RE: ftrace histogram sorting broken on BE architecures

David Laight
2019-12-16T17:09:11Z

From: Steven Rostedt
> Sent: 16 December 2019 16:06
> 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.

I'd sort of guessed that, but then the pointer type passed to tracing_map_cmp_##type()
will always be 'u64 *' (since the field the address is taken of must be that type).
Then the (u64 *) casts are no longer needed.

Possibly you can just pass the u64 values to:
tracing_map_cmp_##type(type a, type b)
{
return a > b ? 1 : a < b ? -1 : 0;
}

The high bit masking and sign extension is then implicit in the call.

David

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