| Message ID | 20230327175446.98151-4-john.fastabend@gmail.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | BPF |
| Headers | show |
| Series | bpf sockmap fixes | expand |
On Mon, Mar 27, 2023 at 10:54 AM -07, John Fastabend wrote: > We noticed some rare sk_buffs were stepping past the queue when system was > under memory pressure. The general theory is to skip enqueueing > sk_buffs when its not necessary which is the normal case with a system > that is properly provisioned for the task, no memory pressure and enough > cpu assigned. > > But, if we can't allocate memory due to an ENOMEM error when enqueueing > the sk_buff into the sockmap receive queue we push it onto a delayed > workqueue to retry later. When a new sk_buff is received we then check > if that queue is empty. However, there is a problem with simply checking > the queue length. When a sk_buff is being processed from the ingress queue > but not yet on the sockmap msg receive queue its possible to also recv > a sk_buff through normal path. It will check the ingress queue which is > zero and then skip ahead of the pkt being processed. > > Previously we used sock lock from both contexts which made the problem > harder to hit, but not impossible. > > To fix also check the 'state' variable where we would cache partially > processed sk_buff. This catches the majority of cases. But, we also > need to use the mutex lock around this check because we can't have both > codes running and check sensibly. We could perhaps do this with atomic > bit checks, but we are already here due to memory pressure so slowing > things down a bit seems OK and simpler to just grab a lock. > > To reproduce issue we run NGINX compliance test with sockmap running and > observe some flakes in our testing that we attributed to this issue. > > Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()") > Tested-by: William Findlay <will@isovalent.com> > Signed-off-by: John Fastabend <john.fastabend@gmail.com> > --- I've got an idea to try, but it'd a bigger change. skb_dequeue is lock, skb_peek, skb_unlink, unlock, right? What if we split up the skb_dequeue in sk_psock_backlog to publish the change to the ingress_skb queue only once an skb has been processed? static void sk_psock_backlog(struct work_struct *work) { ... while ((skb = skb_peek_locked(&psock->ingress_skb))) { ... skb_unlink(skb, &psock->ingress_skb); } ... } Even more - if we hold off the unlinking until an skb has been fully processed, that perhaps opens up the way to get rid of keeping state in sk_psock_work_state. We could just skb_pull the processed data instead. It's just an idea and I don't want to block a tested fix that LGTM so consider this: Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Jakub Sitnicki wrote: > On Mon, Mar 27, 2023 at 10:54 AM -07, John Fastabend wrote: > > We noticed some rare sk_buffs were stepping past the queue when system was > > under memory pressure. The general theory is to skip enqueueing > > sk_buffs when its not necessary which is the normal case with a system > > that is properly provisioned for the task, no memory pressure and enough > > cpu assigned. > > > > But, if we can't allocate memory due to an ENOMEM error when enqueueing > > the sk_buff into the sockmap receive queue we push it onto a delayed > > workqueue to retry later. When a new sk_buff is received we then check > > if that queue is empty. However, there is a problem with simply checking > > the queue length. When a sk_buff is being processed from the ingress queue > > but not yet on the sockmap msg receive queue its possible to also recv > > a sk_buff through normal path. It will check the ingress queue which is > > zero and then skip ahead of the pkt being processed. > > > > Previously we used sock lock from both contexts which made the problem > > harder to hit, but not impossible. > > > > To fix also check the 'state' variable where we would cache partially > > processed sk_buff. This catches the majority of cases. But, we also > > need to use the mutex lock around this check because we can't have both > > codes running and check sensibly. We could perhaps do this with atomic > > bit checks, but we are already here due to memory pressure so slowing > > things down a bit seems OK and simpler to just grab a lock. > > > > To reproduce issue we run NGINX compliance test with sockmap running and > > observe some flakes in our testing that we attributed to this issue. > > > > Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()") > > Tested-by: William Findlay <will@isovalent.com> > > Signed-off-by: John Fastabend <john.fastabend@gmail.com> > > --- > > I've got an idea to try, but it'd a bigger change. > > skb_dequeue is lock, skb_peek, skb_unlink, unlock, right? > > What if we split up the skb_dequeue in sk_psock_backlog to publish the > change to the ingress_skb queue only once an skb has been processed? I think this works now. Early on we tried to run backlog without locking but given this is now locked by mutex I think it works out. We have a few places that work on ingress_skb but those are all enqueue on tail so should be fine to peek here. > > static void sk_psock_backlog(struct work_struct *work) > { > ... > while ((skb = skb_peek_locked(&psock->ingress_skb))) { > ... > skb_unlink(skb, &psock->ingress_skb); > } > ... > } > > Even more - if we hold off the unlinking until an skb has been fully > processed, that perhaps opens up the way to get rid of keeping state in > sk_psock_work_state. We could just skb_pull the processed data instead. Yep. > > It's just an idea and I don't want to block a tested fix that LGTM so > consider this: Did you get a chance to try this? Otherwise I can also give the idea a try next week. > > Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
On Fri, Mar 31, 2023 at 05:59 PM -07, John Fastabend wrote: > Jakub Sitnicki wrote: [...] >> It's just an idea and I don't want to block a tested fix that LGTM so >> consider this: > > Did you get a chance to try this? Otherwise I can also give the idea > a try next week. No. By all means, feel free. You caught me at a busy time. That's also why the review has been going so slow.
diff --git a/net/core/skmsg.c b/net/core/skmsg.c index 96a6a3a74a67..34de0605694e 100644 --- a/net/core/skmsg.c +++ b/net/core/skmsg.c @@ -985,6 +985,7 @@ EXPORT_SYMBOL_GPL(sk_psock_tls_strp_read); static int sk_psock_verdict_apply(struct sk_psock *psock, struct sk_buff *skb, int verdict) { + struct sk_psock_work_state *state; struct sock *sk_other; int err = 0; u32 len, off; @@ -1001,13 +1002,28 @@ static int sk_psock_verdict_apply(struct sk_psock *psock, struct sk_buff *skb, skb_bpf_set_ingress(skb); + /* We need to grab mutex here because in-flight skb is in one of + * the following states: either on ingress_skb, in psock->state + * or being processed by backlog and neither in state->skb and + * ingress_skb may be also empty. The troublesome case is when + * the skb has been dequeued from ingress_skb list or taken from + * state->skb because we can not easily test this case. Maybe we + * could be clever with flags and resolve this but being clever + * got us here in the first place and we note this is done under + * sock lock and backlog conditions mean we are already running + * into ENOMEM or other performance hindering cases so lets do + * the obvious thing and grab the mutex. + */ + mutex_lock(&psock->work_mutex); + state = &psock->work_state; + /* If the queue is empty then we can submit directly * into the msg queue. If its not empty we have to * queue work otherwise we may get OOO data. Otherwise, * if sk_psock_skb_ingress errors will be handled by * retrying later from workqueue. */ - if (skb_queue_empty(&psock->ingress_skb)) { + if (skb_queue_empty(&psock->ingress_skb) && likely(!state->skb)) { len = skb->len; off = 0; if (skb_bpf_strparser(skb)) { @@ -1028,9 +1044,11 @@ static int sk_psock_verdict_apply(struct sk_psock *psock, struct sk_buff *skb, spin_unlock_bh(&psock->ingress_lock); if (err < 0) { skb_bpf_redirect_clear(skb); + mutex_unlock(&psock->work_mutex); goto out_free; } } + mutex_unlock(&psock->work_mutex); break; case __SK_REDIRECT: err = sk_psock_skb_redirect(psock, skb);
We noticed some rare sk_buffs were stepping past the queue when system was under memory pressure. The general theory is to skip enqueueing sk_buffs when its not necessary which is the normal case with a system that is properly provisioned for the task, no memory pressure and enough cpu assigned. But, if we can't allocate memory due to an ENOMEM error when enqueueing the sk_buff into the sockmap receive queue we push it onto a delayed workqueue to retry later. When a new sk_buff is received we then check if that queue is empty. However, there is a problem with simply checking the queue length. When a sk_buff is being processed from the ingress queue but not yet on the sockmap msg receive queue its possible to also recv a sk_buff through normal path. It will check the ingress queue which is zero and then skip ahead of the pkt being processed. Previously we used sock lock from both contexts which made the problem harder to hit, but not impossible. To fix also check the 'state' variable where we would cache partially processed sk_buff. This catches the majority of cases. But, we also need to use the mutex lock around this check because we can't have both codes running and check sensibly. We could perhaps do this with atomic bit checks, but we are already here due to memory pressure so slowing things down a bit seems OK and simpler to just grab a lock. To reproduce issue we run NGINX compliance test with sockmap running and observe some flakes in our testing that we attributed to this issue. Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()") Tested-by: William Findlay <will@isovalent.com> Signed-off-by: John Fastabend <john.fastabend@gmail.com> --- net/core/skmsg.c | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-)