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

[Wangnan (F)]

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.

Thank you.

> - Arnaldo


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/