tree 35759c9544c07c174aa01b5510c41b6853d62a72
parent 2f41503d647629cfafea42cf6f827e4139536703
author Kumar Kartikeya Dwivedi <memxor@gmail.com> 1744366679 -0700
committer Alexei Starovoitov <ast@kernel.org> 1744392506 -0700

bpf: Convert ringbuf map to rqspinlock

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>
Link: https://lore.kernel.org/r/20250411101759.4061366-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>