From: Alexei Starovoitov <firstname.lastname@example.org>
To: Brian Vazquez <email@example.com>
Cc: Song Liu <firstname.lastname@example.org>,
Brian Vazquez <email@example.com>,
Alexei Starovoitov <firstname.lastname@example.org>,
Daniel Borkmann <email@example.com>,
"David S . Miller" <firstname.lastname@example.org>,
Stanislav Fomichev <email@example.com>,
Willem de Bruijn <firstname.lastname@example.org>,
Petar Penkov <email@example.com>,
open list <firstname.lastname@example.org>,
Networking <email@example.com>, bpf <firstname.lastname@example.org>
Subject: Re: [PATCH bpf-next 2/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call
Date: Thu, 25 Jul 2019 16:54:33 -0700 [thread overview]
Message-ID: <20190725235432.lkptx3fafegnm2et@ast-mbp> (raw)
On Thu, Jul 25, 2019 at 04:25:53PM -0700, Brian Vazquez wrote:
> > > > If prev_key is deleted before map_get_next_key(), we get the first key
> > > > again. This is pretty weird.
> > >
> > > Yes, I know. But note that the current scenario happens even for the
> > > old interface (imagine you are walking a map from userspace and you
> > > tried get_next_key the prev_key was removed, you will start again from
> > > the beginning without noticing it).
> > > I tried to sent a patch in the past but I was missing some context:
> > > before NULL was used to get the very first_key the interface relied in
> > > a random (non existent) key to retrieve the first_key in the map, and
> > > I was told what we still have to support that scenario.
> > BPF_MAP_DUMP is slightly different, as you may return the first key
> > multiple times in the same call. Also, BPF_MAP_DUMP is new, so we
> > don't have to support legacy scenarios.
> > Since BPF_MAP_DUMP keeps a list of elements. It is possible to try
> > to look up previous keys. Would something down this direction work?
> I've been thinking about it and I think first we need a way to detect
> that since key was not present we got the first_key instead:
> - One solution I had in mind was to explicitly asked for the first key
> with map_get_next_key(map, NULL, first_key) and while walking the map
> check that map_get_next_key(map, prev_key, key) doesn't return the
> same key. This could be done using memcmp.
> - Discussing with Stan, he mentioned that another option is to support
> a flag in map_get_next_key to let it know that we want an error
> instead of the first_key.
> After detecting the problem we also need to define what we want to do,
> here some options:
> a) Return the error to the caller
> b) Try with previous keys if any (which be limited to the keys that we
> have traversed so far in this dump call)
> c) continue with next entries in the map. array is easy just get the
> next valid key (starting on i+1), but hmap might be difficult since
> starting on the next bucket could potentially skip some keys that were
> concurrently added to the same bucket where key used to be, and
> starting on the same bucket could lead us to return repeated elements.
> Or maybe we could support those 3 cases via flags and let the caller
> decide which one to use?
this type of indecision is the reason why I wasn't excited about
batch dumping in the first place and gave 'soft yes' when Stan
mentioned it during lsf/mm/bpf uconf.
We probably shouldn't do it.
It feels this map_dump makes api more complex and doesn't really
give much benefit to the user other than large map dump becomes faster.
I think we gotta solve this problem differently.
next prev parent reply other threads:[~2019-07-25 23:54 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-24 16:57 [PATCH bpf-next 0/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call Brian Vazquez
2019-07-24 16:57 ` [PATCH bpf-next 1/6] bpf: add bpf_map_value_size and bp_map_copy_value helper functions Brian Vazquez
2019-07-24 20:53 ` Song Liu
2019-07-24 16:57 ` [PATCH bpf-next 2/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call Brian Vazquez
2019-07-24 19:54 ` Willem de Bruijn
2019-07-24 22:26 ` Brian Vazquez
2019-07-24 22:33 ` Willem de Bruijn
2019-07-24 21:40 ` Song Liu
2019-07-24 22:44 ` Brian Vazquez
2019-07-24 23:04 ` Song Liu
2019-07-25 23:25 ` Brian Vazquez
2019-07-25 23:54 ` Alexei Starovoitov [this message]
2019-07-26 1:02 ` Willem de Bruijn
2019-07-26 1:24 ` Brian Vazquez
2019-07-26 1:47 ` Alexei Starovoitov
2019-07-26 6:10 ` Yonghong Song
2019-07-26 23:36 ` Brian Vazquez
2019-07-27 0:02 ` Jakub Kicinski
2019-07-27 17:54 ` Yonghong Song
2019-07-24 16:58 ` [PATCH bpf-next 3/6] bpf: keep bpf.h in sync with tools/ Brian Vazquez
2019-07-24 21:41 ` Song Liu
2019-07-24 23:10 ` Andrii Nakryiko
2019-07-25 23:27 ` Brian Vazquez
2019-07-24 16:58 ` [PATCH bpf-next 4/6] libbpf: support BPF_MAP_DUMP command Brian Vazquez
2019-07-24 19:51 ` Willem de Bruijn
2019-07-24 16:58 ` [PATCH bpf-next 5/6] selftests/bpf: test BPF_MAP_DUMP command on a bpf hashmap Brian Vazquez
2019-07-24 21:58 ` Song Liu
2019-07-24 16:58 ` [PATCH bpf-next 6/6] selftests/bpf: add test to measure performance of BPF_MAP_DUMP Brian Vazquez
2019-07-24 22:01 ` Song Liu
2019-07-24 19:20 ` [PATCH bpf-next 0/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call Song Liu
2019-07-24 22:15 ` Brian Vazquez
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