Message-ID: <CAEf4BzbyHHDrECCEjrSC3A5X39qb_WZaU_3_qNONP+vHAcUzuQ@mail.gmail.com>
Date:   Tue, 15 Dec 2020 14:03:58 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Cong Wang <cong.wang@...edance.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Dongdong Wang <wangdongdong.6@...edance.com>
Subject: Re: [Patch bpf-next v2 2/5] bpf: introduce timeout map

On Tue, Dec 15, 2020 at 12:06 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Tue, Dec 15, 2020 at 11:27 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Mon, Dec 14, 2020 at 12:17 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> > >
> > > From: Cong Wang <cong.wang@...edance.com>
> > >
> > > This borrows the idea from conntrack and will be used for conntrack in
> > > bpf too. Each element in a timeout map has a user-specified timeout
> > > in secs, after it expires it will be automatically removed from the map.
> > >
> > > There are two cases here:
> > >
> > > 1. When the timeout map is idle, that is, no one updates or accesses it,
> > >    we rely on the idle work to scan the whole hash table and remove
> > >    these expired. The idle work is scheduled every 1 sec.
> >
> > Would 1 second be a good period for a lot of cases? Probably would be
> > good to expand on what went into this decision.
>
> Sure, because our granularity is 1 sec, I will add it into changelog.
>

Granularity of a timeout is not that coupled with the period of
garbage collection. In this case, with 1 second period, you can have
some items not garbage collected for up to 2 seconds due to timing and
races. Just keep that in mind.

> >
> > >
> > > 2. When the timeout map is actively accessed, we could reach expired
> > >    elements before the idle work kicks in, we can simply skip them and
> > >    schedule another work to do the actual removal work. We avoid taking
> > >    locks on fast path.
> > >
> > > The timeout of each element can be set or updated via bpf_map_update_elem()
> > > and we reuse the upper 32-bit of the 64-bit flag for the timeout value.
> > >
> > > Cc: Alexei Starovoitov <ast@...nel.org>
> > > Cc: Daniel Borkmann <daniel@...earbox.net>
> > > Cc: Dongdong Wang <wangdongdong.6@...edance.com>
> > > Signed-off-by: Cong Wang <cong.wang@...edance.com>
> > > ---
> > >  include/linux/bpf_types.h      |   1 +
> > >  include/uapi/linux/bpf.h       |   3 +-
> > >  kernel/bpf/hashtab.c           | 244 ++++++++++++++++++++++++++++++++-
> > >  kernel/bpf/syscall.c           |   3 +-
> > >  tools/include/uapi/linux/bpf.h |   1 +
> > >  5 files changed, 248 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/include/linux/bpf_types.h b/include/linux/bpf_types.h
> > > index 99f7fd657d87..00a3b17b6af2 100644
> > > --- a/include/linux/bpf_types.h
> > > +++ b/include/linux/bpf_types.h
> > > @@ -125,6 +125,7 @@ BPF_MAP_TYPE(BPF_MAP_TYPE_STACK, stack_map_ops)
> > >  BPF_MAP_TYPE(BPF_MAP_TYPE_STRUCT_OPS, bpf_struct_ops_map_ops)
> > >  #endif
> > >  BPF_MAP_TYPE(BPF_MAP_TYPE_RINGBUF, ringbuf_map_ops)
> > > +BPF_MAP_TYPE(BPF_MAP_TYPE_TIMEOUT_HASH, htab_timeout_map_ops)
> > >
> > >  BPF_LINK_TYPE(BPF_LINK_TYPE_RAW_TRACEPOINT, raw_tracepoint)
> > >  BPF_LINK_TYPE(BPF_LINK_TYPE_TRACING, tracing)
> > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > index 30b477a26482..dedb47bc3f52 100644
> > > --- a/include/uapi/linux/bpf.h
> > > +++ b/include/uapi/linux/bpf.h
> > > @@ -158,6 +158,7 @@ enum bpf_map_type {
> > >         BPF_MAP_TYPE_RINGBUF,
> > >         BPF_MAP_TYPE_INODE_STORAGE,
> > >         BPF_MAP_TYPE_TASK_STORAGE,
> > > +       BPF_MAP_TYPE_TIMEOUT_HASH,
> > >  };
> > >
> > >  /* Note that tracing related programs such as
> > > @@ -393,7 +394,7 @@ enum bpf_link_type {
> > >   */
> > >  #define BPF_PSEUDO_CALL                1
> > >
> > > -/* flags for BPF_MAP_UPDATE_ELEM command */
> > > +/* flags for BPF_MAP_UPDATE_ELEM command, upper 32 bits are timeout */
> >
> > timeout in what units of time?
>
> 1 sec, should I also add it in this comment (and everywhere else)?

yes, please

>
> >
> > >  enum {
> > >         BPF_ANY         = 0, /* create new element or update existing */
> > >         BPF_NOEXIST     = 1, /* create new element if it didn't exist */
> > > diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> > > index f0b7b54fa3a8..178cb376c397 100644
> > > --- a/kernel/bpf/hashtab.c
> > > +++ b/kernel/bpf/hashtab.c
> > > @@ -8,6 +8,8 @@
> > >  #include <linux/filter.h>
> > >  #include <linux/rculist_nulls.h>
> > >  #include <linux/random.h>
> > > +#include <linux/llist.h>
> > > +#include <linux/workqueue.h>
> > >  #include <uapi/linux/btf.h>
> > >  #include <linux/rcupdate_trace.h>
> > >  #include "percpu_freelist.h"
> > > @@ -84,6 +86,8 @@ struct bucket {
> > >                 raw_spinlock_t raw_lock;
> > >                 spinlock_t     lock;
> > >         };
> > > +       struct llist_node gc_node;
> > > +       atomic_t pending;
> >
> > HASH is an extremely frequently used type of map, and oftentimes with
> > a lot of entries/buckets. I don't think users of normal
> > BPF_MAP_TYPE_HASH should pay the price of way more niche hashmap with
> > timeouts. So I think it's not appropriate to increase the size of the
> > struct bucket here.
>
> I understand that, but what's a better way to do this? I can wrap it up
> on top of struct bucket for sure, but it would need to change a lot of code.
> So, basically code reuse vs. struct bucket size increase. ;)

I think not paying potentially lots of memory for unused features
wins. Some struct embedding might work. Or just better code reuse.
Please think this through, don't wait for me to write the code for
you.

>
> >
> > >  };
> > >
> > >  #define HASHTAB_MAP_LOCK_COUNT 8
> > > @@ -104,6 +108,9 @@ struct bpf_htab {
> > >         u32 hashrnd;
> > >         struct lock_class_key lockdep_key;
> > >         int __percpu *map_locked[HASHTAB_MAP_LOCK_COUNT];
> > > +       struct llist_head gc_list;
> > > +       struct work_struct gc_work;
> > > +       struct delayed_work gc_idle_work;
> > >  };
> > >
> > >  /* each htab element is struct htab_elem + key + value */
> > > @@ -124,6 +131,7 @@ struct htab_elem {
> > >                 struct bpf_lru_node lru_node;
> > >         };
> > >         u32 hash;
> > > +       u64 expires;
> >
> > time units? and similar concerns about wasting a lot of added memory
> > for not benefit to HASH users.
>
> 'expires' is in jiffies, I can add a comment here if necessary.

I think it's really helpful, because everyone reading this would be
wondering and then jumping around the code to figure it out

>
> Similarly, please suggest how to expand struct htab_elem without changing
> a lot of code. I also tried to find some hole in the struct, but I
> couldn't, so I
> ran out of ideas here.

I mentioned above, you can have your own struct and embed htab_elem
inside. It might need some refactoring, of course.

>
> >
> > >         char key[] __aligned(8);
> > >  };
> > >
> > > @@ -143,6 +151,7 @@ static void htab_init_buckets(struct bpf_htab *htab)
> > >
> > >         for (i = 0; i < htab->n_buckets; i++) {
> > >                 INIT_HLIST_NULLS_HEAD(&htab->buckets[i].head, i);
> > > +               atomic_set(&htab->buckets[i].pending, 0);
> > >                 if (htab_use_raw_lock(htab)) {
> > >                         raw_spin_lock_init(&htab->buckets[i].raw_lock);
> > >                         lockdep_set_class(&htab->buckets[i].raw_lock,
> > > @@ -431,6 +440,14 @@ static int htab_map_alloc_check(union bpf_attr *attr)
> > >         return 0;
> > >  }
> > >
> > > +static void htab_sched_gc(struct bpf_htab *htab, struct bucket *b)
> > > +{
> > > +       if (atomic_fetch_or(1, &b->pending))
> > > +               return;
> > > +       llist_add(&b->gc_node, &htab->gc_list);
> > > +       queue_work(system_unbound_wq, &htab->gc_work);
> > > +}
> >
> > I'm concerned about each bucket being scheduled individually... And
> > similarly concerned that each instance of TIMEOUT_HASH will do its own
> > scheduling independently. Can you think about the way to have a
> > "global" gc/purging logic, and just make sure that buckets that need
> > processing would be just internally chained together. So the purging
> > routing would iterate all the scheduled hashmaps, and within each it
> > will have a linked list of buckets that need processing? And all that
> > is done just once each GC period. Not N times for N maps or N*M times
> > for N maps with M buckets in each.
>
> Our internal discussion went to the opposite actually, people here argued
> one work is not sufficient for a hashtable because there would be millions
> of entries (max_entries, which is also number of buckets). ;)

I was hoping that it's possible to expire elements without iterating
the entire hash table every single time, only items that need to be
processed. Hashed timing wheel is one way to do something like this,
kernel has to solve similar problems with timeouts as well, why not
taking inspiration there?

>
> I chose one work per hash table because we could use map-in-map to divide
> the millions of entries.
>
> So, this really depends on how many maps and how many buckets in each
> map. I tend to leave this as it is, because there is no way to satisfy
> all of the
> cases.

But I think some ways are better than others. Please consider all the
options, not just the simplest one.

>
> >
> > > +
> > >  static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
> > >  {
> > >         bool percpu = (attr->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
> > > @@ -732,10 +749,13 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
> > >  static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
> > >  {
> > >         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
> > > +       bool is_timeout = map->map_type == BPF_MAP_TYPE_TIMEOUT_HASH;
> > >         struct hlist_nulls_head *head;
> > >         struct htab_elem *l, *next_l;
> > >         u32 hash, key_size;
> > > +       struct bucket *b;
> > >         int i = 0;
> > > +       u64 now;
> > >
> > >         WARN_ON_ONCE(!rcu_read_lock_held());
> > >
> > > @@ -746,7 +766,8 @@ static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
> > >
> > >         hash = htab_map_hash(key, key_size, htab->hashrnd);
> > >
> > > -       head = select_bucket(htab, hash);
> > > +       b = __select_bucket(htab, hash);
> > > +       head = &b->head;
> > >
> > >         /* lookup the key */
> > >         l = lookup_nulls_elem_raw(head, hash, key, key_size, htab->n_buckets);
> > > @@ -759,6 +780,13 @@ static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
> > >                                   struct htab_elem, hash_node);
> > >
> > >         if (next_l) {
> > > +               if (is_timeout) {
> > > +                       now = get_jiffies_64();
> > > +                       if (time_after_eq64(now, next_l->expires)) {
> > > +                               htab_sched_gc(htab, b);
> > > +                               goto find_first_elem;
> > > +                       }
> > > +               }
> >
> > this piece of logic is repeated verbatim many times, seems like a
> > helper function would make sense here
>
> Except goto or continue, isn't it? ;) Please do share your ideas on
> how to make it a helper for both goto and continue.

So there is no way to make it work like this:

if (htab_elem_expired(htab, next_l))
    goto find_first_elem;

?

>
>
> >
> > >                 /* if next elem in this hash list is non-zero, just return it */
> > >                 memcpy(next_key, next_l->key, key_size);
> > >                 return 0;
> > > @@ -771,12 +799,20 @@ static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
> > >  find_first_elem:
> > >         /* iterate over buckets */
> > >         for (; i < htab->n_buckets; i++) {
> > > -               head = select_bucket(htab, i);
> > > +               b = __select_bucket(htab, i);
> > > +               head = &b->head;
> > >
> > >                 /* pick first element in the bucket */
> > >                 next_l = hlist_nulls_entry_safe(rcu_dereference_raw(hlist_nulls_first_rcu(head)),
> > >                                           struct htab_elem, hash_node);
> > >                 if (next_l) {
> > > +                       if (is_timeout) {
> > > +                               now = get_jiffies_64();
> > > +                               if (time_after_eq64(now, next_l->expires)) {
> > > +                                       htab_sched_gc(htab, b);
> > > +                                       continue;
> > > +                               }
> > > +                       }
> > >                         /* if it's not empty, just return it */
> > >                         memcpy(next_key, next_l->key, key_size);
> > >                         return 0;
> > > @@ -975,18 +1011,31 @@ static int check_flags(struct bpf_htab *htab, struct htab_elem *l_old,
> > >         return 0;
> > >  }
> > >
> > > +static u32 fetch_timeout(u64 *map_flags)
> > > +{
> > > +       u32 timeout = (*map_flags) >> 32;
> > > +
> > > +       *map_flags = (*map_flags) & 0xffffffff;
> > > +       return timeout;
> > > +}
> > > +
> > >  /* Called from syscall or from eBPF program */
> > >  static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
> > >                                 u64 map_flags)
> > >  {
> > >         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
> > > +       bool timeout_map = map->map_type == BPF_MAP_TYPE_TIMEOUT_HASH;
> > >         struct htab_elem *l_new = NULL, *l_old;
> > >         struct hlist_nulls_head *head;
> > >         unsigned long flags;
> > >         struct bucket *b;
> > >         u32 key_size, hash;
> > > +       u32 timeout;
> > > +       u64 now;
> > >         int ret;
> > >
> > > +       timeout = fetch_timeout(&map_flags);
> > > +
> > >         if (unlikely((map_flags & ~BPF_F_LOCK) > BPF_EXIST))
> > >                 /* unknown flags */
> > >                 return -EINVAL;
> > > @@ -1042,6 +1091,10 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
> > >                 copy_map_value_locked(map,
> > >                                       l_old->key + round_up(key_size, 8),
> > >                                       value, false);
> > > +               if (timeout_map) {
> > > +                       now = get_jiffies_64();
> > > +                       l_old->expires = now + HZ * timeout;
> > > +               }
> >
> > Ok, so it seems timeout is at a second granularity. Would it make
> > sense to make it at millisecond granularity instead? I think
> > millisecond would be more powerful and allow more use cases, in the
> > long run. Micro- and nano-second granularity seems like an overkill,
> > though. And would reduce the max timeout to pretty small numbers. With
> > milliseconds, you still will get more than 23 days of timeout, which
> > seems to be plenty.
>
> Sure if you want to pay the price of scheduling the work more often...

See above about timer granularity and GC period. You can have
nanosecond precision timeout and still GC only once every seconds, as
an example. You are checking expiration on lookup, so it can be
handled very precisely. You don't have to GC 1000 times per second to
support millisecond granularity.

>
> For our own use case, second is sufficient. What use case do you have
> for paying this price? I am happy to hear.

I don't have a specific use case. But I also don't see the extra price
we need to pay. You are adding a new *generic* data structure to the
wide BPF infrastructure, so please consider implications beyond your
immediate use case.

>
> Thanks.

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