Hi On Thu, Mar 5, 2020 at 3:46 PM Markus Armbruster <arm...@redhat.com> wrote: > > I tried to observe the main loop keeps running while the screendump does > its work. > > The main loop appears to lack trace points. Alright, if there's no > hammer handy, I'll use a rock: > > diff --git a/softmmu/vl.c b/softmmu/vl.c > index 5549f4b619..b6561a65d7 100644 > --- a/softmmu/vl.c > +++ b/softmmu/vl.c > @@ -1661,6 +1661,7 @@ void qemu_main_loop(void) > #ifdef CONFIG_PROFILER > ti = profile_getclock(); > #endif > + printf("*** main loop\n"); > main_loop_wait(false); > #ifdef CONFIG_PROFILER > dev_time += profile_getclock() - ti; > > > First experiment: does the main loop continue to run when writing out > the screendump blocks / would block? > > Observe qmp_screendump() opens the file without O_EXCL. Great, that > lets me block output by making it open a FIFO. > > Terminal#1: > > $ mkfifo s > > Terminal#2: > > $ upstream-qemu -S -display none -chardev > socket,id=qmp,path=test-qmp,server=on,wait=off -mon mode=control,chardev=qmp > *** main loop > *** main loop > *** main loop > > Keeps printing at a steady pace. > > Terminal#3: > > $ socat "READLINE,history=$HOME/.qmp_history,prompt=QMP>" > UNIX-CONNECT:$HOME/work/images/test-qmp > {"QMP": {"version": {"qemu": {"micro": 50, "minor": 2, "major": 4}, > "package": "v4.2.0-2069-g5e5ae6b644-dirty"}, "capabilities": ["oob"]}} > QMP>{"execute": "qmp_capabilities"} > {"return": {}} > QMP>{"execute": "screendump", "arguments": {"filename": "s"}} > > The printing in terminal#2 stops. This is expected; qemu_open() calls > open(), which blocks, because the FIFO has no reader. > > Terminal#1: > > $ exec 4<s > > Now the FIFO has a reader. Terminal#2 remains quiet. > > We now hang in ppm_save(). Abridged stack backtrace: > > #0 0x00007ffff519d0f5 in writev () at /lib64/libc.so.6 > #1 0x0000555555e15f61 in qio_channel_file_writev > (ioc=0x5555567bf5f0, iov=0x555556a441b0, niov=1, fds=0x0, nfds=0, > errp=0x7fffe9d81d10) at /work/armbru/qemu/io/channel-file.c:123 > #2 0x0000555555e133d3 in qio_channel_writev_full > (ioc=0x5555567bf5f0, iov=0x555556a441b0, niov=1, fds=0x0, nfds=0, > errp=0x7fffe9d81d10) at /work/armbru/qemu/io/channel.c:86 > #3 0x0000555555e137a2 in qio_channel_writev > (ioc=0x5555567bf5f0, iov=0x555556a441b0, niov=1, errp=0x7fffe9d81d10) > at /work/armbru/qemu/io/channel.c:207 > #4 0x0000555555e13696 in qio_channel_writev_all > (ioc=0x5555567bf5f0, iov=0x7fffe9d81bd0, niov=1, errp=0x7fffe9d81d10) > at /work/armbru/qemu/io/channel.c:171 > #5 0x0000555555e139b1 in qio_channel_write_all > (ioc=0x5555567bf5f0, buf=0x555556b05200 "", buflen=1920, > errp=0x7fffe9d81d10) at /work/armbru/qemu/io/channel.c:257 > #6 0x0000555555cd74ff in ppm_save > (fd=22, image=0x5555568ffdd0, errp=0x7fffe9d81d10) > at /work/armbru/qemu/ui/console.c:336 > #7 0x0000555555cd77e6 in qmp_screendump > (filename=0x555556ea0900 "s", has_device=false, device=0x0, > has_head=false, head=0, errp=0x7fffe9d81d10) at > /work/armbru/qemu/ui/console.c:401 > > A brief inspection of qio_channel_file_writev() and > qio_channel_writev_all() suggests this might work if you make the output > file descriptor non-blocking.
Right, the goal was rather originally to fix rhbz#1230527. We got coroutine IO by accident, and I was too optimistic about default behaviour change ;) I will update the patch. > > $ head -c 1 <&4 | hexdump -C > 00000000 50 |P| > 00000001 > > Still quiet. > > $ cat <&4 >/dev/null > > The printing resumes. > > $ exec 4<&- > > > Second experiment: does the main loop continue to run while we wait for > graphic_hw_update_done()? > > Left as an exercise for the patch submitter ;) > > With your main loop printf, one printf in graphic_hw_update() and one in graphic_hw_update_done() ? (rather part of testing commit 4d6316218bf7bf3b8c7c7165b072cc314511a7a7, soon 4y old!) -- Marc-André Lureau