Re: [REGRESSION] osnoise: "eventpoll: Replace rwlock with spinlock" causes ~50µs noise spikes on isolated PREEMPT_RT cores

[Crystal Wood <crwood@redhat.com>]

On Thu, 2026-03-26 at 16:00 +0200, Ionut Nechita (Wind River) wrote:
> Hi,
> 
> I'm reporting a regression introduced by commit 0c43094f8cc9
> ("eventpoll: Replace rwlock with spinlock"), backported to stable 6.12.y.
> 
> On a PREEMPT_RT system with nohz_full isolated cores, this commit causes
> significant osnoise degradation on the isolated CPUs.
> 
> Setup:
>   - Kernel: 6.12.78 with PREEMPT_RT
>   - Hardware: x86_64, dual-socket (CPUs 0-63)
>   - Boot params: nohz_full=1-16,33-48 isolcpus=nohz,domain,managed_irq,1-16,33-48
>     rcu_nocbs=1-31,33-63 kthread_cpus=0,32 irqaffinity=17-31,49-63
>   - Tool: osnoise tracer (./osnoise -c 1-16,33-48)

Is SMT disabled?


> With commit applied (spinlock, kernel 6.12.78-vanilla-0):
> 
>   CPU    RUNTIME   MAX_NOISE   AVAIL%      NOISE  NMI   IRQ   SIRQ  Thread
>   [001]   950000       50163   94.719%        14    0   6864     0    5922
>   [004]   950000       50294   94.705%        14    0   6864     0    5920
>   [007]   950000       49782   94.759%        14    0   6864     1    5921
>   [033]   950000       49528   94.786%        15    0   6864     2    5922
>   [016]   950000       48551   94.889%        20    0   6863    19    5942
>   [008]   950000       44343   95.332%        14    0   6864     0    5925
> 
> With commit reverted (rwlock restored, kernel 6.12.78-vanilla-1):
> 
>   CPU    RUNTIME   MAX_NOISE   AVAIL%      NOISE  NMI   IRQ   SIRQ  Thread
>   [001]   950000           0   100.000%       0    0      6     0       0
>   [004]   950000           0   100.000%       0    0      4     0       0
>   [007]   950000           0   100.000%       0    0      4     0       0
>   [033]   950000           0   100.000%       0    0      4     0       0
>   [016]   950000           0   100.000%       0    0      5     0       0
>   [008]   950000           7    99.999%       7    0      5     0       0
> 
> Summary across all isolated cores (32 CPUs):
> 
>                           With spinlock       With rwlock (reverted)
>   MAX noise (ns):         44,343 - 51,869     0 - 10
>   IRQ count/sample:       ~6,650 - 6,870      3 - 7
>   Thread noise/sample:    ~5,700 - 5,940      0 - 1
>   CPU availability:       94.5% - 95.3%       ~100%
> 
> The regression is roughly 3 orders of magnitude in noise on isolated
> cores. The test was run over many consecutive samples and the pattern
> is consistent: with the spinlock, every isolated core sees thousands
> of IRQs and ~50µs of noise per 950ms sample window. With the rwlock,
> the cores are essentially silent.
> 
> Note that CPU 016 occasionally shows SIRQ noise (softirq) with both
> kernels, which is a separate known issue with the tick on the first
> nohz_full CPU. The eventpoll regression is the dominant noise source.
> 
> My understanding of the root cause: the original rwlock allowed
> ep_poll_callback() (producer side, running from IRQ context on any CPU)
> to use read_lock, which does not cause cross-CPU contention on isolated
> cores when no local epoll activity exists. With the spinlock conversion,
> on PREEMPT_RT spinlock_t becomes an rt_mutex. This means that even if
> the isolated core is not involved in any epoll activity, the lock's
> cacheline bouncing and potential PI-boosted wakeups from housekeeping
> CPUs can inject noise into the isolated cores via IPI or cache
> invalidation traffic.

That sounds like a general isolation problem... it's not a bug for non-
isolated CPUs to bounce cachelines or send IPIs to each other.

Whether it's IPIs or not, osnoise is showing IRQs on the isolated CPUs,
so I'd look into which IRQs and why.  Even with the patch reverted,
there are some IRQs on the isolated CPUs.

> 
> The commit message acknowledges the throughput regression but argues
> real workloads won't notice. However, for RT/latency-sensitive
> deployments with CPU isolation, the impact is severe and measurable
> even with zero local epoll usage.
> 
> I believe this needs either:
>   a) A revert of the backport for stable RT trees, or

Even if the patch weren't trying to address an RT issue in the first
place, this would just be a bandaid rather than a real solution.

>   b) A fix that avoids the spinlock contention path for isolated CPUs

If there's truly no epoll activity on the isolated CPUs, when would you
ever reach that path on an isolated CPU?


-Crystal