Re: ftrace histogram sorting broken on BE architecures

Steven Rostedt
2019-12-16T16:05:43Z

On Mon, 16 Dec 2019 15:47:12 +0000
David Laight <David.Laight@ACULAB.COM> wrote:

> > From: Tom Zanussi
> > Sent: 12 December 2019 19:17
> > On Wed, 2019-12-11 at 11:09 -0500, Steven Rostedt wrote:
> > > On Wed, 11 Dec 2019 10:35:57 -0500
> > > Steven Rostedt <rostedt@goodmis.org> wrote:
> > >
> > > > > Any thoughts on how to fix this? I'm not sure whether i fully
> > > > > understand the
> > > > > ftrace maps... ;-)
> > > >
> > > > Your analysis makes sense. I'll take a deeper look at it.
> > >
> > > Sven,
> > >
> > > Does this patch fix it for you?
> > >
> > > Tom,
> > >
> > > Correct me if I'm wrong, from what I can tell, all sums and keys are
> > > u64 unless they are a string. Thus, I believe this patch should not
> > > have any issues.
> ...
> > > --- a/kernel/trace/tracing_map.c
> > > +++ b/kernel/trace/tracing_map.c
> > > @@ -148,8 +148,8 @@ static int tracing_map_cmp_atomic64(void *val_a,
> > > void *val_b)
> > > #define DEFINE_TRACING_MAP_CMP_FN(type) \
> > > static int tracing_map_cmp_##type(void *val_a, void *val_b) \
> > > { \
> > > - type a = *(type *)val_a; \
> > > - type b = *(type *)val_b; \
> > > + type a = (type)(*(u64 *)val_a); \
> > > + type b = (type)(*(u64 *)val_b); \
> > > \
> > > return (a > b) ? 1 : ((a < b) ? -1 : 0); \
> > > }
>
> That looks so horrid/wrong it can't be right on both BE and LE.

Well, the original is obviously not right for both BE and LE, but the
fix is:

type a = (type)(*(u64 *)val_a);

Which breaks down to:

(u64 *)val_a - make val_a a pointer to a u64 number

all values were written as u64.

u64 data = (u64)original_val_a

Where original_val_a could be a byte, short, int, long or long long.

Now that we have (u64 *)val_a, we dereference it:

*(u64 *)val_a - which gives us a u64 number.

This u64 number should be the same as the u64 data.

Then we convert this as:

(type)(*(u64 *)val_a)

Taking the u64 number we retrieved and converted it back to the
original type that was recorded.

In other words, if the following should work:

type a = x;
u64 data = (u64)a;
u64 *ptr = &data;
u64 b_data = *ptr;
type b = (type)b_data;

If b == a above, then there should be nothing wrong with this patch.

The compiler should know how to map those type conversions properly for
both BE and LE.

-- Steve