biesheuvel wrote:
> On Thu, 27 Nov 2025 at 13:12, Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 27/11/2025 09:22, Ard Biesheuvel wrote:
>>> From: Ard Biesheuvel <ardb@kernel.org>
>>>
>>> Ryan reports that get_random_u16() is dominant in the performance
>>> profiling of syscall entry when kstack randomization is enabled [0].
>>>
>>> This is the reason many architectures rely on a counter instead, and
>>> that, in turn, is the reason for the convoluted way the (pseudo-)entropy
>>> is gathered and recorded in a per-CPU variable.
>>>
>>> Let's try to make the get_random_uXX() fast path faster, and switch to
>>> get_random_u8() so that we'll hit the slow path 2x less often. Then,
>>> wire it up in the syscall entry path, replacing the per-CPU variable,
>>> making the logic at syscall exit redundant.
>>
>> I ran the same set of syscall benchmarks for this series as I've done for my
>> series.
>>
>
> Thanks!
>
>
>> The baseline is v6.18-rc5 with stack randomization turned *off*. So I'm showing
>> performance cost of turning it on without any changes to the implementation,
>> then the reduced performance cost of turning it on with my changes applied, and
>> finally cost of turning it on with Ard's changes applied:
>>
>> arm64 (AWS Graviton3):
>> +-----------------+--------------+-------------+---------------+-----------------+
>> | Benchmark | Result Class | v6.18-rc5 | per-task-prng | fast-get-random |
>> | | | rndstack-on | | |
>> +=================+==============+=============+===============+=================+
>> | syscall/getpid | mean (ns) | (R) 15.62% | (R) 3.43% | (R) 11.93% |
>> | | p99 (ns) | (R) 155.01% | (R) 3.20% | (R) 11.00% |
>> | | p99.9 (ns) | (R) 156.71% | (R) 2.93% | (R) 11.39% |
>> +-----------------+--------------+-------------+---------------+-----------------+
>> | syscall/getppid | mean (ns) | (R) 14.09% | (R) 2.12% | (R) 10.44% |
>> | | p99 (ns) | (R) 152.81% | 1.55% | (R) 9.94% |
>> | | p99.9 (ns) | (R) 153.67% | 1.77% | (R) 9.83% |
>> +-----------------+--------------+-------------+---------------+-----------------+
>> | syscall/invalid | mean (ns) | (R) 13.89% | (R) 3.32% | (R) 10.39% |
>> | | p99 (ns) | (R) 165.82% | (R) 3.51% | (R) 10.72% |
>> | | p99.9 (ns) | (R) 168.83% | (R) 3.77% | (R) 11.03% |
>> +-----------------+--------------+-------------+---------------+-----------------+
>>
>
> What does the (R) mean?
>
>> So this fixes the tail problem. I guess get_random_u8() only takes the slow path
>> every 768 calls, whereas get_random_u16() took it every 384 calls. I'm not sure
>> that fully explains it though.
>>
>> But it's still a 10% cost on average.
>>
>> Personally I think 10% syscall cost is too much to pay for 6 bits of stack
>> randomisation. 3% is better, but still higher than we would all prefer, I'm sure.
>>
>
> Interesting!
>
> So the only thing that get_random_u8() does that could explain the
> delta is calling into the scheduler on preempt_enable(), given that it
> does very little beyond that.
>
> Would you mind repeating this experiment after changing the
> put_cpu_var() to preempt_enable_no_resched(), to test this theory?

This has no impact on performance.