Re: [RFC 0/1] Combining CUs into a single hash table

[Yonghong Song <yhs@fb.com>]

On 2/23/21 12:44 PM, Bill Wendling wrote:
> Bump for exposure.
> 
> On Fri, Feb 12, 2021 at 1:16 PM Bill Wendling <morbo@google.com> wrote:
>>
>> Hey gang,
>>
>> I would like your feedback on this patch.
>>
>> This patch creates one hash table that all CUs share. The impetus for this
>> patch is to support clang's LTO (Link-Time Optimizations). Currently, pahole
>> can't handle the DWARF data that clang produces, because the CUs may refer to
>> tags in other CUs (all of the code having been squozen together).

Hi, Bill,

LTO build support is now in linus tree 5.12 rc2 and also merged in 
latest bpf-next. I tried thin-LTO build and it is fine with latest
trunk llvm (llvm13) until it hits pahole and it stuck there (pahole 
1.20) probably some kind of infinite loop in pahole as pahole is
not ready to handle lto dwarf yet.

I then applied this patch on top of master pahole (1.20) and pahole
seg faulted. I did not debug. Have you hit the same issue?
How did you make pahole work with LTO built kernel?

Thanks!

Yonghong

>>
>> One solution I found is to process the CUs in two steps:
>>
>>    1. add the CUs into a single hash table, and
>>    2. perform the recoding and finalization steps in a a separate step.
>>
>> The issue I'm facing with this patch is that it balloons the runtime from
>> ~11.11s to ~14.27s. It looks like the underlying cause is that some (but not
>> all) hash buckets have thousands of entries each. I've bumped up the
>> HASHTAGS__BITS from 15 to 16, which helped a little. Bumping it up to 17 or
>> above causes a failure.
>>
>> A couple of things I thought of may help. We could increase the number of
>> buckets, which would help with distribution. As I mentioned though, that seemed
>> to cause a failure. Another option is to store the bucket entries in a
>> non-list, e.g. binary search tree.
>>
>> I wanted to get your opinions before I trod down one of these roads.
>>
>> Share and enjoy!
>> -bw
>>
>> Bill Wendling (1):
>>    dwarf_loader: have all CUs use a single hash table
>>
>>   dwarf_loader.c | 45 +++++++++++++++++++++++++++++++++------------
>>   1 file changed, 33 insertions(+), 12 deletions(-)
>>
>> --
>> 2.30.0.478.g8a0d178c01-goog
>>