Messages in this thread | | | Date | Mon, 9 Jan 2023 10:39:22 -0500 | From | Steven Rostedt <> | Subject | Re: [External] Re: [PATCH v4] sock: add tracepoint for send recv length |
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On Mon, 9 Jan 2023 16:20:58 +0100 Eric Dumazet <edumazet@google.com> wrote:
> On Mon, Jan 9, 2023 at 4:08 PM Steven Rostedt <rostedt@goodmis.org> wrote: > > > > On Mon, 9 Jan 2023 15:54:38 +0100 > > Eric Dumazet <edumazet@google.com> wrote: > > > > > > static inline int sock_sendmsg_nosec(struct socket *sock, struct msghdr *msg) > > > > { > > > > int ret = INDIRECT_CALL_INET(sock->ops->sendmsg, inet6_sendmsg, > > > > inet_sendmsg, sock, msg, > > > > msg_data_left(msg)); > > > > BUG_ON(ret == -EIOCBQUEUED); > > > > > > > > if (trace_sock_send_length_enabled()) { > > > > > > A barrier() is needed here, with the current state of affairs. > > > > > > IMO, ftrace/x86 experts should take care of this generic issue ? > > > > trace_*_enabled() is a static_branch() (aka. jump label). > > > > It's a nop, where the if block is in the out-of-line code and skipped. When > > the tracepoint is enabled, it gets turned into a jump to the if block > > (which returns back to this location). > > > > This is not a nop, as shown in the generated assembly, I copied in > this thread earlier.
But I thought that was for the patch before the added noinline helper function to force the tracepoint into its own function, and this now just has the static branch.
> > Compiler does all sorts of things before the point the static branch > is looked at. > > Let's add the extract again with <<*>> tags on added instructions/dereferences. > >
Ug, bad line wrapping
> sock_recvmsg_nosec: > pushq %r12 # > movl %edx, %r12d # tmp123, flags > pushq %rbp # > # net/socket.c:999: int ret = > INDIRECT_CALL_INET(sock->ops->recvmsg, inet6_recvmsg, > movl %r12d, %ecx # flags, > # net/socket.c:998: { > movq %rdi, %rbp # tmp121, sock > pushq %rbx # > # net/socket.c:999: int ret = > INDIRECT_CALL_INET(sock->ops->recvmsg, inet6_recvmsg, > movq 32(%rdi), %rax # sock_19(D)->ops, sock_19(D)->ops > # ./include/linux/uio.h:270: return i->count; > movq 32(%rsi), %rdx # MEM[(const struct iov_iter > *)msg_20(D) + 16B].count, pretmp_48 > # net/socket.c:999: int ret = > INDIRECT_CALL_INET(sock->ops->recvmsg, inet6_recvmsg, > movq 144(%rax), %rax # _1->recvmsg, _2 > cmpq $inet6_recvmsg, %rax #, _2 > jne .L107 #, > call inet6_recvmsg # > <<*>> movl %eax, %ebx # tmp124, <retval> > .L108: > # net/socket.c:1003: trace_sock_recv_length(sock->sk, sock->sk->sk_family, > <<*>> xorl %r8d, %r8d # tmp127 > <<*>> testl %ebx, %ebx # <retval> > # net/socket.c:1004: sock->sk->sk_protocol, > <<*>> movq 24(%rbp), %rsi # sock_19(D)->sk, _10 > # net/socket.c:1003: trace_sock_recv_length(sock->sk, sock->sk->sk_family, > <<*>> cmovle %ebx, %r8d # <retval>,, tmp119 > <<*>> testb $2, %r12b #, flags > # net/socket.c:1004: sock->sk->sk_protocol, > <<*>> movzwl 516(%rsi), %ecx # _10->sk_protocol, > # net/socket.c:1003: trace_sock_recv_length(sock->sk, sock->sk->sk_family, > <<*>> movzwl 16(%rsi), %edx # _10->__sk_common.skc_family, > # net/socket.c:1003: trace_sock_recv_length(sock->sk, sock->sk->sk_family, > <<*>> cmove %ebx, %r8d # tmp119,, <retval>, iftmp.54_16 > # ./arch/x86/include/asm/jump_label.h:27: asm_volatile_goto("1:" > #APP > # 27 "./arch/x86/include/asm/jump_label.h" 1 > 1:jmp .L111 # objtool NOPs this # > .pushsection __jump_table, "aw" > .balign 8 > .long 1b - . > .long .L111 - . # > .quad __tracepoint_sock_recv_length+8 + 2 - . #, > .popsection > > # 0 "" 2 > #NO_APP > .L106: > # net/socket.c:1008: } > <<*>> movl %ebx, %eax # <retval>, > popq %rbx # > popq %rbp # > popq %r12 # > ret > .L111: > # ./include/trace/events/sock.h:308: DEFINE_EVENT(sock_msg_length, > sock_recv_length, > > > That is, when the tracepoint in the block gets enabled so does the above > > branch. Sure, there could be a race between the two being enabled, but I > > don't see any issue if there is. But the process to modify the jump labels, > > does a bunch of synchronization between the CPUs. > > > > What barrier are you expecting? > > Something preventing the compiler being 'smart', forcing expression evaluations > before TP_fast_assign() is eventually called.
There's no good way to generically keep the compiler from being 'smart', that I know of. That's because the tracepoint injection is defined by:
#define __DECLARE_TRACE(name, proto, args, cond, data_proto) \ extern int __traceiter_##name(data_proto); \ DECLARE_STATIC_CALL(tp_func_##name, __traceiter_##name); \ extern struct tracepoint __tracepoint_##name; \ static inline void __trace_##name(proto) \ {
That (proto) is the prototype being passed in. And because macros can't create other macros, I don't know how to have a way to inject a barrier() before and after that call, or better yet, to have the prototype hidden behind the static_branch.
But looking at this tracepoint again, I see a issue that can help with the dereferencing.
Why is family and protocol passed in?
+ trace_sock_send_length(sock->sk, sock->sk->sk_family, + sock->sk->sk_protocol, ret, 0);
Where the TP_fast_assign() is:
+ TP_fast_assign( + __entry->sk = sk; + __entry->family = sk->sk_family; + __entry->protocol = sk->sk_protocol; + __entry->length = ret > 0 ? ret : 0; + __entry->error = ret < 0 ? ret : 0; + __entry->flags = flags; + ),
The family and protocol is taken from the sk, and not the parameters. I bet dropping those would help.
-- Steve
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