Messages in this thread Patch in this message |  | | Date | Mon, 16 Dec 2019 11:05:39 -0500 | From | Steven Rostedt <> | Subject | Re: ftrace histogram sorting broken on BE architecures |
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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
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