Re: [PATCH bpf-next v2 0/4] Add ftrace direct call for arm64

From: Florent Revest
Date: Mon Oct 17 2022 - 13:55:37 EST

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On Thu, Oct 6, 2022 at 6:29 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Thu, 6 Oct 2022 18:19:12 +0200
> Florent Revest <revest@xxxxxxxxxxxx> wrote:
>
> > Sure, we can give this a try, I'll work on a macro that generates the
> > 7 callbacks and we can check how much that helps. My belief right now
> > is that ftrace's iteration over all ops on arm64 is where we lose most
> > time but now that we have numbers it's pretty easy to check hypothesis
> > :)
>
> Ah, I forgot that's what Mark's code is doing. But yes, that needs to be
> fixed first. I forget that arm64 doesn't have the dedicated trampolines yet.
>
> So, let's hold off until that is complete.
>
> -- Steve

Mark finished an implementation of his per-callsite-ops and min-args
branches (meaning that we can now skip the expensive ftrace's saving
of all registers and iteration over all ops if only one is attached)
- https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64-ftrace-call-ops-20221017

And Masami wrote similar patches to what I had originally done to
fprobe in my branch:
- https://github.com/mhiramat/linux/commits/kprobes/fprobe-update

So I could rebase my previous "bpf on fprobe" branch on top of these:
(as before, it's just good enough for benchmarking and to give a
general sense of the idea, not for a thorough code review):
- https://github.com/FlorentRevest/linux/commits/fprobe-min-args-3

And I could run the benchmarks against my rpi4. I have different
baseline numbers as Xu so I ran everything again and tried to keep the
format the same. "indirect call" refers to my branch I just linked and
"direct call" refers to the series this is a reply to (Xu's work)

1. test with dd

1.1 when no bpf prog attached to vfs_write

# dd if=/dev/zero of=/dev/null count=1000000
1000000+0 records in
1000000+0 records out
512000000 bytes (512 MB, 488 MiB) copied, 3.94315 s, 130 MB/s

1.2 attach bpf prog with kprobe, bpftrace -e kprobe:vfs_write {}

# dd if=/dev/zero of=/dev/null count=1000000
1000000+0 records in
1000000+0 records out
512000000 bytes (512 MB, 488 MiB) copied, 5.80493 s, 88.2 MB/s

1.3 attach bpf prog with with direct call, bpftrace -e kfunc:vfs_write {}

# dd if=/dev/zero of=/dev/null count=1000000
1000000+0 records in
1000000+0 records out
512000000 bytes (512 MB, 488 MiB) copied, 4.18579 s, 122 MB/s

1.4 attach bpf prog with with indirect call, bpftrace -e kfunc:vfs_write {}

# dd if=/dev/zero of=/dev/null count=1000000
1000000+0 records in
1000000+0 records out
512000000 bytes (512 MB, 488 MiB) copied, 4.92616 s, 104 MB/s

2. test with bpf/bench

2.1 bench trig-base
Iter 0 ( 86.518us): hits 0.700M/s ( 0.700M/prod), drops
0.000M/s, total operations 0.700M/s
Iter 1 (-26.352us): hits 0.701M/s ( 0.701M/prod), drops
0.000M/s, total operations 0.701M/s
Iter 2 ( 1.092us): hits 0.701M/s ( 0.701M/prod), drops
0.000M/s, total operations 0.701M/s
Iter 3 ( -1.890us): hits 0.701M/s ( 0.701M/prod), drops
0.000M/s, total operations 0.701M/s
Iter 4 ( -2.315us): hits 0.701M/s ( 0.701M/prod), drops
0.000M/s, total operations 0.701M/s
Iter 5 ( 4.184us): hits 0.701M/s ( 0.701M/prod), drops
0.000M/s, total operations 0.701M/s
Iter 6 ( -3.241us): hits 0.701M/s ( 0.701M/prod), drops
0.000M/s, total operations 0.701M/s
Summary: hits 0.701 ± 0.000M/s ( 0.701M/prod), drops 0.000 ±
0.000M/s, total operations 0.701 ± 0.000M/s

2.2 bench trig-kprobe
Iter 0 ( 96.833us): hits 0.290M/s ( 0.290M/prod), drops
0.000M/s, total operations 0.290M/s
Iter 1 (-20.834us): hits 0.291M/s ( 0.291M/prod), drops
0.000M/s, total operations 0.291M/s
Iter 2 ( -2.426us): hits 0.291M/s ( 0.291M/prod), drops
0.000M/s, total operations 0.291M/s
Iter 3 ( 22.332us): hits 0.292M/s ( 0.292M/prod), drops
0.000M/s, total operations 0.292M/s
Iter 4 (-18.204us): hits 0.292M/s ( 0.292M/prod), drops
0.000M/s, total operations 0.292M/s
Iter 5 ( 5.370us): hits 0.292M/s ( 0.292M/prod), drops
0.000M/s, total operations 0.292M/s
Iter 6 ( -7.853us): hits 0.290M/s ( 0.290M/prod), drops
0.000M/s, total operations 0.290M/s
Summary: hits 0.291 ± 0.001M/s ( 0.291M/prod), drops 0.000 ±
0.000M/s, total operations 0.291 ± 0.001M/s

2.3 bench trig-fentry, with direct call
Iter 0 ( 86.481us): hits 0.530M/s ( 0.530M/prod), drops
0.000M/s, total operations 0.530M/s
Iter 1 (-12.593us): hits 0.536M/s ( 0.536M/prod), drops
0.000M/s, total operations 0.536M/s
Iter 2 ( -5.760us): hits 0.532M/s ( 0.532M/prod), drops
0.000M/s, total operations 0.532M/s
Iter 3 ( 1.629us): hits 0.532M/s ( 0.532M/prod), drops
0.000M/s, total operations 0.532M/s
Iter 4 ( -1.945us): hits 0.533M/s ( 0.533M/prod), drops
0.000M/s, total operations 0.533M/s
Iter 5 ( -1.297us): hits 0.532M/s ( 0.532M/prod), drops
0.000M/s, total operations 0.532M/s
Iter 6 ( 0.444us): hits 0.535M/s ( 0.535M/prod), drops
0.000M/s, total operations 0.535M/s
Summary: hits 0.533 ± 0.002M/s ( 0.533M/prod), drops 0.000 ±
0.000M/s, total operations 0.533 ± 0.002M/s

2.3 bench trig-fentry, with indirect call
Iter 0 ( 84.463us): hits 0.404M/s ( 0.404M/prod), drops
0.000M/s, total operations 0.404M/s
Iter 1 (-16.260us): hits 0.405M/s ( 0.405M/prod), drops
0.000M/s, total operations 0.405M/s
Iter 2 ( -1.038us): hits 0.405M/s ( 0.405M/prod), drops
0.000M/s, total operations 0.405M/s
Iter 3 ( -3.797us): hits 0.405M/s ( 0.405M/prod), drops
0.000M/s, total operations 0.405M/s
Iter 4 ( -0.537us): hits 0.402M/s ( 0.402M/prod), drops
0.000M/s, total operations 0.402M/s
Iter 5 ( 3.536us): hits 0.403M/s ( 0.403M/prod), drops
0.000M/s, total operations 0.403M/s
Iter 6 ( 12.203us): hits 0.404M/s ( 0.404M/prod), drops
0.000M/s, total operations 0.404M/s
Summary: hits 0.404 ± 0.001M/s ( 0.404M/prod), drops 0.000 ±
0.000M/s, total operations 0.404 ± 0.001M/s

3. perf report of bench trig-fentry

3.1 with direct call

98.67% 0.27% bench bench
[.] trigger_producer
|
--98.40%--trigger_producer
|
|--96.63%--syscall
| |
| --71.90%--el0t_64_sync
| el0t_64_sync_handler
| el0_svc
| do_el0_svc
| |
| |--70.94%--el0_svc_common
| | |
| |
|--29.55%--invoke_syscall
| | | |
| | |
|--26.23%--__arm64_sys_getpgid
| | | |
|
| | | |
|--18.88%--bpf_trampoline_6442462665_0
| | | |
| |
| | | |
| |--6.85%--__bpf_prog_enter
| | | |
| | |
| | | |
| | --2.68%--migrate_disable
| | | |
| |
| | | |
| |--5.28%--__bpf_prog_exit
| | | |
| | |
| | | |
| | --1.29%--migrate_enable
| | | |
| |
| | | |
|
|--3.96%--bpf_prog_21856463590f61f1_bench_trigger_fentry
| | | |
| |
| | | |
| --0.61%--__rcu_read_lock
| | | |
|
| | | |
--4.42%--find_task_by_vpid
| | | |
|
| | | |
|--2.53%--radix_tree_lookup
| | | |
|
| | | |
--0.61%--idr_find
| | | |
| | |
--0.81%--pid_vnr
| | |
| |
--0.53%--__arm64_sys_getpgid
| |
| --0.95%--invoke_syscall
|
--0.99%--syscall@plt

3.2 with indirect call

98.68% 0.20% bench bench
[.] trigger_producer
|
--98.48%--trigger_producer
|
--97.47%--syscall
|
--76.11%--el0t_64_sync
el0t_64_sync_handler
el0_svc
do_el0_svc
|
|--75.52%--el0_svc_common
| |
|
|--46.35%--invoke_syscall
| | |
| |
--44.06%--__arm64_sys_getpgid
| |
|
| |
|--35.40%--ftrace_caller
| |
| |
| |
| --34.04%--fprobe_handler
| |
| |
| |
| |--15.61%--bpf_fprobe_entry
| |
| | |
| |
| | |--3.79%--__bpf_prog_enter
| |
| | | |
| |
| | |
--0.80%--migrate_disable
| |
| | |
| |
| | |--3.74%--__bpf_prog_exit
| |
| | | |
| |
| | |
--0.77%--migrate_enable
| |
| | |
| |
| |
--2.65%--bpf_prog_21856463590f61f1_bench_trigger_fentry
| |
| |
| |
| |--12.65%--rethook_trampoline_handler
| |
| |
| |
| |--1.70%--rethook_try_get
| |
| | |
| |
| | --1.48%--rcu_is_watching
| |
| |
| |
| |--1.46%--freelist_try_get
| |
| |
| |
| --0.65%--rethook_recycle
| |
|
| |
--6.36%--find_task_by_vpid
| |
|
| |
|--3.64%--radix_tree_lookup
| |
|
| |
--1.74%--idr_find
| |
| --1.05%--ftrace_caller
|
--0.59%--invoke_syscall

This looks slightly better than before but it is actually still a
pretty significant performance hit compared to direct calls.

Note that I can't really make sense of the perf report with indirect
calls. it always reports it spent 12% of the time in
rethook_trampoline_handler but I verified with both a WARN in that
function and a breakpoint with a debugger, this function does *not*
get called when running this "bench trig-fentry" benchmark. Also it
wouldn't make sense for fprobe_handler to call it so I'm quite
confused why perf would report this call and such a long time spent
there. Anyone know what I could be missing here ?

Next message: Gautam Menghani: "[PATCH v2] selftests/kvm: Avoid for loop initialization using vcpu->id in xapic_state_test"
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Next in thread: Steven Rostedt: "Re: [PATCH bpf-next v2 0/4] Add ftrace direct call for arm64"
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