On Mon, Mar 11, 2024 at 2:26 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Mon, Mar 11, 2024 at 10:32:23AM -0700, Andrii Nakryiko wrote:
> > On Mon, Mar 11, 2024 at 3:59 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> > >
> > > On Tue, Mar 05, 2024 at 09:24:08AM +0100, Jiri Olsa wrote:
> > > > On Mon, Mar 04, 2024 at 04:55:33PM -0800, Andrii Nakryiko wrote:
> > > > > On Sun, Mar 3, 2024 at 2:20 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> > > > > >
> > > > > > On Fri, Mar 01, 2024 at 09:26:57AM -0800, Andrii Nakryiko wrote:
> > > > > > > On Fri, Mar 1, 2024 at 9:01 AM Alexei Starovoitov
> > > > > > > <alexei.starovoitov@xxxxxxxxx> wrote:
> > > > > > > >
> > > > > > > > On Fri, Mar 1, 2024 at 12:18 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> > > > > > > > >
> > > > > > > > > On Thu, Feb 29, 2024 at 04:25:17PM -0800, Andrii Nakryiko wrote:
> > > > > > > > > > On Thu, Feb 29, 2024 at 6:39 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> > > > > > > > > > >
> > > > > > > > > > > One of uprobe pain points is having slow execution that involves
> > > > > > > > > > > two traps in worst case scenario or single trap if the original
> > > > > > > > > > > instruction can be emulated. For return uprobes there's one extra
> > > > > > > > > > > trap on top of that.
> > > > > > > > > > >
> > > > > > > > > > > My current idea on how to make this faster is to follow the optimized
> > > > > > > > > > > kprobes and replace the normal uprobe trap instruction with jump to
> > > > > > > > > > > user space trampoline that:
> > > > > > > > > > >
> > > > > > > > > > > - executes syscall to call uprobe consumers callbacks
> > > > > > > > > >
> > > > > > > > > > Did you get a chance to measure relative performance of syscall vs
> > > > > > > > > > int3 interrupt handling? If not, do you think you'll be able to get
> > > > > > > > > > some numbers by the time the conference starts? This should inform the
> > > > > > > > > > decision whether it even makes sense to go through all the trouble.
> > > > > > > > >
> > > > > > > > > right, will do that
> > > > > > > >
> > > > > > > > I believe Yusheng measured syscall vs uprobe performance
> > > > > > > > difference during LPC. iirc it was something like 3x.
> > > > > > >
> > > > > > > Do you have a link to slides? Was it actual uprobe vs just some fast
> > > > > > > syscall (not doing BPF program execution) comparison? Or comparing the
> > > > > > > performance of int3 handling vs equivalent syscall handling.
> > > > > > >
> > > > > > > I suspect it's the former, and so probably not that representative.
> > > > > > > I'm curious about the performance of going
> > > > > > > userspace->kernel->userspace through int3 vs syscall (all other things
> > > > > > > being equal).
> > > > > >
> > > > > > I have a simple test [1] comparing:
> > > > > > - uprobe with 2 traps
> > > > > > - uprobe with 1 trap
> > > > > > - syscall executing uprobe
> > > > > >
> > > > > > the syscall takes uprobe address as argument, finds the uprobe and executes
> > > > > > its consumers, which should be comparable to what the trampoline will do
> > > > > >
> > > > > > test does same amount of loops triggering each uprobe type and measures
> > > > > > the time it took
> > > > > >
> > > > > > # ./test_progs -t uprobe_syscall_bench -v
> > > > > > bpf_testmod.ko is already unloaded.
> > > > > > Loading bpf_testmod.ko...
> > > > > > Successfully loaded bpf_testmod.ko.
> > > > > > test_bench_1:PASS:uprobe_bench__open_and_load 0 nsec
> > > > > > test_bench_1:PASS:uprobe_bench__attach 0 nsec
> > > > > > test_bench_1:PASS:uprobe1_cnt 0 nsec
> > > > > > test_bench_1:PASS:syscalls_uprobe1_cnt 0 nsec
> > > > > > test_bench_1:PASS:uprobe2_cnt 0 nsec
> > > > > > test_bench_1: uprobes (1 trap) in 36.439s
> > > > > > test_bench_1: uprobes (2 trap) in 91.960s
> > > > > > test_bench_1: syscalls in 17.872s
> > > > > > #395/1 uprobe_syscall_bench/bench_1:OK
> > > > > > #395 uprobe_syscall_bench:OK
> > > > > > Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED
> > > > > >
> > > > > > syscall uprobe execution seems to be ~2x faster than 1 trap uprobe
> > > > > > and ~5x faster than 2 traps uprobe
> > > > > >
> > > > >
> > > > > Thanks for running benchmarks! I quickly looked at the selftest and
> > > > > noticed this:
> > > > >
> > > > > +/*
> > > > > + * Assuming following prolog:
> > > > > + *
> > > > > + * 6984ac: 55 push %rbp
> > > > > + * 6984ad: 48 89 e5 mov %rsp,%rbp
> > > > > + */
> > > > > +noinline void uprobe2_bench_trigger(void)
> > > > > +{
> > > > > + asm volatile ("");
> > > > > +}
> > > > >
> > > > > This actually will be optimized out to just ret in -O2 mode (make
> > > > > RELEASE=1 for selftests):
> > > > >
> > > > > 00000000005a0ce0 <uprobe2_bench_trigger>:
> > > > > 5a0ce0: c3 retq
> > > > > 5a0ce1: 66 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:(%rax,%rax)
> > > > > 5a0cec: 0f 1f 40 00 nopl (%rax)
> > > > >
> > > > > So be careful with that.
> > > >
> > > > right, I did not mean for this to be checked in, just wanted to get the
> > > > numbers quickly
> > > >
> > > > >
> > > > > Also, I just updated our existing set of uprobe benchmarks (see [0]),
> > > > > do you mind adding your syscall-based one as another one there and
> > > > > running all of them and sharing the numbers with us? Very curious to
> > > > > see both absolute and relative numbers from that benchmark. (and
> > > > > please do build with RELEASE=1)
> > > > >
> > > > > You should be able to just run benchs/run_bench_uprobes.sh (also don't
> > > > > forget to add your syscall-based benchmark to the list of benchmarks
> > > > > in that shell script).
> > > >
> > > > yes, saw it and was going to run/compare it.. it's good idea to add
> > > > the syscall one and get all numbers together, will do that
> > > >
> > > > >
> > > > > Thank you!
> > > > >
> > > > >
> > > > > BTW, while I think patching multiple instructions for syscall-based
> > > > > uprobe is going to be extremely tricky, I think at least u*ret*probe's
> > > > > int3 can be pretty easily optimized away with syscall, given that the
> > > > > kernel controls code generation there. If anything, it will get the
> > > > > uretprobe case a bit closer to the performance of uprobe. Give it some
> > > > > thought.
> > > >
> > > > hm, right.. the trampoline is there already, but at the moment is global
> > > > and used by all uretprobes.. and int3 code moves userspace (changes rip)
> > > > to the original return address.. maybe we can do that through syscall
> > > > as well
> > >
> > > it seems like good idea, I tried change below (use syscall on return
> > > trampoline) and got some speedup:
> > >
> > > current:
> > > base : 15.817 ± 0.009M/s
> > > uprobe-nop : 2.901 ± 0.000M/s
> > > uprobe-push : 2.743 ± 0.002M/s
> > > uprobe-ret : 1.089 ± 0.001M/s
> > > uretprobe-nop : 1.448 ± 0.001M/s
> > > uretprobe-push : 1.407 ± 0.001M/s
> > > uretprobe-ret : 0.792 ± 0.001M/s
> > >
> > > with syscall:
> > > base : 15.831 ± 0.026M/s
> > > uprobe-nop : 2.904 ± 0.001M/s
> > > uprobe-push : 2.764 ± 0.002M/s
> > > uprobe-ret : 1.082 ± 0.001M/s
> > > uretprobe-nop : 1.785 ± 0.000M/s
> > > uretprobe-push : 1.733 ± 0.001M/s
> > > uretprobe-ret : 0.885 ± 0.004M/s
> > >
> > > ~23% for nop/push (emulated) cases, ~11% for ret (sstep) case
> > >
> > > jirka
> >
> > heh, I tried this as well over weekend, though I cut few more corners
> > (see diff below, I didn't add saving/restoring rax, though that would
> > be required, of course). My test machine is (way) slower, though, so I
> > got a slightly different numbers (up to 15%):
>
> nice :-) btw I just checked on another slower amd server and it's ~10% in
> all 3 cases, my previous results are from intel machine.. I guess the hw
> trap behaviour/speed makes this not proportional across archs
>
> >
> > ### baseline
> > uprobe-base : 79.462 ± 0.058M/s
> > base : 2.920 ± 0.004M/s
> > uprobe-nop : 1.093 ± 0.001M/s
> > uprobe-push : 1.066 ± 0.001M/s
> > uprobe-ret : 0.480 ± 0.001M/s
> > uretprobe-nop : 0.555 ± 0.000M/s
> > uretprobe-push : 0.549 ± 0.000M/s
> > uretprobe-ret : 0.338 ± 0.000M/s
> >
> >
> > ### uretprobe syscall (vs baseline)
> > uprobe-base : 79.488 ± 0.033M/s
> > base : 2.917 ± 0.003M/s
> > uprobe-nop : 1.095 ± 0.001M/s
> > uprobe-push : 1.058 ± 0.000M/s
> > uprobe-ret : 0.483 ± 0.000M/s
> > uretprobe-nop : 0.638 ± 0.000M/s (+15%)
> > uretprobe-push : 0.627 ± 0.000M/s (+14.2%)
> > uretprobe-ret : 0.366 ± 0.000M/s (+8.3%)
> >
> > Either way, yes, we should implement this. Are you planning to send an
> > official patch some time soon? I'm working on other small improvements
> > in uprobe/uretprobe, I'll probably send the first patches
> > today/tomorrow, but they shouldn't interfere with this uretprobe code
> > path.
>
> yes, wanted to finish/post it this week
>
great, looking forward, we can use these speeds up for uretprobe in
our production
> SNIP
>
> > > ---
> > > diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> > > index 7e8d46f4147f..fa5f8a058bc2 100644
> > > --- a/arch/x86/entry/syscalls/syscall_64.tbl
> > > +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> > > @@ -383,6 +383,7 @@
> > > 459 common lsm_get_self_attr sys_lsm_get_self_attr
> > > 460 common lsm_set_self_attr sys_lsm_set_self_attr
> > > 461 common lsm_list_modules sys_lsm_list_modules
> > > +462 64 uprobe sys_uprobe
> > >
> >
> > we should call it "uretprobe", "uprobe" will be a separate thing with
> > different logic.
> >
> > I went with generic "trace", but realized that it would be better to
> > have separate more targeted "special/internal" syscalls (where, if
> > necessary, extra arguments would be passed through stack to avoid
> > storing/restoring user-space registers). We have rg_sigreturn
> > precedent which explicitly states that userspace shouldn't use it and
> > shouldn't rely on any specific arguments conventions.
>
> somehow I thought of syscalls as of scare resource and wanted to add
> arguments/commands to the uprobe syscalls.. but having uretprobe
> dedicated syscall makes things easier
given we are at 462 already, not sure I believe it's scarce :) but
it's also a performance aspect, not using any arguments means we can
avoid saving/restoring user-space registers, so it makes sense to have
2-3 dedicated uprobe/uretprobe syscalls vs 1 more generic on (IMO), at
least from performance POV.
>
> >
> > [...]
> >
> > > /*
> > > * Deprecated system calls which are still defined in
> > > diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> > > index f46e0ca0169c..9ef244c8ff19 100644
> > > --- a/include/linux/uprobes.h
> > > +++ b/include/linux/uprobes.h
> > > @@ -138,6 +138,8 @@ extern bool arch_uretprobe_is_alive(struct return_instance *ret, enum rp_check c
> > > extern bool arch_uprobe_ignore(struct arch_uprobe *aup, struct pt_regs *regs);
> > > extern void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
> > > void *src, unsigned long len);
> > > +extern void uprobe_handle_trampoline(struct pt_regs *regs);
> > > +uprobe_opcode_t* arch_uprobe_trampoline(unsigned long *psize);
> >
> > just `void *` here? it can be a sequence of instructions now
>
> hm, it's pointer to u8, which should be fine no? is there benefit to
> have void* in here instead?
>
Quick grepping initially brought up `typedef u32 uprobe_opcode_t;`,
but that's for non-x86 architectures. I don't think it matters all
that much (in my mind it's just a generated code blob, so just `void
*` memory, we don't have to look at its contents).
> thanks,
> jirka
