Re: [RFC bpf-next 08/16] bpf: Use delayed link free in bpf_link_put

[Jiri Olsa <jolsa@redhat.com>]

On Fri, Oct 23, 2020 at 12:46:15PM -0700, Andrii Nakryiko wrote:
> On Thu, Oct 22, 2020 at 8:01 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Moving bpf_link_free call into delayed processing so we don't
> > need to wait for it when releasing the link.
> >
> > For example bpf_tracing_link_release could take considerable
> > amount of time in bpf_trampoline_put function due to
> > synchronize_rcu_tasks call.
> >
> > It speeds up bpftrace release time in following example:
> >
> > Before:
> >
> >  Performance counter stats for './src/bpftrace -ve kfunc:__x64_sys_s*
> >     { printf("test\n"); } i:ms:10 { printf("exit\n"); exit();}' (5 runs):
> >
> >      3,290,457,628      cycles:k                                 ( +-  0.27% )
> >        933,581,973      cycles:u                                 ( +-  0.20% )
> >
> >              50.25 +- 4.79 seconds time elapsed  ( +-  9.53% )
> >
> > After:
> >
> >  Performance counter stats for './src/bpftrace -ve kfunc:__x64_sys_s*
> >     { printf("test\n"); } i:ms:10 { printf("exit\n"); exit();}' (5 runs):
> >
> >      2,535,458,767      cycles:k                                 ( +-  0.55% )
> >        940,046,382      cycles:u                                 ( +-  0.27% )
> >
> >              33.60 +- 3.27 seconds time elapsed  ( +-  9.73% )
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  kernel/bpf/syscall.c | 8 ++------
> >  1 file changed, 2 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 1110ecd7d1f3..61ef29f9177d 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -2346,12 +2346,8 @@ void bpf_link_put(struct bpf_link *link)
> >         if (!atomic64_dec_and_test(&link->refcnt))
> >                 return;
> >
> > -       if (in_atomic()) {
> > -               INIT_WORK(&link->work, bpf_link_put_deferred);
> > -               schedule_work(&link->work);
> > -       } else {
> > -               bpf_link_free(link);
> > -       }
> > +       INIT_WORK(&link->work, bpf_link_put_deferred);
> > +       schedule_work(&link->work);
> 
> We just recently reverted this exact change. Doing this makes it
> non-deterministic from user-space POV when the BPF program is
> **actually** detached. This makes user-space programming much more
> complicated and unpredictable. So please don't do this. Let's find
> some other way to speed this up.

ok, makes sense

jirka

> 
> >  }
> >
> >  static int bpf_link_release(struct inode *inode, struct file *filp)
> > --
> > 2.26.2
> >
>