Marc-André Lureau <marcandre.lur...@gmail.com> writes:
> 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!)
We might also need to artificially delay graphic_hw_update_done().
