On Thu, Oct 22, 2020 at 8:01 AM Jiri Olsa <jolsa@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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.
> }
>
> static int bpf_link_release(struct inode *inode, struct file *filp)
> --
> 2.26.2
>
