Re: [PATCH 2/2] sched/debug: add sched_update_nr_running tracepoint

From: Joel Fernandes <joel@joelfernandes.org>
To: Qais Yousef <qais.yousef@arm.com>
Cc: "Valentin Schneider" <valentin.schneider@arm.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	linux-kernel@vger.kernel.org, "Ingo Molnar" <mingo@redhat.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Jirka Hladký" <jhladky@redhat.com>,
	"Jiří Vozár" <jvozar@redhat.com>,
	x86@kernel.org
Subject: Re: [PATCH 2/2] sched/debug: add sched_update_nr_running tracepoint
Date: Wed, 4 Sep 2019 11:46:26 -0400	[thread overview]
Message-ID: <20190904154626.GL240514@google.com> (raw)
In-Reply-To: <20190904145759.xljofuqibwbwxzfx@e107158-lin.cambridge.arm.com>

On Wed, Sep 04, 2019 at 03:57:59PM +0100, Qais Yousef wrote:
> On 09/04/19 10:41, Joel Fernandes wrote:
> > On Wed, Sep 04, 2019 at 03:20:17PM +0100, Qais Yousef wrote:
> > > On 09/04/19 09:06, Joel Fernandes wrote:
> > > > > 
> > > > > It is actually true.
> > > > >
> > > > > But you need to make the distinction between a tracepoint
> > > > > and a trace event first.
> > > > 
> > > > I know this distinction well.
> > > > 
> > > > > What Valentin is talking about here is the *bare*
> > > > > tracepoint without any event associated with them like the one I added to the
> > > > > scheduler recently. These ones are not accessible via eBPF, unless something
> > > > > has changed since I last tried.
> > > > 
> > > > Can this tracepoint be registered on with tracepoint_probe_register()?
> > > > Quickly looking at these new tracepoint, they can be otherwise how would they
> > > > even work right? If so, then eBPF can very well access it. Look at
> > > > __bpf_probe_register() and bpf_raw_tracepoint_open() which implement the
> > > > BPF_RAW_TRACEPOINT_OPEN.
> > > 
> > > Humm okay. I tried to use raw tracepoint with bcc but failed to attach. But
> > > maybe I missed something on the way it should be used. AFAICT it was missing
> > > the bits that I implemented in [1]. Maybe the method you mention is lower level
> > > than bcc.
> > 
> > Oh, Ok. Not sure about BCC. I know that facebook folks are using *existing*
> > tracepoints (not trace events) to probe context switches and such (probably
> > not through BCC but some other BPF tracing code). Peter had rejected trace
> > events they were trying to add IIRC, so they added BPF_RAW_TRACEPOINT_OPEN
> > then IIRC.
> 
> Looking at the history BPF_RAW_TRACEPOINT_OPEN was added with the support for
> RAW_TRACEPOINT c4f6699dfcb8 (bpf: introduce BPF_RAW_TRACEPOINT).
> 
> Anyway, if you ever get a chance please try it and let me know. I might have
> done something wrong and you're more of a eBPF guru than I am :-)

eBPF guru and me? no way ;-) I have tried out BPF_RAW_TRACEPOINT_OPEN before
and it works as expected. Are there not any in-kernel samples? Perhaps Alexei
can post some if there are not.

thanks,

 - Joel