- Fix broken direct trampolines being called when another callback is
attached the same function. ARM 64 does not support FTRACE_WITH_REGS, and
when it added direct trampoline calls from ftrace, it removed the
"WITH_REGS" flag from the ftrace_ops for direct trampolines. This broke
x86 as x86 requires direct trampolines to have WITH_REGS. This wasn't
noticed because direct trampolines work as long as the function it is
attached to is not shared with other callbacks (like the function tracer).
When there's other callbacks, a helper trampoline is called, to call all
the non direct callbacks and when it returns, the direct trampoline is
called. For x86, the direct trampoline sets a flag in the regs field to
tell the x86 specific code to call the direct trampoline. But this only
works if the ftrace_ops had WITH_REGS set. ARM does things differently
that does not require this. For now, set WITH_REGS if the arch supports
WITH_REGS (which ARM does not), and this makes it work for both ARM64 and
x86.
- Fix wasted memory in the saved_cmdlines logic.
The saved_cmdlines is a cache that maps PIDs to COMMs that tracing can
use. Most trace events only save the PID in the event. The saved_cmdlines
file lists PIDs to COMMs so that the tracing tools can show an actual name
and not just a PID for each event. There's an array of PIDs that map to a
small set of saved COMM strings. The array is set to PID_MAX_DEFAULT which
is usually set to 32768. When a PID comes in, it will add itself to this
array along with the index into the COMM array (note if the system allows
more than PID_MAX_DEFAULT, this cache is similar to cache lines as an
update of a PID that has the same PID_MAX_DEFAULT bits set will flush out
another task with the same matching bits set).
A while ago, the size of this cache was changed to be dynamic and the
array was moved into a structure and created with kmalloc(). But this
new structure had the size of 131104 bytes, or 0x20020 in hex. As kmalloc
allocates in powers of two, it was actually allocating 0x40000 bytes
(262144) leaving 131040 bytes of wasted memory. The last element of this
structure was a pointer to the COMM string array which defaulted to just
saving 128 COMMs.
By changing the last field of this structure to a variable length string,
and just having it round up to fill the allocated memory, the default
size of the saved COMM cache is now 8190. This not only uses the wasted
space, but actually saves space by removing the extra allocation for the
COMM names.
-----BEGIN PGP SIGNATURE-----
iIoEABYIADIWIQRRSw7ePDh/lE+zeZMp5XQQmuv6qgUCZcYi8RQccm9zdGVkdEBn
b29kbWlzLm9yZwAKCRAp5XQQmuv6qqENAQD6xGE9EPkbHArElKfgpSuQOfGhcyyP
LjgVhqVgmIoqUwD8CeVpxk3VwZIOQYvPn5XictcZgkYSeEWUZcKYg4c/3gs=
=iIBv
-----END PGP SIGNATURE-----
Merge tag 'trace-v6.8-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace
Pull tracing fixes from Steven Rostedt:
- Fix broken direct trampolines being called when another callback is
attached the same function.
ARM 64 does not support FTRACE_WITH_REGS, and when it added direct
trampoline calls from ftrace, it removed the "WITH_REGS" flag from
the ftrace_ops for direct trampolines. This broke x86 as x86 requires
direct trampolines to have WITH_REGS.
This wasn't noticed because direct trampolines work as long as the
function it is attached to is not shared with other callbacks (like
the function tracer). When there are other callbacks, a helper
trampoline is called, to call all the non direct callbacks and when
it returns, the direct trampoline is called.
For x86, the direct trampoline sets a flag in the regs field to tell
the x86 specific code to call the direct trampoline. But this only
works if the ftrace_ops had WITH_REGS set. ARM does things
differently that does not require this. For now, set WITH_REGS if the
arch supports WITH_REGS (which ARM does not), and this makes it work
for both ARM64 and x86.
- Fix wasted memory in the saved_cmdlines logic.
The saved_cmdlines is a cache that maps PIDs to COMMs that tracing
can use. Most trace events only save the PID in the event. The
saved_cmdlines file lists PIDs to COMMs so that the tracing tools can
show an actual name and not just a PID for each event. There's an
array of PIDs that map to a small set of saved COMM strings. The
array is set to PID_MAX_DEFAULT which is usually set to 32768. When a
PID comes in, it will add itself to this array along with the index
into the COMM array (note if the system allows more than
PID_MAX_DEFAULT, this cache is similar to cache lines as an update of
a PID that has the same PID_MAX_DEFAULT bits set will flush out
another task with the same matching bits set).
A while ago, the size of this cache was changed to be dynamic and the
array was moved into a structure and created with kmalloc(). But this
new structure had the size of 131104 bytes, or 0x20020 in hex. As
kmalloc allocates in powers of two, it was actually allocating
0x40000 bytes (262144) leaving 131040 bytes of wasted memory. The
last element of this structure was a pointer to the COMM string array
which defaulted to just saving 128 COMMs.
By changing the last field of this structure to a variable length
string, and just having it round up to fill the allocated memory, the
default size of the saved COMM cache is now 8190. This not only uses
the wasted space, but actually saves space by removing the extra
allocation for the COMM names.
* tag 'trace-v6.8-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
tracing: Fix wasted memory in saved_cmdlines logic
ftrace: Fix DIRECT_CALLS to use SAVE_REGS by default
ca8a66738a