linux-kernel - Re: [PATCH tip 0/9] tracing: attach eBPF programs to tracepoints/syscalls/kprobe

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Date:	Wed, 21 Jan 2015 17:49:08 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH tip 0/9] tracing: attach eBPF programs to tracepoints/syscalls/kprobe

On Wed, Jan 21, 2015 at 5:03 PM, Namhyung Kim <namhyung@...nel.org> wrote:
>
> AFAIK a trigger can be fired before allocating a ring buffer if it
> doesn't use the event record (i.e. has filter) or ->post_trigger bit
> set (stacktrace).  Please see ftrace_trigger_soft_disabled().

yes, but such trigger has no arguments, so I would have to hack
ftrace_trigger_soft_disabled() to pass 'ctx' further down
and through all pointer dereferences and list walking.
Also there is no return value, so I have to add it as well
similar to post-triggers.
that's quite a bit of overhead that I would like to avoid.

Actually now I'm thinking to move condition
if (ftrace_file->flags & TRACE_EVENT_FL_BPF)
before ftrace_trigger_soft_disabled() check.
So programs always run first and if they return non-zero
then all standard processing will follow.
May be return value from the program can influence triggers.
That will nicely replace bpf_dump_stack()...the program
will return ETT_STACKTRACE constant to trigger dump.

> This also makes it keeping events in the soft-disabled state.

I was never able to figure out the use case for soft-disabled state.
Probably historical before static_key was done.
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