[PATCH] tracing: add cond_resched to ftrace_replace_code()

[PATCH] tracing: add cond_resched to ftrace_replace_code()

[Anders Roxell]

When running in qemu on an kernel built with allmodconfig and debug
options (in particular kcov and ubsan) enabled, ftrace_replace_code
function call take minutes. The ftrace selftest calls
ftrace_replace_code to look >40000 through
ftrace_make_call/ftrace_make_nop, and these end up calling
__aarch64_insn_write/aarch64_insn_patch_text_nosync.

Microseconds add up because this is called in a loop for each dyn_ftrace
record, and this triggers the softlockup watchdog unless we let it sleep
occasionally.

Rework so that we call cond_resched() if !irqs_disabled() && !preempt_count().

Suggested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
---
 kernel/trace/ftrace.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index c375e33239f7..582e3441e318 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -2419,11 +2419,14 @@ void __weak ftrace_replace_code(int enable)
 {
 	struct dyn_ftrace *rec;
 	struct ftrace_page *pg;
+	bool schedulable;
 	int failed;
 
 	if (unlikely(ftrace_disabled))
 		return;
 
+	schedulable = !irqs_disabled() && !preempt_count();
+
 	do_for_each_ftrace_rec(pg, rec) {
 
 		if (rec->flags & FTRACE_FL_DISABLED)
@@ -2435,6 +2438,13 @@ void __weak ftrace_replace_code(int enable)
 			/* Stop processing */
 			return;
 		}
+		/*
+		 * Some archs calls this function with interrupts or preemption
+		 * disabled. Howeve, other archs don't and this can cause a
+		 * tremendous unneeded latency.
+		 */
+		if (schedulable)
+			cond_resched();
 	} while_for_each_ftrace_rec();
 }
 
-- 
2.19.2

Re: [PATCH] tracing: add cond_resched to ftrace_replace_code()

[Steven Rostedt]

On Tue,  4 Dec 2018 20:12:28 +0100
Anders Roxell <anders.roxell@linaro.org> wrote:

> When running in qemu on an kernel built with allmodconfig and debug
> options (in particular kcov and ubsan) enabled, ftrace_replace_code
> function call take minutes. The ftrace selftest calls
> ftrace_replace_code to look >40000 through
> ftrace_make_call/ftrace_make_nop, and these end up calling
> __aarch64_insn_write/aarch64_insn_patch_text_nosync.
> 
> Microseconds add up because this is called in a loop for each dyn_ftrace
> record, and this triggers the softlockup watchdog unless we let it sleep
> occasionally.
> 
> Rework so that we call cond_resched() if !irqs_disabled() && !preempt_count().
> 
> Suggested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
> ---
>  kernel/trace/ftrace.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index c375e33239f7..582e3441e318 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2419,11 +2419,14 @@ void __weak ftrace_replace_code(int enable)
>  {
>  	struct dyn_ftrace *rec;
>  	struct ftrace_page *pg;
> +	bool schedulable;
>  	int failed;
>  
>  	if (unlikely(ftrace_disabled))
>  		return;
>  
> +	schedulable = !irqs_disabled() && !preempt_count();
> +
>  	do_for_each_ftrace_rec(pg, rec) {
>  
>  		if (rec->flags & FTRACE_FL_DISABLED)
> @@ -2435,6 +2438,13 @@ void __weak ftrace_replace_code(int enable)
>  			/* Stop processing */
>  			return;
>  		}
> +		/*
> +		 * Some archs calls this function with interrupts or preemption
> +		 * disabled. Howeve, other archs don't and this can cause a
> +		 * tremendous unneeded latency.
> +		 */

Actually, could you move the comment up where schedulable gets set?

Thanks!

-- Steve

> +		if (schedulable)
> +			cond_resched();
>  	} while_for_each_ftrace_rec();
>  }
>  

Re: [PATCH] tracing: add cond_resched to ftrace_replace_code()

[Steven Rostedt]

On Tue, 4 Dec 2018 14:19:08 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> > @@ -2435,6 +2438,13 @@ void __weak ftrace_replace_code(int enable)
> >  			/* Stop processing */
> >  			return;
> >  		}
> > +		/*
> > +		 * Some archs calls this function with interrupts or preemption
> > +		 * disabled. Howeve, other archs don't and this can cause a

typo "However". But could you write it this way:

	"However, for other archs that can preempt, this can cause an
	tremendous unneeded latency."

Thanks!

-- Steve

> > +		 * tremendous unneeded latency.
> > +		 */  
> 
> Actually, could you move the comment up where schedulable gets set?
> 
> Thanks!
> 
> -- Steve
> 
> > +		if (schedulable)
> > +			cond_resched();
> >  	} while_for_each_ftrace_rec();
> >  }
> >    
> 

Re: [PATCH] tracing: add cond_resched to ftrace_replace_code()

[Anders Roxell]

On Tue, 4 Dec 2018 at 20:21, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Tue, 4 Dec 2018 14:19:08 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > > @@ -2435,6 +2438,13 @@ void __weak ftrace_replace_code(int enable)
> > >                     /* Stop processing */
> > >                     return;
> > >             }
> > > +           /*
> > > +            * Some archs calls this function with interrupts or preemption
> > > +            * disabled. Howeve, other archs don't and this can cause a
>
> typo "However". But could you write it this way:
>
>         "However, for other archs that can preempt, this can cause an
>         tremendous unneeded latency."
>

I'll fix this and move it up (where I added it in the first place but
moved it) =)

Cheers,
Anders

> Thanks!
>
> -- Steve
>
> > > +            * tremendous unneeded latency.
> > > +            */
> >
> > Actually, could you move the comment up where schedulable gets set?
> >
> > Thanks!
> >
> > -- Steve
> >
> > > +           if (schedulable)
> > > +                   cond_resched();
> > >     } while_for_each_ftrace_rec();
> > >  }
> > >
> >
>

Re: [PATCH] tracing: add cond_resched to ftrace_replace_code()

[Steven Rostedt]

On Tue, 4 Dec 2018 20:25:31 +0100
Anders Roxell <anders.roxell@linaro.org> wrote:

> On Tue, 4 Dec 2018 at 20:21, Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > On Tue, 4 Dec 2018 14:19:08 -0500
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> >  
> > > > @@ -2435,6 +2438,13 @@ void __weak ftrace_replace_code(int enable)
> > > >                     /* Stop processing */
> > > >                     return;
> > > >             }
> > > > +           /*
> > > > +            * Some archs calls this function with interrupts or preemption
> > > > +            * disabled. Howeve, other archs don't and this can cause a  
> >
> > typo "However". But could you write it this way:
> >
> >         "However, for other archs that can preempt, this can cause an
> >         tremendous unneeded latency."
> >  
> 
> I'll fix this and move it up (where I added it in the first place but
> moved it) =)
>

I also noticed a grammar issue:

	"Some archs call this function .."

-- Steve

Re: [PATCH] tracing: add cond_resched to ftrace_replace_code()

[Anders Roxell]

On Tue, 4 Dec 2018 at 20:29, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Tue, 4 Dec 2018 20:25:31 +0100
> Anders Roxell <anders.roxell@linaro.org> wrote:
>
> > On Tue, 4 Dec 2018 at 20:21, Steven Rostedt <rostedt@goodmis.org> wrote:
> > >
> > > On Tue, 4 Dec 2018 14:19:08 -0500
> > > Steven Rostedt <rostedt@goodmis.org> wrote:
> > >
> > > > > @@ -2435,6 +2438,13 @@ void __weak ftrace_replace_code(int enable)
> > > > >                     /* Stop processing */
> > > > >                     return;
> > > > >             }
> > > > > +           /*
> > > > > +            * Some archs calls this function with interrupts or preemption
> > > > > +            * disabled. Howeve, other archs don't and this can cause a
> > >
> > > typo "However". But could you write it this way:
> > >
> > >         "However, for other archs that can preempt, this can cause an
> > >         tremendous unneeded latency."
> > >
> >
> > I'll fix this and move it up (where I added it in the first place but
> > moved it) =)
> >
>
> I also noticed a grammar issue:
>
>         "Some archs call this function .."

I will fix that.

Cheers,
Anders