Re: [PATCH bpf-next 0/2] bpf, riscv: use BPF prog pack allocator in BPF JIT

Puranjay Mohan
2023-08-14T12:06:02Z

On Mon, Aug 14, 2023 at 12:40 PM Björn Töpel <bjorn@kernel.org> wrote:
>
> Björn Töpel <bjorn@kernel.org> writes:
>
> > Puranjay Mohan <puranjay12@gmail.com> writes:
> >
> >> BPF programs currently consume a page each on RISCV. For systems with many BPF
> >> programs, this adds significant pressure to instruction TLB. High iTLB pressure
> >> usually causes slow down for the whole system.
> >>
> >> Song Liu introduced the BPF prog pack allocator[1] to mitigate the above issue.
> >> It packs multiple BPF programs into a single huge page. It is currently only
> >> enabled for the x86_64 BPF JIT.
> >>
> >> I enabled this allocator on the ARM64 BPF JIT[2]. It is being reviewed now.
> >>
> >> This patch series enables the BPF prog pack allocator for the RISCV BPF JIT.
> >> This series needs a patch[3] from the ARM64 series to work.
> >>
> >> ======================================================
> >> Performance Analysis of prog pack allocator on RISCV64
> >> ======================================================
> >>
> >> Test setup:
> >> ===========
> >>
> >> Host machine: Debian GNU/Linux 11 (bullseye)
> >> Qemu Version: QEMU emulator version 8.0.3 (Debian 1:8.0.3+dfsg-1)
> >> u-boot-qemu Version: 2023.07+dfsg-1
> >> opensbi Version: 1.3-1
> >>
> >> To test the performance of the BPF prog pack allocator on RV, a stresser
> >> tool[4] linked below was built. This tool loads 8 BPF programs on the system and
> >> triggers 5 of them in an infinite loop by doing system calls.
> >>
> >> The runner script starts 20 instances of the above which loads 8*20=160 BPF
> >> programs on the system, 5*20=100 of which are being constantly triggered.
> >> The script is passed a command which would be run in the above environment.
> >>
> >> The script was run with following perf command:
> >> ./run.sh "perf stat -a \
> >> -e iTLB-load-misses \
> >> -e dTLB-load-misses \
> >> -e dTLB-store-misses \
> >> -e instructions \
> >> --timeout 60000"
> >>
> >> The output of the above command is discussed below before and after enabling the
> >> BPF prog pack allocator.
> >>
> >> The tests were run on qemu-system-riscv64 with 8 cpus, 16G memory. The rootfs
> >> was created using Bjorn's riscv-cross-builder[5] docker container linked below.
> >>
> >> Results
> >> =======
> >>
> >> Before enabling prog pack allocator:
> >> ------------------------------------
> >>
> >> Performance counter stats for 'system wide':
> >>
> >> 4939048 iTLB-load-misses
> >> 5468689 dTLB-load-misses
> >> 465234 dTLB-store-misses
> >> 1441082097998 instructions
> >>
> >> 60.045791200 seconds time elapsed
> >>
> >> After enabling prog pack allocator:
> >> -----------------------------------
> >>
> >> Performance counter stats for 'system wide':
> >>
> >> 3430035 iTLB-load-misses
> >> 5008745 dTLB-load-misses
> >> 409944 dTLB-store-misses
> >> 1441535637988 instructions
> >>
> >> 60.046296600 seconds time elapsed
> >>
> >> Improvements in metrics
> >> =======================
> >>
> >> It was expected that the iTLB-load-misses would decrease as now a single huge
> >> page is used to keep all the BPF programs compared to a single page for each
> >> program earlier.
> >>
> >> --------------------------------------------
> >> The improvement in iTLB-load-misses: -30.5 %
> >> --------------------------------------------
> >>
> >> I repeated this expriment more than 100 times in different setups and the
> >> improvement was always greater than 30%.
> >>
> >> This patch series is boot tested on the Starfive VisionFive 2 board[6].
> >> The performance analysis was not done on the board because it doesn't
> >> expose iTLB-load-misses, etc. The stresser program was run on the board to test
> >> the loading and unloading of BPF programs
> >>
> >> [1] https://lore.kernel.org/bpf/20220204185742.271030-1-song@kernel.org/
> >> [2] https://lore.kernel.org/all/20230626085811.3192402-1-puranjay12@gmail.com/
> >> [3] https://lore.kernel.org/all/20230626085811.3192402-2-puranjay12@gmail.com/
> >> [4] https://github.com/puranjaymohan/BPF-Allocator-Bench
> >> [5] https://github.com/bjoto/riscv-cross-builder
> >> [6] https://www.starfivetech.com/en/site/boards
> >>
> >> Puranjay Mohan (2):
> >> riscv: Extend patch_text_nosync() for multiple pages
> >> bpf, riscv: use prog pack allocator in the BPF JIT
> >
> > I get a hang for "test_tag", but it's not directly related to your
> > series, but rather "remote fence.i".
> >
> > | rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> > | rcu: 0-....: (1400 ticks this GP) idle=d5e4/1/0x4000000000000000 softirq=5542/5542 fqs=1862
> > | rcu: (detected by 1, t=5252 jiffies, g=10253, q=195 ncpus=4)
> > | Task dump for CPU 0:
> > | task:kworker/0:5 state:R running task stack:0 pid:319 ppid:2 flags:0x00000008
> > | Workqueue: events bpf_prog_free_deferred
> > | Call Trace:
> > | [<ffffffff80cbc444>] __schedule+0x2d0/0x940
> > | watchdog: BUG: soft lockup - CPU#0 stuck for 21s! [kworker/0:5:319]
> > | Modules linked in: nls_iso8859_1 drm fuse i2c_core drm_panel_orientation_quirks backlight dm_mod configfs ip_tables x_tables
> > | CPU: 0 PID: 319 Comm: kworker/0:5 Not tainted 6.5.0-rc5 #1
> > | Hardware name: riscv-virtio,qemu (DT)
> > | Workqueue: events bpf_prog_free_deferred
> > | epc : __sbi_rfence_v02_call.isra.0+0x74/0x11a
> > | ra : __sbi_rfence_v02+0xda/0x1a4
> > | epc : ffffffff8000ab4c ra : ffffffff8000accc sp : ff20000001c9bbd0
> > | gp : ffffffff82078c48 tp : ff600000888e6a40 t0 : ff20000001c9bd44
> > | t1 : 0000000000000000 t2 : 0000000000000040 s0 : ff20000001c9bbf0
> > | s1 : 0000000000000010 a0 : 0000000000000000 a1 : 0000000000000000
> > | a2 : 0000000000000000 a3 : 0000000000000000 a4 : 0000000000000000
> > | a5 : 0000000000000000 a6 : 0000000000000000 a7 : 0000000052464e43
> > | s2 : 000000000000ffff s3 : 00000000ffffffff s4 : ffffffff81667528
> > | s5 : 0000000000000000 s6 : 0000000000000000 s7 : 0000000000000000
> > | s8 : 0000000000000001 s9 : 0000000000000003 s10: 0000000000000040
> > | s11: ffffffff8207d240 t3 : 000000000000000f t4 : 000000000000002a
> > | t5 : ff600000872df140 t6 : ffffffff81e26828
> > | status: 0000000200000120 badaddr: 0000000000000000 cause: 8000000000000005
> > | [<ffffffff8000ab4c>] __sbi_rfence_v02_call.isra.0+0x74/0x11a
> > | [<ffffffff8000accc>] __sbi_rfence_v02+0xda/0x1a4
> > | [<ffffffff8000a886>] sbi_remote_fence_i+0x1e/0x26
> > | [<ffffffff8000cee2>] flush_icache_all+0x1a/0x48
> > | [<ffffffff80007736>] patch_text_nosync+0x6c/0x8c
> > | [<ffffffff8000f0f8>] bpf_arch_text_invalidate+0x62/0xac
> > | [<ffffffff8016c538>] bpf_prog_pack_free+0x9c/0x1b2
> > | [<ffffffff8016c84a>] bpf_jit_binary_pack_free+0x20/0x4a
> > | [<ffffffff8000f198>] bpf_jit_free+0x56/0x9e
> > | [<ffffffff8016b43a>] bpf_prog_free_deferred+0x15a/0x182
> > | [<ffffffff800576c4>] process_one_work+0x1b6/0x3d6
> > | [<ffffffff80057d52>] worker_thread+0x84/0x378
> > | [<ffffffff8005fc2c>] kthread+0xe8/0x108
> > | [<ffffffff80003ffa>] ret_from_fork+0xe/0x20
> >
> > I'm digging into that now, and I would appreciate if you could run the
> > test_tag on VF2 or similar (I'm missing that HW).
> >
> > It seems like we're hitting a bug with this series, so let's try to
> > figure out where the problems is, prior merging it.
>
> Hmm, it looks like the bpf_arch_text_invalidate() implementation is a
> bit problematic:
>
> +int bpf_arch_text_invalidate(void *dst, size_t len)
> +{
> + __le32 *ptr;
> + int ret = 0;
> + u32 inval = 0;
> +
> + for (ptr = dst; ret == 0 && len >= sizeof(u32); len -= sizeof(u32)) {
> + mutex_lock(&text_mutex);
> + ret = patch_text_nosync(ptr++, &inval, sizeof(u32));
> + mutex_unlock(&text_mutex);
> + }
> +
> + return ret;
> +}
>
> Each patch_text_nosync() is a remote fence.i, and for a big "len", we'll
> be flooded with remote fences.

I understand this now, thanks for debugging this.

We are calling patch_text_nosync() for each word (u32) which calls
flush_icache_range() and therefore "fence.i" is inserted after every word.

I still don't fully understand how it causes this bug because I lack
the prerequisite
knowledge about test_tag and what the failing test is doing.

But to solve this issue we would need a function like the x86
text_poke_set() that will only
insert a single "fence.i" after setting the whole memory area. This
can be done by
implementing a wrapper around patch_insn_write() which would set the memory area
and at the end call flush_icache_range().

Something like:

void *text_set_nosync(void *dst, int c, size_t len)
{
__le32 *ptr;
int ret = 0;

for (ptr = dst; ret == 0 && len >= sizeof(u32); len -= sizeof(u32)) {
ret = patch_insn_write(ptr++, &c, sizeof(u32));
}
if(!ret)
flush_icache_range((uintptr_t) dst, (uintptr_t) dst + len);

return ret;
}

Let me know if this looks correct or we need more details here.
I will then send v2 with this implemented as a separate patch.

>
> I think that's exactly what we hit with "test_tag".
>
>
> Björn

Thanks,
Puranjay