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

Ard Biesheuvel
2025-11-27T12:28:13Z

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?