Date:   Tue, 3 Sep 2019 23:07:54 +0000
From:   Yonghong Song <yhs@...com>
To:     Stanislav Fomichev <sdf@...ichev.me>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Brian Vazquez <brianvv@...gle.com>,
        Alexei Starovoitov <ast@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 00/13] bpf: adding map batch processing support



On 9/3/19 3:30 PM, Stanislav Fomichev wrote:
> On 09/03, Alexei Starovoitov wrote:
>> On Fri, Aug 30, 2019 at 02:18:09PM -0700, Stanislav Fomichev wrote:
>>>>>
>>>>> I personally like Jakub's/Quentin's proposal more. So if I get to choose
>>>>> between this series and Jakub's filter+dump in BPF, I'd pick filter+dump
>>>>> (pending per-cpu issue which we actually care about).
>>>>>
>>>>> But if we can have both, I don't have any objections; this patch
>>
>> I think we need to have both.
>> imo Jakub's and Yonghong's approach are solving slightly different cases.
>>
>> filter+dump via program is better suited for LRU map walks where filter prog
>> would do some non-trivial logic.
>> Whereas plain 'delete all' or 'dump all' is much simpler to use without
>> loading yet another prog just to dump it.
>> bpf infra today isn't quite ready for this very short lived auxiliary progs.
>> At prog load pages get read-only mapping, tlbs across cpus flushed,
>> kallsyms populated, FDs allocated, etc.
>> Loading the prog is a heavy operation. There was a chatter before to have
>> built-in progs. This filter+dump could benefit from builtin 'allow all'
>> or 'delete all' progs, but imo that complicates design and asks even
>> more questions than it answers. Should this builtin progs show up
>> in 'bpftool prog show' ? When do they load/unload? Same safety requirements
>> as normal progs? etc.
>> imo it's fine to have little bit overlap between apis.
>> So I think we should proceed with both batching apis.
> We don't need to load filter+dump every time we need a dump, right?
> We, internally, want to have this 'batch dump' only for long running daemons
> (I think the same applies to bcc), we can load this filter+dump once and
> then have a sys_bpf() command to trigger it.
> 
> Also, related, if we add this batch dump, it doesn't mean that
> everything should switch to it. For example, I feel like we
> are perfectly fine if bpftool still uses get_next_key+lookup
> since we use it only for debugging.
> 
>> Having said that I think both are suffering from the important issue pointed out
>> by Brian: when kernel deletes an element get_next_key iterator over hash/lru
>> map will produce duplicates.
>> The amount of duplicates can be huge. When batched iterator is slow and
>> bpf prog is doing a lot of update/delete, there could be 10x worth of duplicates,
>> since walk will resume from the beginning.
>> User space cannot be tasked to deal with it.
>> I think this issue has to be solved in the kernel first and it may require
>> different batching api.
>>
>> One idea is to use bucket spin_lock and batch process it bucket-at-a-time.
>>  From api pov the user space will tell kernel:
>> - here is the buffer for N element. start dump from the beginning.
>> - kernel will return <= N elements and an iterator.
>> - user space will pass this opaque iterator back to get another batch
>> For well behaved hash/lru map there will be zero or one elements per bucket.
>> When there are 2+ the batching logic can process them together.
>> If 'lookup' is requested the kernel can check whether user space provided
>> enough space for these 2 elements. If not abort the batch earlier.
>> get_next_key won't be used. Instead some sort of opaque iterator
>> will be returned to user space, so next batch lookup can start from it.
>> This iterator could be the index of the last dumped bucket.
>> This idea won't work for pathological hash tables though.
>> A lot of elements in a single bucket may be more than room for single batch.
>> In such case iterator will get stuck, since num_of_elements_in_bucket > batch_buf_size.
>> May be special error code can be used to solve that?
> This all requires new per-map implementations unfortunately :-(
> We were trying to see if we can somehow improve the existing bpf_map_ops
> to be more friendly towards batching.

The below is a link to folly current hashmap implementation:
https://github.com/facebook/folly/blob/master/folly/concurrency/ConcurrentHashMap.h
It uses segment level batching and each segment can contain multiple 
buckets. It support concurrent iterating vs. deletion.
I am yet to fully understand its implementation. But my guess is
that old elements are somehow kept long enough until all the
queries, read, etc. done and then garbage collection kicks in
and removes those old elements.

Kernel bucket-level locking should be okay. Indeed, per-map 
implementation or tweaking existing per-map interfaces will be necessary.

> 
> You also bring a valid point with regard to well behaved hash/lru,
> we might be optimizing for the wrong case :-)
> 
>> I hope we can come up with other ideas to have a stable iterator over hash table.
>> Let's use email to describe the ideas and upcoming LPC conference to
>> sort out details and finalize the one to use.

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