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Message-ID: <CA+khW7hvLgCKVA0kiKhREW-PZ4aOYvkGHoEqKAggEdyY9TRp7Q@mail.gmail.com>
Date: Thu, 28 Jul 2022 10:25:04 -0700
From: Hao Luo <haoluo@...gle.com>
To: Yonghong Song <yhs@...com>
Cc: Yosry Ahmed <yosryahmed@...gle.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Tejun Heo <tj@...nel.org>,
Zefan Li <lizefan.x@...edance.com>,
Johannes Weiner <hannes@...xchg.org>,
Shuah Khan <shuah@...nel.org>,
Michal Hocko <mhocko@...nel.org>,
KP Singh <kpsingh@...nel.org>,
Benjamin Tissoires <benjamin.tissoires@...hat.com>,
John Fastabend <john.fastabend@...il.com>,
Michal Koutný <mkoutny@...e.com>,
Roman Gushchin <roman.gushchin@...ux.dev>,
David Rientjes <rientjes@...gle.com>,
Stanislav Fomichev <sdf@...gle.com>,
Greg Thelen <gthelen@...gle.com>,
Shakeel Butt <shakeelb@...gle.com>,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
bpf@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH bpf-next v5 4/8] bpf: Introduce cgroup iter
On Wed, Jul 27, 2022 at 10:49 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 7/22/22 10:48 AM, Yosry Ahmed wrote:
> > From: Hao Luo <haoluo@...gle.com>
> >
> > Cgroup_iter is a type of bpf_iter. It walks over cgroups in three modes:
> >
> > - walking a cgroup's descendants in pre-order.
> > - walking a cgroup's descendants in post-order.
> > - walking a cgroup's ancestors.
> >
> > When attaching cgroup_iter, one can set a cgroup to the iter_link
> > created from attaching. This cgroup is passed as a file descriptor and
> > serves as the starting point of the walk. If no cgroup is specified,
> > the starting point will be the root cgroup.
> >
> > For walking descendants, one can specify the order: either pre-order or
> > post-order. For walking ancestors, the walk starts at the specified
> > cgroup and ends at the root.
> >
> > One can also terminate the walk early by returning 1 from the iter
> > program.
> >
> > Note that because walking cgroup hierarchy holds cgroup_mutex, the iter
> > program is called with cgroup_mutex held.
> >
> > Currently only one session is supported, which means, depending on the
> > volume of data bpf program intends to send to user space, the number
> > of cgroups that can be walked is limited. For example, given the current
> > buffer size is 8 * PAGE_SIZE, if the program sends 64B data for each
> > cgroup, the total number of cgroups that can be walked is 512. This is
>
> PAGE_SIZE needs to be 4KB in order to conclude that the total number of
> walked cgroups is 512.
>
Sure. Will change that.
> > a limitation of cgroup_iter. If the output data is larger than the
> > buffer size, the second read() will signal EOPNOTSUPP. In order to work
> > around, the user may have to update their program to reduce the volume
> > of data sent to output. For example, skip some uninteresting cgroups.
> > In future, we may extend bpf_iter flags to allow customizing buffer
> > size.
> >
> > Signed-off-by: Hao Luo <haoluo@...gle.com>
> > Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> > Acked-by: Yonghong Song <yhs@...com>
> > ---
> > include/linux/bpf.h | 8 +
> > include/uapi/linux/bpf.h | 30 +++
> > kernel/bpf/Makefile | 3 +
> > kernel/bpf/cgroup_iter.c | 252 ++++++++++++++++++
> > tools/include/uapi/linux/bpf.h | 30 +++
> > .../selftests/bpf/prog_tests/btf_dump.c | 4 +-
> > 6 files changed, 325 insertions(+), 2 deletions(-)
> > create mode 100644 kernel/bpf/cgroup_iter.c
>
> This patch cannot apply to bpf-next cleanly, so please rebase
> and post again.
>
Sorry about that. Will do.
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index a97751d845c9..9061618fe929 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -47,6 +47,7 @@ struct kobject;
> > struct mem_cgroup;
> > struct module;
> > struct bpf_func_state;
> > +struct cgroup;
> >
> > extern struct idr btf_idr;
> > extern spinlock_t btf_idr_lock;
> > @@ -1717,7 +1718,14 @@ int bpf_obj_get_user(const char __user *pathname, int flags);
> > int __init bpf_iter_ ## target(args) { return 0; }
> >
> > struct bpf_iter_aux_info {
> > + /* for map_elem iter */
> > struct bpf_map *map;
> > +
> > + /* for cgroup iter */
> > + struct {
> > + struct cgroup *start; /* starting cgroup */
> > + int order;
> > + } cgroup;
> > };
> >
> > typedef int (*bpf_iter_attach_target_t)(struct bpf_prog *prog,
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index ffcbf79a556b..fe50c2489350 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -87,10 +87,30 @@ struct bpf_cgroup_storage_key {
> > __u32 attach_type; /* program attach type (enum bpf_attach_type) */
> > };
> >
> > +enum bpf_iter_cgroup_traversal_order {
> > + BPF_ITER_CGROUP_PRE = 0, /* pre-order traversal */
> > + BPF_ITER_CGROUP_POST, /* post-order traversal */
> > + BPF_ITER_CGROUP_PARENT_UP, /* traversal of ancestors up to the root */
> > +};
> > +
> > union bpf_iter_link_info {
> > struct {
> > __u32 map_fd;
> > } map;
> > +
> > + /* cgroup_iter walks either the live descendants of a cgroup subtree, or the
> > + * ancestors of a given cgroup.
> > + */
> > + struct {
> > + /* Cgroup file descriptor. This is root of the subtree if walking
> > + * descendants; it's the starting cgroup if walking the ancestors.
> > + * If it is left 0, the traversal starts from the default cgroup v2
> > + * root. For walking v1 hierarchy, one should always explicitly
> > + * specify the cgroup_fd.
> > + */
>
> I did see how the above cgroup v1/v2 scenarios are enforced.
>
Do you mean _not_ see? Yosry and I experimented a bit. We found even
on systems where v2 is not enabled, cgroup v2 root always exists and
can be attached to, and can be iterated on (only trivially). We didn't
find a way to tell v1 and v2 apart and deemed a comment to instruct v1
users is fine?
> > + __u32 cgroup_fd;
> > + __u32 traversal_order;
> > + } cgroup;
> > };
> >
> > /* BPF syscall commands, see bpf(2) man-page for more details. */
> > @@ -6136,6 +6156,16 @@ struct bpf_link_info {
> > __u32 map_id;
> > } map;
> > };
> > + union {
> > + struct {
> > + __u64 cgroup_id;
> > + __u32 traversal_order;
> > + } cgroup;
> > + };
> > + /* For new iters, if the first field is larger than __u32,
> > + * the struct should be added in the second union. Otherwise,
> > + * it will create holes before map_id, breaking uapi.
> > + */
>
> Please put the comment above the union. Let us just say, if
> the iter specific field is __u32, it can be put in the first or
> second union. Otherwise, it is put in second union.
>
Ok.
> > } iter;
> > struct {
> > __u32 netns_ino;
> > diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> > index 057ba8e01e70..00e05b69a4df 100644
> > --- a/kernel/bpf/Makefile
> > +++ b/kernel/bpf/Makefile
> > @@ -24,6 +24,9 @@ endif
> > ifeq ($(CONFIG_PERF_EVENTS),y)
> > obj-$(CONFIG_BPF_SYSCALL) += stackmap.o
> > endif
> > +ifeq ($(CONFIG_CGROUPS),y)
> > +obj-$(CONFIG_BPF_SYSCALL) += cgroup_iter.o
> > +endif
> > obj-$(CONFIG_CGROUP_BPF) += cgroup.o
> > ifeq ($(CONFIG_INET),y)
> > obj-$(CONFIG_BPF_SYSCALL) += reuseport_array.o
> > diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
> > new file mode 100644
> > index 000000000000..1027faed0b8b
> > --- /dev/null
> > +++ b/kernel/bpf/cgroup_iter.c
> > @@ -0,0 +1,252 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/* Copyright (c) 2022 Google */
> > +#include <linux/bpf.h>
> > +#include <linux/btf_ids.h>
> > +#include <linux/cgroup.h>
> > +#include <linux/kernel.h>
> > +#include <linux/seq_file.h>
> > +
> > +#include "../cgroup/cgroup-internal.h" /* cgroup_mutex and cgroup_is_dead */
> > +
> > +/* cgroup_iter provides three modes of traversal to the cgroup hierarchy.
> > + *
> > + * 1. Walk the descendants of a cgroup in pre-order.
> > + * 2. Walk the descendants of a cgroup in post-order.
> > + * 2. Walk the ancestors of a cgroup.
> > + *
> > + * For walking descendants, cgroup_iter can walk in either pre-order or
> > + * post-order. For walking ancestors, the iter walks up from a cgroup to
> > + * the root.
> > + *
> > + * The iter program can terminate the walk early by returning 1. Walk
> > + * continues if prog returns 0.
> > + *
> > + * The prog can check (seq->num == 0) to determine whether this is
> > + * the first element. The prog may also be passed a NULL cgroup,
> > + * which means the walk has completed and the prog has a chance to
> > + * do post-processing, such as outputing an epilogue.
> > + *
> > + * Note: the iter_prog is called with cgroup_mutex held.
> > + *
> > + * Currently only one session is supported, which means, depending on the
> > + * volume of data bpf program intends to send to user space, the number
> > + * of cgroups that can be walked is limited. For example, given the current
> > + * buffer size is 8 * PAGE_SIZE, if the program sends 64B data for each
> > + * cgroup, the total number of cgroups that can be walked is 512. This is
>
> Again, let us specify PAGE_SIZE = 4KB here.
>
Ok.
> > + * a limitation of cgroup_iter. If the output data is larger than the
> > + * buffer size, the second read() will signal EOPNOTSUPP. In order to work
> > + * around, the user may have to update their program to reduce the volume
> > + * of data sent to output. For example, skip some uninteresting cgroups.
> > + */
> > +
> > +struct bpf_iter__cgroup {
> > + __bpf_md_ptr(struct bpf_iter_meta *, meta);
> > + __bpf_md_ptr(struct cgroup *, cgroup);
> > +};
> > +
> > +struct cgroup_iter_priv {
> > + struct cgroup_subsys_state *start_css;
> > + bool terminate;
> > + int order;
> > +};
> > +
> > +static void *cgroup_iter_seq_start(struct seq_file *seq, loff_t *pos)
> > +{
> > + struct cgroup_iter_priv *p = seq->private;
> > +
> > + mutex_lock(&cgroup_mutex);
> > +
> > + /* cgroup_iter doesn't support read across multiple sessions. */
> > + if (*pos > 0)
> > + return ERR_PTR(-EOPNOTSUPP);
>
> This is not quite right. Let us say, the number of cgroups is 1,
> after bpf program run, pos = 1, and the control return to user
> space. Now the second read() will return -EOPNOTSUPP which is not
> right. -EOPNOTSUPP should be returned ONLY if the previous cgroup
> iterations do not traverse all cgroups.
> So you might need to record additional information in cgroup_iter_priv
> to record such information.
>
Oh, I missed seeing this scenario. Then we need a flag in
cgroup_iter_priv indicating whether iter is happening. Only when
during iter hasn't ended, we return -EOPNOTSUPP.
> > +
> > + ++*pos;
> > + p->terminate = false;
> > + if (p->order == BPF_ITER_CGROUP_PRE)
> > + return css_next_descendant_pre(NULL, p->start_css);
> > + else if (p->order == BPF_ITER_CGROUP_POST)
> > + return css_next_descendant_post(NULL, p->start_css);
> > + else /* BPF_ITER_CGROUP_PARENT_UP */
> > + return p->start_css;
> > +}
> > +
> > +static int __cgroup_iter_seq_show(struct seq_file *seq,
> > + struct cgroup_subsys_state *css, int in_stop);
> > +
> > +static void cgroup_iter_seq_stop(struct seq_file *seq, void *v)
> > +{
> > + /* pass NULL to the prog for post-processing */
> > + if (!v)
> > + __cgroup_iter_seq_show(seq, NULL, true);
> > + mutex_unlock(&cgroup_mutex);
> > +}
> > +
> > +static void *cgroup_iter_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> > +{
> > + struct cgroup_subsys_state *curr = (struct cgroup_subsys_state *)v;
> > + struct cgroup_iter_priv *p = seq->private;
> > +
> > + ++*pos;
> > + if (p->terminate)
> > + return NULL;
> > +
> > + if (p->order == BPF_ITER_CGROUP_PRE)
> > + return css_next_descendant_pre(curr, p->start_css);
> > + else if (p->order == BPF_ITER_CGROUP_POST)
> > + return css_next_descendant_post(curr, p->start_css);
> > + else
> > + return curr->parent;
> > +}
> > +
> [...]
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