| Message ID | 20200925211206.423598568@goodmis.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | tracing/mm: Add tracepoint_enabled() helper function for headers | expand |
Bah, My cut-and-paste of my "quilt mail --send" chopped off Mathieu's email. Mathieu, I didn't meant to not Cc you on this. Do you need me to bounce the rest to you or you can get it from lore? -- Steve On Fri, 25 Sep 2020 17:12:06 -0400 Steven Rostedt <rostedt@goodmis.org> wrote: > Tracepoints are not safe to be called directly from header files as they may > be included by C code that has CREATE_TRACE_POINTS defined, and this would > cause side effects and possibly break the build in hard to debug ways. Not > to mention it also will bloat the code being in commonly used inline > functions. > > Instead, it is recommended to call a tracepoint helper function that is > defined in a C file that calls the tracepoint. But we would only want this > function to be called if the tracepoint is enabled, as function calls add > overhead. > > The trace_<tracepoint>_enabled() function is also not safe to be called in a > header file as it is created by the tracepoint header, which suffers the > same fate if CREATE_TRACE_POINTS is defined. Instead, the tracepoint needs > to be declared as an extern, and the helper function can test the static key > to call the helper function that calls the tracepoint. > > This has been done by open coding the tracepoint extern and calling the > static key directly: > > commit 95813b8faa0cd ("mm/page_ref: add tracepoint to track down page reference manipulation") > commit 7f47d8cc039f ("x86, tracing, perf: Add trace point for MSR accesses") > > does this (back in 2015). Now we have another use case, so a helper function > should be created to keep the internals of the tracepoints from being spread > out in other subsystems. > > Link: https://lore.kernel.org/r/20200922125113.12ef1e03@gandalf.local.home > > This adds tracepoint_enabled() helper macro and DECLARE_TRACEPOINT() macro > to allow this to be done without exposing the internals of the tracepoints. > > The first patch adds the infrastructure, the second converts page_ref over > to it, and third converts over msr.h. > > Steven Rostedt (VMware) (3): > tracepoints: Add helper to test if tracepoint is enabled in a header > mm/page_ref: Convert the open coded tracepoint enabled to the new helper > x86: Use tracepoint_enabled() for msr tracepoints instead of open coding it > > ---- > > Changes since v1 (https://lore.kernel.org/r/20200924170928.466191266@goodmis.org): > > - Fixed using "trace_enabled()" instead of "tracepoint_enabled()" > (Mathieu Desnoyers reported) > > - Reworded to include comments about bloating the kernel when tracepoints > are used in commonly used inlined functions. > > - Added the msr update as well. > > > Documentation/trace/tracepoints.rst | 27 ++++++++++++++++++++++++ > arch/x86/include/asm/msr.h | 18 +++++++--------- > include/linux/page_ref.h | 42 ++++++++++++++++++------------------- > include/linux/tracepoint-defs.h | 34 ++++++++++++++++++++++++++++++ > 4 files changed, 90 insertions(+), 31 deletions(-)
No worries, I'll get it from lore. Thanks, Mathieu ----- Steven Rostedt <rostedt@goodmis.org> wrote: > > Bah, My cut-and-paste of my "quilt mail --send" chopped off Mathieu's email. > > Mathieu, I didn't meant to not Cc you on this. Do you need me to bounce > the rest to you or you can get it from lore? > > -- Steve > > > On Fri, 25 Sep 2020 17:12:06 -0400 > Steven Rostedt <rostedt@goodmis.org> wrote: > > > Tracepoints are not safe to be called directly from header files as they may > > be included by C code that has CREATE_TRACE_POINTS defined, and this would > > cause side effects and possibly break the build in hard to debug ways. Not > > to mention it also will bloat the code being in commonly used inline > > functions. > > > > Instead, it is recommended to call a tracepoint helper function that is > > defined in a C file that calls the tracepoint. But we would only want this > > function to be called if the tracepoint is enabled, as function calls add > > overhead. > > > > The trace_<tracepoint>_enabled() function is also not safe to be called in a > > header file as it is created by the tracepoint header, which suffers the > > same fate if CREATE_TRACE_POINTS is defined. Instead, the tracepoint needs > > to be declared as an extern, and the helper function can test the static key > > to call the helper function that calls the tracepoint. > > > > This has been done by open coding the tracepoint extern and calling the > > static key directly: > > > > commit 95813b8faa0cd ("mm/page_ref: add tracepoint to track down page reference manipulation") > > commit 7f47d8cc039f ("x86, tracing, perf: Add trace point for MSR accesses") > > > > does this (back in 2015). Now we have another use case, so a helper function > > should be created to keep the internals of the tracepoints from being spread > > out in other subsystems. > > > > Link: https://lore.kernel.org/r/20200922125113.12ef1e03@gandalf.local.home > > > > This adds tracepoint_enabled() helper macro and DECLARE_TRACEPOINT() macro > > to allow this to be done without exposing the internals of the tracepoints. > > > > The first patch adds the infrastructure, the second converts page_ref over > > to it, and third converts over msr.h. > > > > Steven Rostedt (VMware) (3): > > tracepoints: Add helper to test if tracepoint is enabled in a header > > mm/page_ref: Convert the open coded tracepoint enabled to the new helper > > x86: Use tracepoint_enabled() for msr tracepoints instead of open coding it > > > > ---- > > > > Changes since v1 (https://lore.kernel.org/r/20200924170928.466191266@goodmis.org): > > > > - Fixed using "trace_enabled()" instead of "tracepoint_enabled()" > > (Mathieu Desnoyers reported) > > > > - Reworded to include comments about bloating the kernel when tracepoints > > are used in commonly used inlined functions. > > > > - Added the msr update as well. > > > > > > Documentation/trace/tracepoints.rst | 27 ++++++++++++++++++++++++ > > arch/x86/include/asm/msr.h | 18 +++++++--------- > > include/linux/page_ref.h | 42 ++++++++++++++++++------------------- > > include/linux/tracepoint-defs.h | 34 ++++++++++++++++++++++++++++++ > > 4 files changed, 90 insertions(+), 31 deletions(-) >
Tracepoints are not safe to be called directly from header files as they may be included by C code that has CREATE_TRACE_POINTS defined, and this would cause side effects and possibly break the build in hard to debug ways. Not to mention it also will bloat the code being in commonly used inline functions. Instead, it is recommended to call a tracepoint helper function that is defined in a C file that calls the tracepoint. But we would only want this function to be called if the tracepoint is enabled, as function calls add overhead. The trace_<tracepoint>_enabled() function is also not safe to be called in a header file as it is created by the tracepoint header, which suffers the same fate if CREATE_TRACE_POINTS is defined. Instead, the tracepoint needs to be declared as an extern, and the helper function can test the static key to call the helper function that calls the tracepoint. This has been done by open coding the tracepoint extern and calling the static key directly: commit 95813b8faa0cd ("mm/page_ref: add tracepoint to track down page reference manipulation") commit 7f47d8cc039f ("x86, tracing, perf: Add trace point for MSR accesses") does this (back in 2015). Now we have another use case, so a helper function should be created to keep the internals of the tracepoints from being spread out in other subsystems. Link: https://lore.kernel.org/r/20200922125113.12ef1e03@gandalf.local.home This adds tracepoint_enabled() helper macro and DECLARE_TRACEPOINT() macro to allow this to be done without exposing the internals of the tracepoints. The first patch adds the infrastructure, the second converts page_ref over to it, and third converts over msr.h. Steven Rostedt (VMware) (3): tracepoints: Add helper to test if tracepoint is enabled in a header mm/page_ref: Convert the open coded tracepoint enabled to the new helper x86: Use tracepoint_enabled() for msr tracepoints instead of open coding it ---- Changes since v1 (https://lore.kernel.org/r/20200924170928.466191266@goodmis.org): - Fixed using "trace_enabled()" instead of "tracepoint_enabled()" (Mathieu Desnoyers reported) - Reworded to include comments about bloating the kernel when tracepoints are used in commonly used inlined functions. - Added the msr update as well. Documentation/trace/tracepoints.rst | 27 ++++++++++++++++++++++++ arch/x86/include/asm/msr.h | 18 +++++++--------- include/linux/page_ref.h | 42 ++++++++++++++++++------------------- include/linux/tracepoint-defs.h | 34 ++++++++++++++++++++++++++++++ 4 files changed, 90 insertions(+), 31 deletions(-)