Re: [syzbot] Re: [PATCH bpf v1] bpf: Convert ringbuf.c to rqspinlock

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Subject: Re: [PATCH bpf v1] bpf: Convert ringbuf.c to rqspinlock
Author: memxor@gmail.com

On Fri, 11 Apr 2025 at 12:18, Kumar Kartikeya Dwivedi <memxor@gmail.com> wrote:
>
> Convert the raw spinlock used by BPF ringbuf to rqspinlock. Currently,
> we have an open syzbot report of a potential deadlock. In addition, the
> ringbuf can fail to reserve spuriously under contention from NMI
> context.
>
> It is potentially attractive to enable unconstrained usage (incl. NMIs)
> while ensuring no deadlocks manifest at runtime, perform the conversion
> to rqspinlock to achieve this.
>
> This change was benchmarked for BPF ringbuf's multi-producer contention
> case on an Intel Sapphire Rapids server, with hyperthreading disabled
> and performance governor turned on. 5 warm up runs were done for each
> case before obtaining the results.
>
> Before (raw_spinlock_t):
>
> Ringbuf, multi-producer contention
> ==================================
> rb-libbpf nr_prod 1  11.440 ± 0.019M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 2  2.706 ± 0.010M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 3  3.130 ± 0.004M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 4  2.472 ± 0.003M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 8  2.352 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 12 2.813 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 16 1.988 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 20 2.245 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 24 2.148 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 28 2.190 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 32 2.490 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 36 2.180 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 40 2.201 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 44 2.226 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 48 2.164 ± 0.001M/s (drops 0.000 ± 0.000M/s)
> rb-libbpf nr_prod 52 1.874 ± 0.001M/s (drops 0.000 ± 0.000M/s)
>
> After (rqspinlock_t):
>
> Ringbuf, multi-producer contention
> ==================================
> rb-libbpf nr_prod 1  11.078 ± 0.019M/s (drops 0.000 ± 0.000M/s) (-3.16%)
> rb-libbpf nr_prod 2  2.801 ± 0.014M/s (drops 0.000 ± 0.000M/s) (3.51%)
> rb-libbpf nr_prod 3  3.454 ± 0.005M/s (drops 0.000 ± 0.000M/s) (10.35%)
> rb-libbpf nr_prod 4  2.567 ± 0.002M/s (drops 0.000 ± 0.000M/s) (3.84%)
> rb-libbpf nr_prod 8  2.468 ± 0.001M/s (drops 0.000 ± 0.000M/s) (4.93%)
> rb-libbpf nr_prod 12 2.510 ± 0.001M/s (drops 0.000 ± 0.000M/s) (-10.77%)
> rb-libbpf nr_prod 16 2.075 ± 0.001M/s (drops 0.000 ± 0.000M/s) (4.38%)
> rb-libbpf nr_prod 20 2.640 ± 0.001M/s (drops 0.000 ± 0.000M/s) (17.59%)
> rb-libbpf nr_prod 24 2.092 ± 0.001M/s (drops 0.000 ± 0.000M/s) (-2.61%)
> rb-libbpf nr_prod 28 2.426 ± 0.005M/s (drops 0.000 ± 0.000M/s) (10.78%)
> rb-libbpf nr_prod 32 2.331 ± 0.004M/s (drops 0.000 ± 0.000M/s) (-6.39%)
> rb-libbpf nr_prod 36 2.306 ± 0.003M/s (drops 0.000 ± 0.000M/s) (5.78%)
> rb-libbpf nr_prod 40 2.178 ± 0.002M/s (drops 0.000 ± 0.000M/s) (-1.04%)
> rb-libbpf nr_prod 44 2.293 ± 0.001M/s (drops 0.000 ± 0.000M/s) (3.01%)
> rb-libbpf nr_prod 48 2.022 ± 0.001M/s (drops 0.000 ± 0.000M/s) (-6.56%)
> rb-libbpf nr_prod 52 1.809 ± 0.001M/s (drops 0.000 ± 0.000M/s) (-3.47%)
>
> There's a fair amount of noise in the benchmark, with numbers on reruns
> going up and down by 10%, so all changes are in the range of this
> disturbance, and we see no major regressions.
>
> Reported-by: syzbot+850aaf14624dc0c6d366@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/0000000000004aa700061379547e@google.com
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---

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