<s>
Kernel	B-Application
markers	I-Application
were	O
a	O
static	O
kernel	B-Operating_System
instrumentation	O
support	O
mechanism	O
for	O
Linux	B-Operating_System
kernel	I-Operating_System
source	O
code	O
,	O
allowing	O
special	O
tools	O
such	O
as	O
LTTng	B-Application
or	O
SystemTap	B-Application
to	O
trace	O
information	O
exposed	O
by	O
these	O
probe	O
points	O
.	O
</s>
<s>
Kernel	B-Application
markers	I-Application
were	O
declared	O
in	O
the	O
kernel	B-Operating_System
code	O
by	O
one-liners	O
of	O
the	O
form	O
:	O
</s>
<s>
Using	O
markers	O
has	O
a	O
negligible	O
overhead	O
thanks	O
in	O
part	O
to	O
Immediate	O
Values	O
,	O
another	O
support	O
mechanism	O
that	O
embeds	O
switches	O
in	O
the	O
code	O
that	O
can	O
be	O
dynamically	O
turned	O
on	O
and	O
off	O
,	O
without	O
using	O
a	O
memory	O
reference	O
and	O
thus	O
saving	O
cache	B-General_Concept
lines	I-General_Concept
.	O
</s>
<s>
Kernel	B-Application
Markers	I-Application
consisted	O
essentially	O
of	O
a	O
C	B-Language
preprocessing	O
macro	O
which	O
added	O
,	O
in	O
the	O
instrumented	O
function	O
,	O
a	O
branch	O
over	O
a	O
function	O
call	O
.	O
</s>
<s>
By	O
doing	O
so	O
,	O
neither	O
the	O
stack	B-General_Concept
setup	O
nor	O
the	O
function	O
call	O
are	O
executed	O
when	O
instrumentation	O
is	O
not	O
enabled	O
.	O
</s>
<s>
function	O
call	O
as	O
unlikely	O
(	O
using	O
the	O
gcc	B-Application
built-in	O
expect( )	O
)	O
,	O
a	O
hint	O
is	O
given	O
to	O
the	O
compiler	B-Language
to	O
position	O
the	O
tracing	O
instructions	O
away	O
from	O
cache	B-General_Concept
lines	I-General_Concept
involved	O
in	O
standard	O
kernel	B-Operating_System
execution	O
.	O
</s>
<s>
Two	O
Kernel	B-Application
Markers	I-Application
drawbacks	O
were	O
identified	O
which	O
led	O
to	O
its	O
replacement	O
by	O
Tracepoints	B-Application
:	O
</s>
<s>
This	O
makes	O
namespace	O
conventions	O
and	O
tracking	O
of	O
instrumentation	O
modification	O
difficult	O
unless	O
the	O
whole	O
kernel	B-Operating_System
tree	O
is	O
monitored	O
.	O
</s>
<s>
To	O
address	O
issues	O
regarding	O
kernel	B-Application
markers	I-Application
,	O
Mathieu	O
Desnoyers	O
,	O
their	O
original	O
author	O
,	O
implemented	O
a	O
simpler	O
and	O
more	O
type-safe	O
version	O
of	O
static	O
probe	O
points	O
named	O
Tracepoints	B-Application
.	O
</s>
<s>
A	O
patch-set	O
implementing	O
Tracepoints	B-Application
was	O
merged	O
into	O
version	O
2.6.28	O
,	O
which	O
was	O
released	O
on	O
December	O
25	O
,	O
2008	O
.	O
</s>
<s>
Starting	O
then	O
,	O
kernel	B-Application
markers	I-Application
were	O
slowly	O
removed	O
from	O
kernel	B-Operating_System
sources	O
and	O
eventually	O
fully	O
removed	O
in	O
Linux	O
kernel	B-Operating_System
2.6.32	O
,	O
which	O
was	O
released	O
on	O
December	O
3	O
,	O
2009	O
.	O
</s>
