On 16 Nov (07:18:45), 草木秀 wrote: > >Indeed, from a sessiond point of view, this will typically use the same FD > >number, but it's really associated with a different posix SHM each time. > > > Yes,what confused me the most is that sessiond sends the same FD number to > UST every time,but UST receives the different FD number each time and can map > it to the right per-cpu share buffer sucessfully.
Yes, in per UID buffers, for every application using UST we send the FD corresponding to the already allocated buffer (being per UID). Please note that sending a FD through a Unix socket dups it so basically UST receives that, access the shared memory buffer and can then operate correctly. Cheers! David > > > Thanks > MuXiu.Cao > > > > > > From: "草木秀" <[email protected]> > To: "mathieu.desnoyers" <[email protected]> > Sent: Friday, November 15, 2013 11:37:11 AM > Subject: how does ust tracer map share lttng buffer > > > Hi,Mathieu > > > After lttng sub-command enable-event is issued at first time, > lttng-consumerd will be informed by lttng-sessiond to open and mmap share > memory regions .Then lttng-sessiond get all shmfds from lttng-consumerd.But > how does ust tracer map to these share memory regions? > > > AFAIK,lttng-sessiond will call do_consumer_create_channel() ,then > send_channel_uid_to_ust(). What's confused me is in > send_channel_uid_to_ust(),lttng-sessiond dup() one shmfd , then > ust_consumer_send_stream_to_ust() to ust tracer, then close the shmfd and > loop all shmfds. So all duped fds are the same fd-number (say A)and send to > ust tracer through > ust_consumer_send_stream_to_ust()->ustctl_send_stream_to_ust()->ustctl_send_stream()->ustcomm_send_fds_unix_sock()(unix > socket is $HOME/.lttng/lttng-ust-sock-5).Why does it send the same fds to > ust tracer? > At ust tracer side,it calls > handle_message()->ustcomm_recv_stream_from_sessiond() and mmap every received > fd to share memory region.However,every received fd is different from A.So I > must miss something anywhere.How does ust tracer map share lttng buffer? > > > In lttng-sessiond, we send file descriptors for each posix SHM associated to > every per-cpu buffer to the application over unix socket. So it's more or > less : receive FD from consumer, send FD over unix socket to UST, close FD > (we loop on this). Indeed, from a sessiond point of view, this will typically > use the same FD number, but it's really associated with a different posix SHM > each time. David can probably answer your question in more detail. > > > > Thanks, > > > > Mathieu > > > > > > > > PS:code is from lttng-2.3.0 . > > > Thanks > MuXiu.Cao > > > > > > > > -- > > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com
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