* [RFC PATCH 0/2] Introduce ftrace_disabled @ 2021-06-19 6:29 Tiezhu Yang 2021-06-19 6:29 ` [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace_disabled Tiezhu Yang 2021-06-19 6:29 ` [RFC PATCH 2/2] docs: kernel-parameters: Add ftrace_disabled Tiezhu Yang 0 siblings, 2 replies; 7+ messages in thread From: Tiezhu Yang @ 2021-06-19 6:29 UTC (permalink / raw) To: Steven Rostedt, Ingo Molnar, Jonathan Corbet Cc: linux-doc, linux-kernel, Xuefeng Li Tiezhu Yang (2): ftrace: Introduce cmdline argument ftrace_disabled docs: kernel-parameters: Add ftrace_disabled Documentation/admin-guide/kernel-parameters.txt | 4 ++++ kernel/trace/ftrace.c | 8 ++++++++ 2 files changed, 12 insertions(+) -- 2.1.0 ^ permalink raw reply [flat|nested] 7+ messages in thread
* [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace_disabled 2021-06-19 6:29 [RFC PATCH 0/2] Introduce ftrace_disabled Tiezhu Yang @ 2021-06-19 6:29 ` Tiezhu Yang 2021-06-19 15:22 ` Steven Rostedt 2021-06-19 6:29 ` [RFC PATCH 2/2] docs: kernel-parameters: Add ftrace_disabled Tiezhu Yang 1 sibling, 1 reply; 7+ messages in thread From: Tiezhu Yang @ 2021-06-19 6:29 UTC (permalink / raw) To: Steven Rostedt, Ingo Molnar, Jonathan Corbet Cc: linux-doc, linux-kernel, Xuefeng Li If set CONFIG_FUNCTION_TRACER=y, we can use the following command to disable ftrace after boot up: echo 0 > /proc/sys/kernel/ftrace_enabled ftrace_disabled is much stronger than ftrace_enabled, introduce a new cmdline argument ftrace_disabled for user to control whether to disable ftrace when boot up. Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn> --- kernel/trace/ftrace.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c index 72ef4dc..a015699 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -5517,6 +5517,14 @@ static char ftrace_filter_buf[FTRACE_FILTER_SIZE] __initdata; /* Used by function selftest to not test if filter is set */ bool ftrace_filter_param __initdata; +static int __init set_ftrace_disabled(char *str) +{ + pr_info("Set ftrace_disabled to disable ftrace\n"); + ftrace_disabled = 1; + return 1; +} +__setup("ftrace_disabled", set_ftrace_disabled); + static int __init set_ftrace_notrace(char *str) { ftrace_filter_param = true; -- 2.1.0 ^ permalink raw reply related [flat|nested] 7+ messages in thread
* Re: [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace_disabled 2021-06-19 6:29 ` [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace_disabled Tiezhu Yang @ 2021-06-19 15:22 ` Steven Rostedt 2021-06-21 1:21 ` Tiezhu Yang 0 siblings, 1 reply; 7+ messages in thread From: Steven Rostedt @ 2021-06-19 15:22 UTC (permalink / raw) To: Tiezhu Yang Cc: Ingo Molnar, Jonathan Corbet, linux-doc, linux-kernel, Xuefeng Li On Sat, 19 Jun 2021 14:29:19 +0800 Tiezhu Yang <yangtiezhu@loongson.cn> wrote: > If set CONFIG_FUNCTION_TRACER=y, we can use the following command to > disable ftrace after boot up: > echo 0 > /proc/sys/kernel/ftrace_enabled I'd love to just remove that. It's original purpose was to stop function tracing when the latency tracers were active. That's now done via a tracefs option. The "ftrace_enabled" really has no use anymore. > > ftrace_disabled is much stronger than ftrace_enabled, introduce a new > cmdline argument ftrace_disabled for user to control whether to disable > ftrace when boot up. "ftrace_disabled" is triggered when an anomaly is detected, and for the safety of the system, ftrace shuts down. It was never meant to be a user triggered event. You have no rationale for this change. What's the purpose of this? -- Steve > > Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn> > --- > kernel/trace/ftrace.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c > index 72ef4dc..a015699 100644 > --- a/kernel/trace/ftrace.c > +++ b/kernel/trace/ftrace.c > @@ -5517,6 +5517,14 @@ static char ftrace_filter_buf[FTRACE_FILTER_SIZE] __initdata; > /* Used by function selftest to not test if filter is set */ > bool ftrace_filter_param __initdata; > > +static int __init set_ftrace_disabled(char *str) > +{ > + pr_info("Set ftrace_disabled to disable ftrace\n"); > + ftrace_disabled = 1; > + return 1; > +} > +__setup("ftrace_disabled", set_ftrace_disabled); > + > static int __init set_ftrace_notrace(char *str) > { > ftrace_filter_param = true; ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace_disabled 2021-06-19 15:22 ` Steven Rostedt @ 2021-06-21 1:21 ` Tiezhu Yang 2021-06-21 14:00 ` Steven Rostedt 0 siblings, 1 reply; 7+ messages in thread From: Tiezhu Yang @ 2021-06-21 1:21 UTC (permalink / raw) To: Steven Rostedt Cc: Ingo Molnar, Jonathan Corbet, linux-doc, linux-kernel, Xuefeng Li On 06/19/2021 11:22 PM, Steven Rostedt wrote: > On Sat, 19 Jun 2021 14:29:19 +0800 > Tiezhu Yang <yangtiezhu@loongson.cn> wrote: > >> If set CONFIG_FUNCTION_TRACER=y, we can use the following command to >> disable ftrace after boot up: >> echo 0 > /proc/sys/kernel/ftrace_enabled > I'd love to just remove that. It's original purpose was to stop function > tracing when the latency tracers were active. That's now done via a tracefs > option. The "ftrace_enabled" really has no use anymore. > >> ftrace_disabled is much stronger than ftrace_enabled, introduce a new >> cmdline argument ftrace_disabled for user to control whether to disable >> ftrace when boot up. > "ftrace_disabled" is triggered when an anomaly is detected, and for the > safety of the system, ftrace shuts down. It was never meant to be a user > triggered event. Thanks for your explanation. > > You have no rationale for this change. What's the purpose of this? The "System Benchmarks Index Score" of UnixBench under FUNCTION_TRACER is lower than !FUNCTION_TRACER, I want to use this new cmdline argument ftrace_disabled to test it, this is the original intention. I see the following help info of "config FUNCTION_TRACER": [If it's runtime disabled (the bootup default), then the overhead of the instructions is very small and not measurable even in micro-benchmarks.] I am not quite understand the above description, could you tell me how to avoid the runtime performance overhead under FUNCTION_TRACER? Thanks, Tiezhu > > -- Steve > > >> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn> >> --- >> kernel/trace/ftrace.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> >> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c >> index 72ef4dc..a015699 100644 >> --- a/kernel/trace/ftrace.c >> +++ b/kernel/trace/ftrace.c >> @@ -5517,6 +5517,14 @@ static char ftrace_filter_buf[FTRACE_FILTER_SIZE] __initdata; >> /* Used by function selftest to not test if filter is set */ >> bool ftrace_filter_param __initdata; >> >> +static int __init set_ftrace_disabled(char *str) >> +{ >> + pr_info("Set ftrace_disabled to disable ftrace\n"); >> + ftrace_disabled = 1; >> + return 1; >> +} >> +__setup("ftrace_disabled", set_ftrace_disabled); >> + >> static int __init set_ftrace_notrace(char *str) >> { >> ftrace_filter_param = true; ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace_disabled 2021-06-21 1:21 ` Tiezhu Yang @ 2021-06-21 14:00 ` Steven Rostedt 2021-06-22 1:19 ` Tiezhu Yang 0 siblings, 1 reply; 7+ messages in thread From: Steven Rostedt @ 2021-06-21 14:00 UTC (permalink / raw) To: Tiezhu Yang Cc: Ingo Molnar, Jonathan Corbet, linux-doc, linux-kernel, Xuefeng Li On Mon, 21 Jun 2021 09:21:31 +0800 Tiezhu Yang <yangtiezhu@loongson.cn> wrote: > > > > You have no rationale for this change. What's the purpose of this? > > The "System Benchmarks Index Score" of UnixBench under FUNCTION_TRACER > is lower than !FUNCTION_TRACER, I want to use this new cmdline argument > ftrace_disabled to test it, this is the original intention. > > I see the following help info of "config FUNCTION_TRACER": > > [If it's runtime disabled (the bootup default), then the overhead of the > instructions is very small and not measurable even in micro-benchmarks.] Those benchmarks were done a long time ago, and they may be measurable today :-/ > > I am not quite understand the above description, could you tell me how to > avoid the runtime performance overhead under FUNCTION_TRACER? Anyway, your patch wont do anything to change the benchmarks. When CONFIG_FUNCTION_TRACER is enabled, on x86_64, most functions will start with a call to fentry. At boot up, these functions will be converted over to become a nop. And thinking about it, "ftrace_disable" stops all conversions, so if you add that to the kernel command line, those calls to fentry, wont be converted to nops, and you'll make things much worse! Now, some versions of gcc (and perhaps clang) can do the conversion to nops at compile time (in which case, your patch would keep the nops and not the calls to fentry). The overhead that FUNCTION_TRACER adds is the 5 byte nop at the start of most functions. This causes a slight hit to instruction cache, and a minuscule amount of time in the instruction pipeline of the CPU. This is the "overhead" that is talked about. Your patch doesn't do anything to address it. The only way to remove that overhead is to compile the kernel without CONFIG_FUNCTION_TRACER. -- Steve ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace_disabled 2021-06-21 14:00 ` Steven Rostedt @ 2021-06-22 1:19 ` Tiezhu Yang 0 siblings, 0 replies; 7+ messages in thread From: Tiezhu Yang @ 2021-06-22 1:19 UTC (permalink / raw) To: Steven Rostedt Cc: Ingo Molnar, Jonathan Corbet, linux-doc, linux-kernel, Xuefeng Li On 06/21/2021 10:00 PM, Steven Rostedt wrote: > On Mon, 21 Jun 2021 09:21:31 +0800 > Tiezhu Yang <yangtiezhu@loongson.cn> wrote: > >>> You have no rationale for this change. What's the purpose of this? >> The "System Benchmarks Index Score" of UnixBench under FUNCTION_TRACER >> is lower than !FUNCTION_TRACER, I want to use this new cmdline argument >> ftrace_disabled to test it, this is the original intention. >> >> I see the following help info of "config FUNCTION_TRACER": >> >> [If it's runtime disabled (the bootup default), then the overhead of the >> instructions is very small and not measurable even in micro-benchmarks.] > Those benchmarks were done a long time ago, and they may be measurable today :-/ > >> I am not quite understand the above description, could you tell me how to >> avoid the runtime performance overhead under FUNCTION_TRACER? > Anyway, your patch wont do anything to change the benchmarks. > > When CONFIG_FUNCTION_TRACER is enabled, on x86_64, most functions will > start with a call to fentry. At boot up, these functions will be > converted over to become a nop. And thinking about it, "ftrace_disable" > stops all conversions, so if you add that to the kernel command line, > those calls to fentry, wont be converted to nops, and you'll make > things much worse! > > Now, some versions of gcc (and perhaps clang) can do the conversion to > nops at compile time (in which case, your patch would keep the nops and > not the calls to fentry). > > The overhead that FUNCTION_TRACER adds is the 5 byte nop at the start > of most functions. This causes a slight hit to instruction cache, and a > minuscule amount of time in the instruction pipeline of the CPU. This > is the "overhead" that is talked about. Your patch doesn't do anything > to address it. The only way to remove that overhead is to compile the > kernel without CONFIG_FUNCTION_TRACER. > > -- Steve OK, I see, thank you very much. Thanks, Tiezhu ^ permalink raw reply [flat|nested] 7+ messages in thread
* [RFC PATCH 2/2] docs: kernel-parameters: Add ftrace_disabled 2021-06-19 6:29 [RFC PATCH 0/2] Introduce ftrace_disabled Tiezhu Yang 2021-06-19 6:29 ` [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace_disabled Tiezhu Yang @ 2021-06-19 6:29 ` Tiezhu Yang 1 sibling, 0 replies; 7+ messages in thread From: Tiezhu Yang @ 2021-06-19 6:29 UTC (permalink / raw) To: Steven Rostedt, Ingo Molnar, Jonathan Corbet Cc: linux-doc, linux-kernel, Xuefeng Li Explain the cmdline argument ftrace_disabled in the file Documentation/admin-guide/kernel-parameters.txt Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn> --- Documentation/admin-guide/kernel-parameters.txt | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index cb89dbd..f255aff 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -1392,6 +1392,10 @@ dump only the buffer of the CPU that triggered the oops. + ftrace_disabled + Set ftrace_disabled in cmdline to disable ftrace when + boot up. + ftrace_filter=[function-list] [FTRACE] Limit the functions traced by the function tracer at boot up. function-list is a comma-separated -- 2.1.0 ^ permalink raw reply related [flat|nested] 7+ messages in thread
end of thread, other threads:[~2021-06-22 1:19 UTC | newest] Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2021-06-19 6:29 [RFC PATCH 0/2] Introduce ftrace_disabled Tiezhu Yang 2021-06-19 6:29 ` [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace_disabled Tiezhu Yang 2021-06-19 15:22 ` Steven Rostedt 2021-06-21 1:21 ` Tiezhu Yang 2021-06-21 14:00 ` Steven Rostedt 2021-06-22 1:19 ` Tiezhu Yang 2021-06-19 6:29 ` [RFC PATCH 2/2] docs: kernel-parameters: Add ftrace_disabled Tiezhu Yang
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