Re: [PATCH] tracing: Fix trace entry and trace common fields for preempt_lazy_count

Jiri Olsa
2020-02-21T16:10:45Z

On Fri, Feb 21, 2020 at 10:49:22AM -0500, Steven Rostedt wrote:
> On Fri, 21 Feb 2020 16:35:41 +0100
> Jiri Olsa <jolsa@kernel.org> wrote:
>
> > When commit 65fd07df3588 added preempt_lazy_count into 'struct trace_entry'
> > it did not add 4 bytes padding. Also we need to update the common fields
> > for tracepoint, otherwise some tools (bpftrace) stop working due to missing
> > common fields.
> >
> > Fixes: 65fd07df3588 ("x86: Support for lazy preemption")
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > include/linux/trace_events.h | 2 ++
> > kernel/trace/trace_events.c | 3 +++
> > 2 files changed, 5 insertions(+)
> >
> > diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
> > index f3b1ef07e4a5..51a3f5188923 100644
> > --- a/include/linux/trace_events.h
> > +++ b/include/linux/trace_events.h
> > @@ -65,6 +65,8 @@ struct trace_entry {
> > unsigned short migrate_disable;
> > unsigned short padding;
> > unsigned char preempt_lazy_count;
> > + unsigned char padding1;
> > + unsigned short padding2;
>
>
> Wait! I don't have these changes in my tree, nor do I see them in
> Linus's. This really bloats the trace events! This header is very
> sensitive to size and just willy nilly adding to it is unacceptable.
> It's like adding to the page_struct. This gets added to *every* event,
> and a single byte added, causes 1M extra for a million events (very
> common in tracing). It causes 1G extra for a billion events.

I'm on top of:
git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
v5.4.19-rt11-rebase

>
> Let's find a better way to handle this.

I can fix the bpftrace tool I guess, through it's not
so convenient the way it's used in it

jirka

>
> -- Steve
>
>
> > };
> >
> > #define TRACE_EVENT_TYPE_MAX \
> > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> > index accaae59a762..1fe37b7aeaff 100644
> > --- a/kernel/trace/trace_events.c
> > +++ b/kernel/trace/trace_events.c
> > @@ -183,6 +183,9 @@ static int trace_define_common_fields(void)
> > __common_field(int, pid);
> > __common_field(unsigned short, migrate_disable);
> > __common_field(unsigned short, padding);
> > + __common_field(unsigned char, preempt_lazy_count);
> > + __common_field(unsigned char, padding1);
> > + __common_field(unsigned short, padding2);
> >
> > return ret;
> > }
>