Re: [PATCH RFC bpf-next 4/3] uprobe: ensure sys_uretprobe uses sysret

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Mar 19, 2024 at 12:08:35PM +0100, Jiri Olsa wrote:
> On Tue, Mar 19, 2024 at 11:25:24AM +0100, Oleg Nesterov wrote:
> > Obviously not for inclusion yet ;) untested, lacks the comments, and I am not
> > sure it makes sense.
> > 
> > But I am wondering if this change can speedup uretprobes a bit more. Any chance
> > you can test it?
> > 
> > With 1/3 sys_uretprobe() changes regs->r11/cx, this is correct but implies iret.
> > See the /* SYSRET requires RCX == RIP and R11 == EFLAGS */ code in do_syscall_64().
> 
> nice idea, looks like sysexit should be faster
> 
> > 
> > With this patch uretprobe_syscall_entry restores rcx/r11 itself and does retq,
> > sys_uretprobe() needs to hijack regs->ip after uprobe_handle_trampoline() to
> > make it possible.
> > 
> > Comments?
> > 
> > Oleg.
> > ---
> > 
> > diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> > index 069371e86180..b99f1d80a8c8 100644
> > --- a/arch/x86/kernel/uprobes.c
> > +++ b/arch/x86/kernel/uprobes.c
> > @@ -319,6 +319,9 @@ asm (
> >  	"pushq %r11\n"
> >  	"movq $462, %rax\n"
> >  	"syscall\n"
> > +	"popq %r11\n"
> > +	"popq %rcx\n"
> > +	"retq\n"
> 
> using rax space on stack for return pointer, cool :)
> 
> I'll run the test with this change

I got bigger speed up on intel, amd stays the same (I'll double check that)

current:
  base           :   16.133 ± 0.035M/s
  uprobe-nop     :    3.003 ± 0.002M/s
  uprobe-push    :    2.829 ± 0.001M/s
  uprobe-ret     :    1.101 ± 0.001M/s
  uretprobe-nop  :    1.485 ± 0.001M/s
  uretprobe-push :    1.447 ± 0.000M/s
  uretprobe-ret  :    0.812 ± 0.000M/s

fix:
  base           :   16.522 ± 0.103M/s
  uprobe-nop     :    2.920 ± 0.034M/s
  uprobe-push    :    2.749 ± 0.047M/s
  uprobe-ret     :    1.094 ± 0.003M/s
  uretprobe-nop  :    2.004 ± 0.006M/s  < ~34% speed up
  uretprobe-push :    1.940 ± 0.003M/s  < ~34% speed up
  uretprobe-ret  :    0.921 ± 0.050M/s  < ~13% speed up

original fix:
  base           :   15.704 ± 0.076M/s
  uprobe-nop     :    2.841 ± 0.008M/s
  uprobe-push    :    2.666 ± 0.029M/s
  uprobe-ret     :    1.037 ± 0.008M/s
  uretprobe-nop  :    1.718 ± 0.010M/s  < ~25% speed up
  uretprobe-push :    1.658 ± 0.008M/s  < ~23% speed up
  uretprobe-ret  :    0.853 ± 0.004M/s  < ~14% speed up


jirka