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

["Björn Töpel" <bjorn@kernel.org>]

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


Björn

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