| Message ID | 1532746900-11710-1-git-send-email-laoar.shao@gmail.com |
|---|---|
| State | Changes Requested, archived |
| Delegated to: | David Miller |
| Headers | show |
| Series | [net-next,1/2] tcp: call tcp_drop() in tcp collapse | expand |
On Fri, Jul 27, 2018 at 8:02 PM Yafang Shao <laoar.shao@gmail.com> wrote: > > When this SKB is dropped, we should add the counter sk_drops. > That could help us better tracking this behavior. > > Signed-off-by: Yafang Shao <laoar.shao@gmail.com> > --- > net/ipv4/tcp_input.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c > index d51fa35..90f83eb 100644 > --- a/net/ipv4/tcp_input.c > +++ b/net/ipv4/tcp_input.c > @@ -4802,7 +4802,7 @@ static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb, > else > rb_erase(&skb->rbnode, root); > > - __kfree_skb(skb); > + tcp_drop(sk, skb); Absolutely not. We do not drop the packet, we have simply lowered the memory overhead.
On Sat, Jul 28, 2018 at 11:06 AM, Eric Dumazet <edumazet@google.com> wrote: > On Fri, Jul 27, 2018 at 8:02 PM Yafang Shao <laoar.shao@gmail.com> wrote: >> >> When this SKB is dropped, we should add the counter sk_drops. >> That could help us better tracking this behavior. >> >> Signed-off-by: Yafang Shao <laoar.shao@gmail.com> >> --- >> net/ipv4/tcp_input.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c >> index d51fa35..90f83eb 100644 >> --- a/net/ipv4/tcp_input.c >> +++ b/net/ipv4/tcp_input.c >> @@ -4802,7 +4802,7 @@ static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb, >> else >> rb_erase(&skb->rbnode, root); >> >> - __kfree_skb(skb); >> + tcp_drop(sk, skb); > > > Absolutely not. > > We do not drop the packet, we have simply lowered the memory overhead. So what about LINUX_MIB_TCPOFOMERGE ? Regarding LINUX_MIB_TCPOFOMERGE, a skb is already covered by another skb, is that dropping the packet or simply lowering the memory overhead ? Thanks Yafang
On Fri, Jul 27, 2018 at 8:35 PM Yafang Shao <laoar.shao@gmail.com> wrote: > So what about LINUX_MIB_TCPOFOMERGE ? > Regarding LINUX_MIB_TCPOFOMERGE, a skb is already covered by another > skb, is that dropping the packet or simply lowering the memory > overhead ? What do you think ? If you receive two times the same payload, don't you have to drop one of the duplicate ? There is a a big difference between the two cases.
On Sat, Jul 28, 2018 at 11:38 AM, Eric Dumazet <edumazet@google.com> wrote: > On Fri, Jul 27, 2018 at 8:35 PM Yafang Shao <laoar.shao@gmail.com> wrote: > >> So what about LINUX_MIB_TCPOFOMERGE ? >> Regarding LINUX_MIB_TCPOFOMERGE, a skb is already covered by another >> skb, is that dropping the packet or simply lowering the memory >> overhead ? > > What do you think ? > > If you receive two times the same payload, don't you have to drop one > of the duplicate ? > > There is a a big difference between the two cases. If the drop caused some data lost (which may then cause retransmition or something), then this is a really DROP. While if the drop won't cause any data lost, meaning it is a non-harmful behavior, I think it should not be defined as DROP. This is my suggestion anyway. Thanks Yafang
On Sat, Jul 28, 2018 at 12:43 AM Yafang Shao <laoar.shao@gmail.com> wrote: > > On Sat, Jul 28, 2018 at 11:38 AM, Eric Dumazet <edumazet@google.com> wrote: > > On Fri, Jul 27, 2018 at 8:35 PM Yafang Shao <laoar.shao@gmail.com> wrote: > > > >> So what about LINUX_MIB_TCPOFOMERGE ? > >> Regarding LINUX_MIB_TCPOFOMERGE, a skb is already covered by another > >> skb, is that dropping the packet or simply lowering the memory > >> overhead ? > > > > What do you think ? > > > > If you receive two times the same payload, don't you have to drop one > > of the duplicate ? > > > > There is a a big difference between the two cases. > > If the drop caused some data lost (which may then cause retransmition > or something), then this is a really DROP. > While if the drop won't cause any data lost, meaning it is a > non-harmful behavior, I think it should not be defined as DROP. > This is my suggestion anyway. Sigh. We count drops, not because they are ' bad or something went wrong'. If TCP stack receives twice the same sequence (same payload), we _drop_ one of the duplicate, so we account for this event. When ' collapsing' we reorganize our own storage, not because we have to drop a payload, but for some memory pressure reason. We have specific SNMP counters to account for these, we do not want to pretend a packet was ' dropped' since it was not. If we have to _drop_ some packets, it is called Pruning, and we do properly account for these drops.
On Sun, Jul 29, 2018 at 12:28 AM, Eric Dumazet <edumazet@google.com> wrote: > On Sat, Jul 28, 2018 at 12:43 AM Yafang Shao <laoar.shao@gmail.com> wrote: >> >> On Sat, Jul 28, 2018 at 11:38 AM, Eric Dumazet <edumazet@google.com> wrote: >> > On Fri, Jul 27, 2018 at 8:35 PM Yafang Shao <laoar.shao@gmail.com> wrote: >> > >> >> So what about LINUX_MIB_TCPOFOMERGE ? >> >> Regarding LINUX_MIB_TCPOFOMERGE, a skb is already covered by another >> >> skb, is that dropping the packet or simply lowering the memory >> >> overhead ? >> > >> > What do you think ? >> > >> > If you receive two times the same payload, don't you have to drop one >> > of the duplicate ? >> > >> > There is a a big difference between the two cases. >> >> If the drop caused some data lost (which may then cause retransmition >> or something), then this is a really DROP. >> While if the drop won't cause any data lost, meaning it is a >> non-harmful behavior, I think it should not be defined as DROP. >> This is my suggestion anyway. > > Sigh. > > We count drops, not because they are ' bad or something went wrong'. > > If TCP stack receives twice the same sequence (same payload), we > _drop_ one of the duplicate, so we account for this event. > > When ' collapsing' we reorganize our own storage, not because we have > to drop a payload, > but for some memory pressure reason. Thanks for you clarification. So what about LINUX_MIB_TCPOFODROP ? if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) { NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFODROP); tcp_drop(sk, skb); return; } It is also because of our own memory pressure, but we call tcp_drop() here. I am not mean to disagree with you. I am just confused and want to make it clear. > We have specific SNMP counters to account for these, we do not want to > pretend a packet was ' dropped' since it was not. > > If we have to _drop_ some packets, it is called Pruning, and we do > properly account for these drops. Agreed. Thanks Yafang
On Sun, Jul 29, 2018 at 7:06 PM Yafang Shao <laoar.shao@gmail.com> wrote: > > On Sun, Jul 29, 2018 at 12:28 AM, Eric Dumazet <edumazet@google.com> wrote: > > On Sat, Jul 28, 2018 at 12:43 AM Yafang Shao <laoar.shao@gmail.com> wrote: > >> > >> On Sat, Jul 28, 2018 at 11:38 AM, Eric Dumazet <edumazet@google.com> wrote: > >> > On Fri, Jul 27, 2018 at 8:35 PM Yafang Shao <laoar.shao@gmail.com> wrote: > >> > > >> >> So what about LINUX_MIB_TCPOFOMERGE ? > >> >> Regarding LINUX_MIB_TCPOFOMERGE, a skb is already covered by another > >> >> skb, is that dropping the packet or simply lowering the memory > >> >> overhead ? > >> > > >> > What do you think ? > >> > > >> > If you receive two times the same payload, don't you have to drop one > >> > of the duplicate ? > >> > > >> > There is a a big difference between the two cases. > >> > >> If the drop caused some data lost (which may then cause retransmition > >> or something), then this is a really DROP. > >> While if the drop won't cause any data lost, meaning it is a > >> non-harmful behavior, I think it should not be defined as DROP. > >> This is my suggestion anyway. > > > > Sigh. > > > > We count drops, not because they are ' bad or something went wrong'. > > > > If TCP stack receives twice the same sequence (same payload), we > > _drop_ one of the duplicate, so we account for this event. > > > > When ' collapsing' we reorganize our own storage, not because we have > > to drop a payload, > > but for some memory pressure reason. > > Thanks for you clarification. > So what about LINUX_MIB_TCPOFODROP ? > > if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) { > NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFODROP); > tcp_drop(sk, skb); > return; > } > > > It is also because of our own memory pressure, but we call tcp_drop() here. Yes, we _drop_ a packet. That is pretty clear that the payload is dropped, and that the sender will have to _retransmit_. > > I am not mean to disagree with you. I am just confused and want to > make it clear. Collapsing is : For (a bunch of packets) Try (to compress them in order to reduce memory overhead) No drop of payload happens here. Sender wont have to retransmit.
On Mon, Jul 30, 2018 at 10:27 AM, Eric Dumazet <edumazet@google.com> wrote: > On Sun, Jul 29, 2018 at 7:06 PM Yafang Shao <laoar.shao@gmail.com> wrote: >> >> On Sun, Jul 29, 2018 at 12:28 AM, Eric Dumazet <edumazet@google.com> wrote: >> > On Sat, Jul 28, 2018 at 12:43 AM Yafang Shao <laoar.shao@gmail.com> wrote: >> >> >> >> On Sat, Jul 28, 2018 at 11:38 AM, Eric Dumazet <edumazet@google.com> wrote: >> >> > On Fri, Jul 27, 2018 at 8:35 PM Yafang Shao <laoar.shao@gmail.com> wrote: >> >> > >> >> >> So what about LINUX_MIB_TCPOFOMERGE ? >> >> >> Regarding LINUX_MIB_TCPOFOMERGE, a skb is already covered by another >> >> >> skb, is that dropping the packet or simply lowering the memory >> >> >> overhead ? >> >> > >> >> > What do you think ? >> >> > >> >> > If you receive two times the same payload, don't you have to drop one >> >> > of the duplicate ? >> >> > >> >> > There is a a big difference between the two cases. >> >> >> >> If the drop caused some data lost (which may then cause retransmition >> >> or something), then this is a really DROP. >> >> While if the drop won't cause any data lost, meaning it is a >> >> non-harmful behavior, I think it should not be defined as DROP. >> >> This is my suggestion anyway. >> > >> > Sigh. >> > >> > We count drops, not because they are ' bad or something went wrong'. >> > >> > If TCP stack receives twice the same sequence (same payload), we >> > _drop_ one of the duplicate, so we account for this event. >> > >> > When ' collapsing' we reorganize our own storage, not because we have >> > to drop a payload, >> > but for some memory pressure reason. >> >> Thanks for you clarification. >> So what about LINUX_MIB_TCPOFODROP ? >> >> if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) { >> NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFODROP); >> tcp_drop(sk, skb); >> return; >> } >> >> >> It is also because of our own memory pressure, but we call tcp_drop() here. > > Yes, we _drop_ a packet. > > That is pretty clear that the payload is dropped, and that the sender > will have to _retransmit_. > >> >> I am not mean to disagree with you. I am just confused and want to >> make it clear. > > > Collapsing is : > > For (a bunch of packets) > Try (to compress them in order to reduce memory overhead) > > No drop of payload happens here. Sender wont have to retransmit. OK. Thanks for your patient. Should we put NET_INC_STATS(sock_net(sk), mib_idx) into the funtion tcp_drop() ? Then we could easily relate the sk_drops with the SNMP counters. Something like that, static void tcp_drop(struct sock *sk, struct sk_buff *skb, int mib_idx) { int segs = max_t(u16, 1, skb_shinfo(skb)->gso_segs); atomic_add(segs, &sk->sk_drops); NET_ADD_STATS(sock_net(sk), mib_idx, segs); __kfree_skb(skb); } Thanks Yafang
On Sun, Jul 29, 2018 at 10:40 PM Yafang Shao <laoar.shao@gmail.com> wrote: > Should we put NET_INC_STATS(sock_net(sk), mib_idx) into the funtion > tcp_drop() ? > Then we could easily relate the sk_drops with the SNMP counters. > > Something like that, > > static void tcp_drop(struct sock *sk, struct sk_buff *skb, int mib_idx) > { > int segs = max_t(u16, 1, skb_shinfo(skb)->gso_segs); > > atomic_add(segs, &sk->sk_drops); > NET_ADD_STATS(sock_net(sk), mib_idx, segs); > __kfree_skb(skb); > } We had a discussion during netconf, and Brendan Gregg was working on an idea like that, so that distinct events could be traced/reported. I prefer letting Brendan submit his patch, which not only refactors things, but add new functionality. Thanks.
On Mon, Jul 30, 2018 at 11:56 PM, Eric Dumazet <edumazet@google.com> wrote: > On Sun, Jul 29, 2018 at 10:40 PM Yafang Shao <laoar.shao@gmail.com> wrote: > >> Should we put NET_INC_STATS(sock_net(sk), mib_idx) into the funtion >> tcp_drop() ? >> Then we could easily relate the sk_drops with the SNMP counters. >> >> Something like that, >> >> static void tcp_drop(struct sock *sk, struct sk_buff *skb, int mib_idx) >> { >> int segs = max_t(u16, 1, skb_shinfo(skb)->gso_segs); >> >> atomic_add(segs, &sk->sk_drops); >> NET_ADD_STATS(sock_net(sk), mib_idx, segs); >> __kfree_skb(skb); >> } > > We had a discussion during netconf, and Brendan Gregg was working on > an idea like that, > so that distinct events could be traced/reported. > Oh yes, introducing a new tracepoint for it should be better. trace_tcp_probe(sk, skb, mib_idx); > I prefer letting Brendan submit his patch, which not only refactors > things, but add new functionality. > OK.
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index d51fa35..90f83eb 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -4802,7 +4802,7 @@ static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb, else rb_erase(&skb->rbnode, root); - __kfree_skb(skb); + tcp_drop(sk, skb); NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED); return next;
When this SKB is dropped, we should add the counter sk_drops. That could help us better tracking this behavior. Signed-off-by: Yafang Shao <laoar.shao@gmail.com> --- net/ipv4/tcp_input.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)