Re: [PATCH] reduce the time of finding symbols for module

[zhouchengming]

On 2017/3/28 19:16, Miroslav Benes wrote:
> On Tue, 28 Mar 2017, zhouchengming wrote:
>
> > On 2017/3/28 17:00, Miroslav Benes wrote:
> > > Hi,
> > >
> > > On Tue, 28 Mar 2017, Zhou Chengming wrote:
> > >
> > > > It's reported that the time of insmoding a klp.ko for one of our
> > > > out-tree modules is too long.
> > > >
> > > > ~ time sudo insmod klp.ko
> > > > real	0m23.799s
> > > > user	0m0.036s
> > > > sys	0m21.256s
> > >
> > > Is this stable through several (>=10) runs? 23 seconds are really
> > > suspicious. Yes, there is a linear search through all the kallsyms in
> > > kallsyms_on_each_symbol(), but there are something like 70k symbols on my
> > > machine (that is, way less than 1M). 23 seconds are somewhat unexpected.
> >
> > Yes, it's stable through several runs.
> >
> > I think the big reason is that our out-tree module used a lot of static local
> > variables. We can see '.rela.kpatch.dynrelas' contains many entries, so it
> > will
> > waste a lot of time if we use kallsyms_on_each_symbol() to find these symbols
> > of module.
>
> Ok, it means that you have a lot of relocation records which reference
> your out-of-tree module. Then for each such entry klp_resolve_symbol()
> is called and then klp_find_object_symbol() to actually resolve it. So if
> you have 20k entries, you walk through vmlinux kallsyms table 20k times.
> It is unneeded and that is why your fix works.
>
> But if there were 20k modules loaded, the problem would still be there.

Yes, vmlinux kallsyms table is too big, but modules loaded are always few.

> I think it would be really nice to fix kallsyms :). Replace ordinary array
> and the linear search with a hash table.
>
> > Relocation section '.rela.kpatch.funcs' at offset 0x382e0 contains 3 entries:
> >    Offset          Info           Type           Sym. Value    Sym. Name +
> > Addend
> > 000000000000  003300000101 R_AARCH64_ABS64   0000000000000000 value_show + 0
> > 000000000020  000b00000101 R_AARCH64_ABS64   0000000000000000 .kpatch.strings
> > + 8
> > 000000000028  000b00000101 R_AARCH64_ABS64   0000000000000000 .kpatch.strings
> > + 0
>
> Hm, we do not have aarch64 support in upstream (yet). There is even no
> dynamic ftrace with regs yet (if I am not mistaken).
>
> > Relocation section '.rela.kpatch.dynrelas' at offset 0x38328 contains 2562
> > entries:
> >    Offset          Info           Type           Sym. Value    Sym. Name +
> > Addend
> > 000000000000  003300000101 R_AARCH64_ABS64   0000000000000000 value_show + 14
> > 000000000018  000b00000101 R_AARCH64_ABS64   0000000000000000 .kpatch.strings
> > + 13
> > 000000000020  000b00000101 R_AARCH64_ABS64   0000000000000000 .kpatch.strings
> > + 0
> > 000000000040  003300000101 R_AARCH64_ABS64   0000000000000000 value_show + 20
> > 000000000058  000b00000101 R_AARCH64_ABS64   0000000000000000 .kpatch.strings
> > + 13
> > 000000000060  000b00000101 R_AARCH64_ABS64   0000000000000000 .kpatch.strings
> > + 0
> >
> > > If it is a problem, can we fix kallsyms_on_each_symbol() and replace the
> > > linear search with something better? All users would benefit...
> >
> > Yes, it's better if we can improve the linear search, but I can't think of
> > that...
>
> I don't understand. Fixing kallsyms is of course much more work but
> everyone would benefit from that.
>
> If there is an agreement, we could accept your solution as temporary. In
> such case, please prefix the subject with 'livepatch: ' and use capital
> letter in 'Reduce'. Please also improve the changelog and describe where
> the problem really is.

Ok, if there are no oppositions, I will send a patch-v2 with improved changelog.
This is really a temporary solution, and others can go on to fix the kallsyms
if needed later.

Thanks.

> Thanks,
> Miroslav
>
> .