Re: [RFC PATCH 5/7] perf tools: Support setting different slots in a BPF map separately

["Wangnan (F)" <wangnan0@huawei.com>]

On 2015/11/23 10:01, Wangnan (F) wrote:
>
>
> On 2015/11/20 23:34, Arnaldo Carvalho de Melo wrote:
>> Em Fri, Nov 20, 2015 at 09:25:36PM +0800, Wangnan (F) escreveu:
>>>> +        case BPF_MAP_PRIV_KEY_INDICS:
>>>> +            for (i = 0; i < priv->key.indics.nr_indics; i++) {
>>>> +                u64 _idx = priv->key.indics.indics[i];
>>>> +                unsigned int idx = (unsigned int)(_idx);
>>>> +
>>>> +                err = (*func)(name, map_fd, &def,
>>>> +                          priv, &idx, arg);
>>>> +                if (err) {
>>>> +                    pr_debug("ERROR: failed to insert value to 
>>>> %s[%u]\n",
>>>> +                         name, idx);
>>>> +                    return err;
>>>> +                }
>>>> +            }
>>> This for-loop has a potential problem that, if perf's user want to
>>> set a very big array using indices, for example:
>>>
>>>   # perf record -e
>>> mybpf.c/maps:mymap:values[1,2,3,10-100000,200000-400000]=3/
>>> mybpf.c/maps:mymap:values[100000-200000]=3/ ...
>>>
>>> Perf would alloc nearly 300000 slots for indices array, consume too 
>>> much
>>> memory.
>>>
>>> I will fix this problem by reinterprete indices array, makes negative
>>> value represent range start and use next slot to store range size. For
>>> example, the above perf cmdline can be converted to:
>>>
>>> {1,2,3,-10, 99991,-200000,200001} and {-100000,100001}.
>> Why is that changing the way you specify what entries should be set to
>> a value will make it not allocate too much memory?
>
> It is actually a problem in the next patch, in which it expand all range
> into a series of indices. If user wants 1-10000, it creates an array as
> [1,2,3,4,...10000], so user is possible to use a simple cmdline to 
> consume
> all of available memory.
>
> However, the method I described above is not the best way to solve 
> this probelm.
> I thought yesterday that we should not insist on indices array. We can
> make parser always return ranges. For example, [1,2,3-5] can be represent
> using [(1,1), (2,1), (3,3)], so we don't need the above ugly negative
> indicators.
>
>> I found the first form of representing ( start-end ) to be better than (
>> -start, size ), but I would use what the C language uses for expressing
>> ranges in switch case ranges, which is familiar and doesn't reuses the
>> minus arithmetic operator to express a range, i.e.:
>>
>>   # perf record -e \
>> mybpf.c/maps:mymap:values[1,2,3,10..100000,200000..400000]=3/
>>
>>   # perf record -e \
>>     mybpf.c/maps:mymap:values[100000..200000]=3/ ...
>
> '..' is better.
>

One problem: the case range syntax is introduced by gcc extension, not a
part of standard, and should be '...'.

Please see: https://gcc.gnu.org/onlinedocs/gcc/Case-Ranges.html

So I'll use '...' also.

Thank you.



> Thank you.
>
>> - Arnaldo
>
>
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