Re: [PATCH 3/3] console: make QMP/HMP screendump run in coroutine

[Markus Armbruster <armbru@redhat.com>]

marcandre.lureau@redhat.com writes:

> From: Marc-André Lureau <marcandre.lureau@redhat.com>
>
> Thanks to the monitors coroutine support, the screendump handler can

monitors'

Suggest to add (merge commit b7092cda1b3) right before the comma.

> trigger a graphic_hw_update(), yield and let the main loop run until
> update is done. Then the handler is resumed, and ppm_save() will write
> the screen image to disk in the coroutine context (thus non-blocking).
>
> Potentially, during non-blocking write, some new graphic update could
> happen, and thus the image may have some glitches. Whether that
> behaviour is acceptable is discutable. Allocating new memory may not be

s/discutable/debatable/

> a good idea, as framebuffers can be quite large. Even then, QEMU may
> become less responsive as it requires paging in etc.

Tradeoff.  I'm okay with "simple & efficient, but might glitch".  It
should be documented, though.  Followup patch is fine.

> Related to:
> https://bugzilla.redhat.com/show_bug.cgi?id=1230527
>
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  hmp-commands.hx    |  1 +
>  monitor/hmp-cmds.c |  3 ++-
>  qapi/ui.json       |  3 ++-
>  ui/console.c       | 27 ++++++++++++++++++++++++---
>  4 files changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/hmp-commands.hx b/hmp-commands.hx
> index cd068389de..ff2d7aa8f3 100644
> --- a/hmp-commands.hx
> +++ b/hmp-commands.hx
> @@ -254,6 +254,7 @@ ERST
>          .help       = "save screen from head 'head' of display device 'device' "
>                        "into PPM image 'filename'",
>          .cmd        = hmp_screendump,
> +        .coroutine  = true,
>      },
>  
>  SRST
> diff --git a/monitor/hmp-cmds.c b/monitor/hmp-cmds.c
> index 9789f4277f..91608bac6d 100644
> --- a/monitor/hmp-cmds.c
> +++ b/monitor/hmp-cmds.c
> @@ -1756,7 +1756,8 @@ err_out:
>      goto out;
>  }
>  
> -void hmp_screendump(Monitor *mon, const QDict *qdict)
> +void coroutine_fn
> +hmp_screendump(Monitor *mon, const QDict *qdict)
>  {
>      const char *filename = qdict_get_str(qdict, "filename");
>      const char *id = qdict_get_try_str(qdict, "device");
> diff --git a/qapi/ui.json b/qapi/ui.json
> index 9d6721037f..6c7b33cb72 100644
> --- a/qapi/ui.json
> +++ b/qapi/ui.json
> @@ -98,7 +98,8 @@
>  #
>  ##
>  { 'command': 'screendump',
> -  'data': {'filename': 'str', '*device': 'str', '*head': 'int'} }
> +  'data': {'filename': 'str', '*device': 'str', '*head': 'int'},
> +  'coroutine': true }
>  
>  ##
>  # == Spice
> diff --git a/ui/console.c b/ui/console.c
> index a56fe0dd26..0118f70d9a 100644
> --- a/ui/console.c
> +++ b/ui/console.c
> @@ -168,6 +168,7 @@ struct QemuConsole {
>      QEMUFIFO out_fifo;
>      uint8_t out_fifo_buf[16];
>      QEMUTimer *kbd_timer;
> +    CoQueue dump_queue;
>  
>      QTAILQ_ENTRY(QemuConsole) next;
>  };
> @@ -263,6 +264,7 @@ static void gui_setup_refresh(DisplayState *ds)
>  
>  void graphic_hw_update_done(QemuConsole *con)
>  {
> +    qemu_co_queue_restart_all(&con->dump_queue);
>  }
>  
>  void graphic_hw_update(QemuConsole *con)
> @@ -340,8 +342,15 @@ static bool ppm_save(int fd, pixman_image_t *image, Error **errp)
>      return true;
>  }
>  
> -void qmp_screendump(const char *filename, bool has_device, const char *device,
> -                    bool has_head, int64_t head, Error **errp)
> +static void graphic_hw_update_bh(void *con)
> +{
> +    graphic_hw_update(con);
> +}
> +
> +/* Safety: coroutine-only, concurrent-coroutine safe, main thread only */
> +void coroutine_fn
> +qmp_screendump(const char *filename, bool has_device, const char *device,
> +               bool has_head, int64_t head, Error **errp)
>  {
>      g_autoptr(pixman_image_t) image = NULL;
>      QemuConsole *con;
> @@ -366,7 +375,15 @@ void qmp_screendump(const char *filename, bool has_device, const char *device,
>          }
>      }
>  
> -    graphic_hw_update(con);
> +    if (qemu_co_queue_empty(&con->dump_queue)) {
> +        /* Defer the update, it will restart the pending coroutines */
> +        aio_bh_schedule_oneshot(qemu_get_aio_context(),
> +                                graphic_hw_update_bh, con);
> +    }
> +    qemu_co_queue_wait(&con->dump_queue, NULL);
> +
> +    /* All pending coroutines are woken up, while BQL taken, no further graphic
> +     * update are possible until it is released, take an image ref before that. */

"while BQL taken": I guess you mean "while the BQL is held".

Style nit: CODING_STYLE.rst asks for wings.

Recommend to limit comment line length for readability.

Recommend to turn a few commas into periods.

Together:

    /*
     * All pending coroutines are woken up, while the BQL is held.  No
     * further graphic update are possible until it is released.  Take
     * an image ref before that.
     */

>      surface = qemu_console_surface(con);
>      if (!surface) {
>          error_setg(errp, "no surface");
> @@ -381,6 +398,9 @@ void qmp_screendump(const char *filename, bool has_device, const char *device,
>          return;
>      }
>  
> +    /* The image content could potentially be updated as the coroutine yields
> +     * and releases the BQL. It could produce corrupted dump, but it should be
> +     * otherwise safe. */

Similar nit.

    /*
     * The image content could potentially be updated as the coroutine
     * yields and releases the BQL. It could produce corrupted dump, but
     * it should be otherwise safe.
     */

>      if (!ppm_save(fd, image, errp)) {
>          qemu_unlink(filename);
>      }
> @@ -1297,6 +1317,7 @@ static QemuConsole *new_console(DisplayState *ds, console_type_t console_type,
>  
>      obj = object_new(TYPE_QEMU_CONSOLE);
>      s = QEMU_CONSOLE(obj);
> +    qemu_co_queue_init(&s->dump_queue);
>      s->head = head;
>      object_property_add_link(obj, "device", TYPE_DEVICE,
>                               (Object **)&s->device,

Simpler than v1 thanks to coroutine support for HMP, and the use of
CoQueue.


Let's revisit the experiment I did for v1: "observe the main loop keeps
running while the screendump does its work".

Message-ID: <87a74ueudt.fsf@dusky.pond.sub.org>
https://lists.nongnu.org/archive/html/qemu-devel/2020-03/msg01235.html

I repeated the first experiment "does the main loop continue to run when
writing out the screendump blocks / would block?"  Same result: main
loop remains blocked.

Back then, you replied

    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.

I'm unsure what exactly the update is.  Is it the change from

    Fixes:
    https://bugzilla.redhat.com/show_bug.cgi?id=1230527

to

    Related to:
    https://bugzilla.redhat.com/show_bug.cgi?id=1230527

?

The commit message says "ppm_save() will write the screen image to disk
in the coroutine context (thus non-blocking)."  Sure reads like a claim
the main loop is no longer blocked.  It is blocked, at least for me.
Please clarify.

Back then, I proposed a second experiment: "does the main loop continue
to run while we wait for graphic_hw_update_done()?"  I don't know the
result.  Do you?

The commit message claims "the screendump handler can trigger a
graphic_hw_update(), yield and let the main loop run until update is
done."