Re: [PATCH] time/sched_clock: mark sched_clock_read_begin as notrace

[Quanyang Wang]

Hi Peter,

On 9/28/20 6:58 PM, Peter Zijlstra wrote:
> On Mon, Sep 28, 2020 at 06:49:52PM +0800, quanyang.wang@xxxxxxxxxxxxx wrote:
> > From: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> >
> > Since sched_clock_read_begin is called by notrace function sched_clock,
> > it shouldn't be traceable either, or else __ftrace_graph_caller will
> > run into a dead loop on the path (arm for instance):
> >
> >    ftrace_graph_caller
> >      prepare_ftrace_return
> >        function_graph_enter
> >          ftrace_push_return_trace
> >            trace_clock_local
> >              sched_clock
> >                sched_clock_read_begin
> >
> > Fixes: 1b86abc1c645 ("sched_clock: Expose struct clock_read_data")
> > Signed-off-by: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> > ---
> >   kernel/time/sched_clock.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
> > index 1c03eec6ca9b..58459e1359d7 100644
> > --- a/kernel/time/sched_clock.c
> > +++ b/kernel/time/sched_clock.c
> > @@ -68,7 +68,7 @@ static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
> >   	return (cyc * mult) >> shift;
> >   }
> >   
> > -struct clock_read_data *sched_clock_read_begin(unsigned int *seq)
> > +notrace struct clock_read_data *sched_clock_read_begin(unsigned int *seq)
> >   {
> >   	*seq = raw_read_seqcount_latch(&cd.seq);
> >   	return cd.read_data + (*seq & 1);
> At the very least sched_clock_read_retry() should also be marked such.

In fact, the sched_clock_read_retry is treated as a "inline" function, so

it doesn't trigger the  dead loop. But for safe, add notrace to it is 
better.

I will send a V2 patch.

Thanks,

Quanyang


> But Steve, how come x86 works? Our sched_clock() doesn't have notrace on
> at all.