Re: [PATCH for-4.19] tools/xentop: fix cpu% sort order

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.05.2024 14:07, Andrew Cooper wrote:
> On 14/05/2024 9:13 am, Leigh Brown wrote:
>> Although using integer comparison to compare doubles kind of
>> works, it's annoying to see domains slightly out of order when
>> sorting by cpu%.
>>
>> Add a compare_dbl() function and update compare_cpu_pct() to
>> call it.
>>
>> Signed-off-by: Leigh Brown <leigh@xxxxxxxxxxxxx>
>> ---
>>  tools/xentop/xentop.c | 13 ++++++++++++-
>>  1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/xentop/xentop.c b/tools/xentop/xentop.c
>> index 545bd5e96d..99199caec9 100644
>> --- a/tools/xentop/xentop.c
>> +++ b/tools/xentop/xentop.c
>> @@ -85,6 +85,7 @@ static void set_delay(const char *value);
>>  static void set_prompt(const char *new_prompt, void (*func)(const char *));
>>  static int handle_key(int);
>>  static int compare(unsigned long long, unsigned long long);
>> +static int compare_dbl(double, double);
>>  static int compare_domains(xenstat_domain **, xenstat_domain **);
>>  static unsigned long long tot_net_bytes( xenstat_domain *, int);
>>  static bool tot_vbd_reqs(xenstat_domain *, int, unsigned long long *);
>> @@ -422,6 +423,16 @@ static int compare(unsigned long long i1, unsigned long 
>> long i2)
>>      return 0;
>>  }
>>  
>> +/* Compares two double precision numbers, returning -1,0,1 for <,=,> */
>> +static int compare_dbl(double d1, double d2)
>> +{
>> +    if(d1 < d2)
>> +            return -1;
>> +    if(d1 > d2)
>> +            return 1;
>> +    return 0;
>> +}
>> +
>>  /* Comparison function for use with qsort.  Compares two domains using the
>>   * current sort field. */
>>  static int compare_domains(xenstat_domain **domain1, xenstat_domain 
>> **domain2)
>> @@ -523,7 +534,7 @@ static double get_cpu_pct(xenstat_domain *domain)
>>  
>>  static int compare_cpu_pct(xenstat_domain *domain1, xenstat_domain *domain2)
>>  {
>> -    return -compare(get_cpu_pct(domain1), get_cpu_pct(domain2));
>> +    return -compare_dbl(get_cpu_pct(domain1), get_cpu_pct(domain2));
> 
> Oh, we were doing an implicit double->unsigned long long conversion. 
> Over the range 0.0 to 100.0, that ought to work as expected.  What kind
> of out-of-order are you seeing?
> 
> Nevertheless, this should comparison should clearly be done using
> doubles.  AFACT, get_cpu_pct() shouldn't ever return a NaN, so I think
> this simple form is fine.

Just for completeness: INF would be similarly an issue, but hopefully cannot
come back from get_cpu_pct() either.

Jan