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

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Message-ID: <CAMEtUuz9bcaYN0if1w1O01vwc593P2ACUX-KK++6Jeqz7MwFsg@mail.gmail.com>
Date:	Wed, 21 Jan 2015 18:13:23 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Namhyung Kim <namhyung@...nel.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 5:56 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed, 21 Jan 2015 17:49:08 -0800
> Alexei Starovoitov <ast@...mgrid.com> wrote:
>
>
>> > 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.
>
> No, it's not historical at all. The "soft-disable" is a way to enable
> from any context. You can't enable a static key from NMI or interrupt
> context, but you can enable a "soft-disable" there.
>
> As you can enable or disable events from any function that the function
> tracer may trace, I needed a way to enable them (make the tracepoint
> active), but do nothing until something else turns them on.

Thanks for explanation. Makes sense.
Speaking of nmi... I think I will add a check that if (in_nmi())
just skip running the program, since supporting this use
case is not needed at the moment.
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