Re: [PATCH] bpf: Put perf_events check ahead of bpf prog

[Masami Hiramatsu]

On 2015/06/27 15:44, He Kuang wrote:
> When we add a kprobe point and record events by perf, the execution path
> of all threads on each cpu will enter this point, but perf may only
> record events on a particular thread or cpu at this kprobe point, a
> check on call->perf_events list filters out the threads which perf is
> not recording.
> 
> Currently, bpf_prog will be entered at the beginning of
> kprobe_perf_func() before the above check, which makes bpf_prog be
> executed in every threads including determined not to be recorded
> threads. A simple test can demonstrate this:
> 
> 'bpf_prog_on_write.o' contains a bpf prog which outputs to trace buffer
> when it is entered. Run a background thread 'another-dd' and 'dd'
> simultaneously, but only record 'dd' thread by perf. The result shows
> all threads trigger bpf_prog.
> 
>   $ another-dd if=/dev/zero of=test1 bs=4k count=1000000
>   $ perf record -v --event bpf_prog_on_write.o -- dd if=/dev/zero of=test2 bs=4k count=3
>   $ cat /sys/kernel/debug/tracing/trace
>   another-dd-1007  [000] d...   120.225835: : generic_perform_write: tgid=1007, pid=1007
>   another-dd-1007  [000] d...   120.227123: : generic_perform_write: tgid=1007, pid=1007
>   [repeat many times...]
>   another-dd-1007  [000] d...   120.412395: : generic_perform_write: tgid=1007, pid=1007
>   another-dd-1007  [000] d...   120.412524: : generic_perform_write: tgid=1007, pid=1007
>           dd-1009  [000] d...   120.413080: : generic_perform_write: tgid=1009, pid=1009
>           dd-1009  [000] d...   120.414846: : generic_perform_write: tgid=1009, pid=1009
>           dd-1009  [000] d...   120.415013: : generic_perform_write: tgid=1009, pid=1009
>   another-dd-1007  [000] d...   120.416128: : generic_perform_write: tgid=1007, pid=1007
>   another-dd-1007  [000] d...   120.416295: : generic_perform_write: tgid=1007, pid=1007
> 
> This patch moves the check on perf_events list ahead and skip running
> bpf_prog on threads perf not care.
> 
> After this patch:
> 
>   $ another-dd if=/dev/zero of=test1 bs=4k count=1000000
>   $ perf record -v --event bpf_prog_on_write.o -- dd if=/dev/zero of=test2 bs=4k count=3
>   $ cat /sys/kernel/debug/tracing/trace
>   dd-994   [000] d...    46.386754: : generic_perform_write: tgid=994, pid=994
>   dd-994   [000] d...    46.389167: : generic_perform_write: tgid=994, pid=994
>   dd-994   [000] d...    46.389551: : generic_perform_write: tgid=994, pid=994

This looks nice :)

Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>

Thank you!
> 
> Signed-off-by: He Kuang <hekuang@xxxxxxxxxx>
> ---
>  kernel/trace/trace_kprobe.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index d0ce590..5600df8 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -1141,13 +1141,13 @@ kprobe_perf_func(struct trace_kprobe *tk, struct pt_regs *regs)
>  	int size, __size, dsize;
>  	int rctx;
>  
> -	if (prog && !trace_call_bpf(prog, regs))
> -		return;
> -
>  	head = this_cpu_ptr(call->perf_events);
>  	if (hlist_empty(head))
>  		return;
>  
> +	if (prog && !trace_call_bpf(prog, regs))
> +		return;
> +
>  	dsize = __get_data_size(&tk->tp, regs);
>  	__size = sizeof(*entry) + tk->tp.size + dsize;
>  	size = ALIGN(__size + sizeof(u32), sizeof(u64));
> @@ -1176,13 +1176,13 @@ kretprobe_perf_func(struct trace_kprobe *tk, struct kretprobe_instance *ri,
>  	int size, __size, dsize;
>  	int rctx;
>  
> -	if (prog && !trace_call_bpf(prog, regs))
> -		return;
> -
>  	head = this_cpu_ptr(call->perf_events);
>  	if (hlist_empty(head))
>  		return;
>  
> +	if (prog && !trace_call_bpf(prog, regs))
> +		return;
> +
>  	dsize = __get_data_size(&tk->tp, regs);
>  	__size = sizeof(*entry) + tk->tp.size + dsize;
>  	size = ALIGN(__size + sizeof(u32), sizeof(u64));
> 


-- 
Masami HIRAMATSU
Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
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