Re: [PATCH v2 4/4] net/colo: Match is-enabled probe to tracepoint

["Daniel P. Berrangé" <berrange@redhat.com>]

On Sat, Jul 18, 2020 at 05:58:56PM +0000, Zhang, Chen wrote:
> 
> 
> > -----Original Message-----
> > From: Roman Bolshakov <r.bolshakov@yadro.com>
> > Sent: Friday, July 17, 2020 5:35 PM
> > To: qemu-devel@nongnu.org
> > Cc: Daniel P. Berrangé <berrange@redhat.com>; Stefan Hajnoczi
> > <stefanha@redhat.com>; Cameron Esfahani <dirty@apple.com>; Roman
> > Bolshakov <r.bolshakov@yadro.com>; Philippe Mathieu-Daudé
> > <philmd@redhat.com>; Zhang, Chen <chen.zhang@intel.com>; Li Zhijian
> > <lizhijian@cn.fujitsu.com>; Jason Wang <jasowang@redhat.com>
> > Subject: [PATCH v2 4/4] net/colo: Match is-enabled probe to tracepoint
> > 
> > Build of QEMU with dtrace fails on macOS:
> > 
> >   LINK    x86_64-softmmu/qemu-system-x86_64
> > error: probe colo_compare_miscompare doesn't exist
> > error: Could not register probes
> > ld: error creating dtrace DOF section for architecture x86_64
> > 
> > The reason of the error is explained by Adam Leventhal [1]:
> > 
> >   Note that is-enabled probes don't have the stability magic so I'm not
> >   sure how things would work if only is-enabled probes were used.
> > 
> > net/colo code uses is-enabled probes to determine if other probes should be
> > used but colo_compare_miscompare itself is not used explicitly.
> > Linker doesn't include the symbol and build fails.
> > 
> > The issue can be resolved if is-enabled probe matches the actual trace point
> > that is used inside the test. Packet dump toggle is replaced with a compile-
> > time conditional definition.
> > 
> > 1. http://markmail.org/message/6grq2ygr5nwdwsnb
> > 
> > Fixes: f4b618360e ("colo-compare: add TCP, UDP, ICMP packet comparison")
> > Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
> > Cc: Cameron Esfahani <dirty@apple.com>
> > Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
> > ---
> >  net/colo-compare.c    | 42 ++++++++++++++++++++++--------------------
> >  net/filter-rewriter.c | 10 ++++++++--
> >  net/trace-events      |  2 --
> >  3 files changed, 30 insertions(+), 24 deletions(-)


> > (trace_event_get_state_backends(TRACE_COLO_COMPARE_MISCOMPARE))
> > {
> > +    if (trace_event_get_state_backends(TRACE_COLO_COMPARE_IP_INFO))
> > {
> >          char pri_ip_src[20], pri_ip_dst[20], sec_ip_src[20], sec_ip_dst[20];
> > 
> >          strcpy(pri_ip_src, inet_ntoa(ppkt->ip->ip_src)); @@ -492,12 +494,12
> > @@ sec:
> >          g_queue_push_head(&conn->primary_list, ppkt);
> >          g_queue_push_head(&conn->secondary_list, spkt);
> > 
> > -        if
> > (trace_event_get_state_backends(TRACE_COLO_COMPARE_MISCOMPARE))
> > {
> > -            qemu_hexdump((char *)ppkt->data, stderr,
> > -                        "colo-compare ppkt", ppkt->size);
> > -            qemu_hexdump((char *)spkt->data, stderr,
> > -                        "colo-compare spkt", spkt->size);
> > -        }
> > +#ifdef DEBUG_COLO_PACKETS
> > +        qemu_hexdump((char *)ppkt->data, stderr,
> > +                     "colo-compare ppkt", ppkt->size);
> > +        qemu_hexdump((char *)spkt->data, stderr,
> > +                     "colo-compare spkt", spkt->size); #endif
> > 
> >          colo_compare_inconsistency_notify(s);
> >      }
> > @@ -533,12 +535,12 @@ static int colo_packet_compare_udp(Packet *spkt,
> > Packet *ppkt)
> >                                      ppkt->size - offset)) {
> >          trace_colo_compare_udp_miscompare("primary pkt size", ppkt->size);
> >          trace_colo_compare_udp_miscompare("Secondary pkt size", spkt-
> > >size);
> > -        if
> > (trace_event_get_state_backends(TRACE_COLO_COMPARE_MISCOMPARE))
> > {
> > -            qemu_hexdump((char *)ppkt->data, stderr, "colo-compare pri pkt",
> > -                         ppkt->size);
> > -            qemu_hexdump((char *)spkt->data, stderr, "colo-compare sec pkt",
> > -                         spkt->size);
> > -        }
> > +#ifdef DEBUG_COLO_PACKETS
> > +        qemu_hexdump((char *)ppkt->data, stderr, "colo-compare pri pkt",
> > +                     ppkt->size);
> > +        qemu_hexdump((char *)spkt->data, stderr, "colo-compare sec pkt",
> > +                     spkt->size);
> > +#endif
> 
> Hi Roman,
> 
> I think change the " trace_event_get_state_backends()" to
> "trace_colo_compare_main("Dump packet hex: ")" is a better choice here.
> It will keep the original code logic and avoid the problem here.

That may workaround the immediate bug, but this is still a misuse of the
tracing code. Use of any trace point should only trigger actions in the
trace infrastructure.

If I'm using dtrace backend to monitor events I don't want to see QEMU
dumping stuff to stderr. Anything written to stderr is going to trigger
disk I/O writing to the VM's logfile, and is also liable to trigger rate
limiting which can impact the guest performance.

Regards,
Daniel
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