Re: [RFC bpf-next 3/4] bpf: Resolve symbols with kallsyms_lookup_names for kprobe multi link

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Apr 08, 2022 at 04:26:10PM -0700, Alexei Starovoitov wrote:
> On Thu, Apr 07, 2022 at 02:52:23PM +0200, Jiri Olsa wrote:
> > Using kallsyms_lookup_names function to speed up symbols lookup in
> > kprobe multi link attachment and replacing with it the current
> > kprobe_multi_resolve_syms function.
> > 
> > This speeds up bpftrace kprobe attachment:
> > 
> >   # perf stat -r 5 -e cycles ./src/bpftrace -e 'kprobe:x* {  } i:ms:1 { exit(); }'
> >   ...
> >   6.5681 +- 0.0225 seconds time elapsed  ( +-  0.34% )
> > 
> > After:
> > 
> >   # perf stat -r 5 -e cycles ./src/bpftrace -e 'kprobe:x* {  } i:ms:1 { exit(); }'
> >   ...
> >   0.5661 +- 0.0275 seconds time elapsed  ( +-  4.85% )
> > 
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  kernel/trace/bpf_trace.c | 123 +++++++++++++++++++++++----------------
> >  1 file changed, 73 insertions(+), 50 deletions(-)
> > 
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index b26f3da943de..2602957225ba 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -2226,6 +2226,72 @@ struct bpf_kprobe_multi_run_ctx {
> >  	unsigned long entry_ip;
> >  };
> >  
> > +struct user_syms {
> > +	const char **syms;
> > +	char *buf;
> > +};
> > +
> > +static int copy_user_syms(struct user_syms *us, void __user *usyms, u32 cnt)
> > +{
> > +	const char __user **usyms_copy = NULL;
> > +	const char **syms = NULL;
> > +	char *buf = NULL, *p;
> > +	int err = -EFAULT;
> > +	unsigned int i;
> > +	size_t size;
> > +
> > +	size = cnt * sizeof(*usyms_copy);
> > +
> > +	usyms_copy = kvmalloc(size, GFP_KERNEL);
> > +	if (!usyms_copy)
> > +		return -ENOMEM;
> > +
> > +	if (copy_from_user(usyms_copy, usyms, size))
> > +		goto error;
> > +
> > +	err = -ENOMEM;
> > +	syms = kvmalloc(size, GFP_KERNEL);
> > +	if (!syms)
> > +		goto error;
> > +
> > +	/* TODO this potentially allocates lot of memory (~6MB in my tests
> > +	 * with attaching ~40k functions). I haven't seen this to fail yet,
> > +	 * but it could be changed to allocate memory gradually if needed.
> > +	 */
> 
> Why would 6MB kvmalloc fail?
> If we don't have such memory the kernel will be ooming soon anyway.
> I don't think we'll see this kvmalloc triggering oom in practice.
> The verifier allocates a lot more memory to check large programs.
> 
> imo this approach is fine. It's simple.
> Trying to do gradual alloc with realloc would be just guessing.
> 
> Another option would be to ask user space (libbpf) to do the sort.
> There are pros and cons.
> This vmalloc+sort is slightly better imo.

ok, makes sense, will keep it

jirka