linux-kernel - Re: [PATCH] sched/clock: cleanup, remove wrap

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Date:	Wed, 7 May 2014 15:38:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dongsheng Yang <yangds.fnst@...fujitsu.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/clock: cleanup, remove wrap_{max|min}().

On Wed, May 07, 2014 at 09:04:19PM +0900, Dongsheng Yang wrote:
> On 05/07/2014 09:55 PM, Peter Zijlstra wrote:
> >On Wed, May 07, 2014 at 08:48:58PM +0900, Dongsheng Yang wrote:
> >>I am not sure why we need the wrap_{max|min}() in kernel/sched/clock.c.
> >>But I checked the implementation of max() and min() in linux/kernel.h, I think
> >>we can reuse them here rather than introduce a new function named
> >>wrap_{max|min}().
> >wrap is a good hint there.. they're supposed to deal with the clock
> >wrapping. Of course 2^64 ns is a rather long time (~584 years in fact),
> >but that doesn't mean we shouldn't care.
> >
> >And no, min/max don't do the right thing.
> 
> IMMO,  max() in kernel.h is checking and returning the u64 with typeof() if
> we are passing a u64 parameter to max(). And I checked current callers of
> wrap_max(),
> they are all passing parameters of u64 type. So I think max/min works well
> at
> the place of wrap_max/min().
> 
> Maybe I am missing something here, please correct me if I understand it
> incorrectly.

Trivial test prog; you really should do these things yourself.

$ ./wrap
wrap_min: 18446744073709551615 wrap_max: 0
min:      0 max:      18446744073709551615


---
#include <stdint.h>
#include <stdio.h>

typedef uint64_t u64;
typedef int64_t  s64;

static inline u64 wrap_min(u64 x, u64 y)
{
        return (s64)(x - y) < 0 ? x : y;
}

static inline u64 wrap_max(u64 x, u64 y)
{
        return (s64)(x - y) > 0 ? x : y;
}

#define min(x, y) ({                            \
        typeof(x) _min1 = (x);                  \
        typeof(y) _min2 = (y);                  \
        (void) (&_min1 == &_min2);              \
        _min1 < _min2 ? _min1 : _min2; })

#define max(x, y) ({                            \
        typeof(x) _max1 = (x);                  \
        typeof(y) _max2 = (y);                  \
        (void) (&_max1 == &_max2);              \
        _max1 > _max2 ? _max1 : _max2; })

void main (void)
{
        u64 a = UINT64_MAX, b = a + 1;

        printf("wrap_min: %lu wrap_max: %lu\n", wrap_min(a, b), wrap_max(a,b));
        printf("min:      %lu max:      %lu\n", min(a, b), max(a,b));
}

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