linux-kernel - Re: [PATCH v2] tracing: add cond_resched to ftrace_replace

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Date:   Wed, 5 Dec 2018 11:43:12 +0100
From:   Anders Roxell <anders.roxell@...aro.org>
To:     Will Deacon <will.deacon@....com>
Cc:     rostedt@...dmis.org, mingo@...hat.com,
        Catalin Marinas <catalin.marinas@....com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] tracing: add cond_resched to ftrace_replace_code()

On Wed, 5 Dec 2018 at 10:54, Will Deacon <will.deacon@....com> wrote:
>
> Hi Anders, Steve,
>
> On Tue, Dec 04, 2018 at 08:29:03PM +0100, Anders Roxell 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@...dmis.org>
> > Signed-off-by: Anders Roxell <anders.roxell@...aro.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..7080eb464983 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -2419,11 +2419,19 @@ void __weak ftrace_replace_code(int enable)
> >  {
> >       struct dyn_ftrace *rec;
> >       struct ftrace_page *pg;
> > +     bool schedulable;
> >       int failed;
> >
> >       if (unlikely(ftrace_disabled))
> >               return;
> >
> > +     /*
> > +      * Some archs calls this function with interrupts or preemption
> > +      * disabled. However, for other archs that can preempt, this can cause
> > +      * an tremendous unneeded latency.
> > +      */
> > +     schedulable = !irqs_disabled() && !preempt_count();
>
> Is there a reason not to use preemptible() here?

As I understand it preemptible() is defined to 0 if
CONFIG_PREEMPT_COUNT is disabled.
Thats no good right ?

Cheers,
Anders