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

Ard Biesheuvel
2025-11-27T15:41:02Z

On Thu, 27 Nov 2025 at 16:03, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Thu, 27 Nov 2025 at 15:18, Ryan Roberts <ryan.roberts@arm.com> wrote:
> >
> > 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.
> >
>
> Thanks. But this is really rather surprising: what else could be
> taking up that time, given that on the fast path, there are only some
> loads and stores to the buffer, and a cmpxchg64_local(). Could it be
> the latter that is causing so much latency? I suppose the local
> cmpxchg() semantics don't really exist on arm64, and this uses the
> exact same LSE instruction that would be used for an ordinary
> cmpxchg(), unlike on x86 where it appears to omit the LOCK prefix.
>

FWIW, my naive get_random_u8() benchmark slows down by 3x on x86 if I
replace cmpxchg64_local() with cmpxchg64(), so I suspect the above
comparison will look different on x86 too.