lkml.org 
[lkml]   [2019]   [Dec]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    SubjectRE: ftrace histogram sorting broken on BE architecures
    Date
    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

    -
    Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
    Registration No: 1397386 (Wales)

    \
     
     \ /
      Last update: 2019-12-16 18:09    [W:2.118 / U:0.088 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site