Re: [PATCH] monitor: Fix slow reading

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Cc'ing the chardev maintainers.

On 11/22/19 10:23 AM, Yury Kotov wrote:
> The monitor_can_read (as a callback of qemu_chr_fe_set_handlers)
> should return size of buffer which monitor_qmp_read or monitor_read
> can process.
> Currently, monitor_can_read returns 1 as a result of logical not.
> Thus, for each QMP command, len(QMD) iterations of the main loop
> are required to handle a command.
> In fact, these both functions can process any buffer size.
> So, return 1024 as a reasonable size which is enough to process
> the most QMP commands, but not too big to block the main loop for
> a long time.
> 
> Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>
> ---
>   monitor/monitor.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/monitor/monitor.c b/monitor/monitor.c
> index 12898b6448..cac3f39727 100644
> --- a/monitor/monitor.c
> +++ b/monitor/monitor.c
> @@ -50,6 +50,13 @@ typedef struct {
>       int64_t rate;       /* Minimum time (in ns) between two events */
>   } MonitorQAPIEventConf;
>   
> +/*
> + * The maximum buffer size which the monitor can process in one iteration
> + * of the main loop. We don't want to block the loop for a long time
> + * because of JSON parser, so use a reasonable value.
> + */
> +#define MONITOR_READ_LEN_MAX 1024

This looks reasonable.
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

> +
>   /* Shared monitor I/O thread */
>   IOThread *mon_iothread;
>   
> @@ -498,7 +505,7 @@ int monitor_can_read(void *opaque)
>   {
>       Monitor *mon = opaque;
>   
> -    return !atomic_mb_read(&mon->suspend_cnt);
> +    return atomic_mb_read(&mon->suspend_cnt) ? 0 : MONITOR_READ_LEN_MAX;
>   }
>   
>   void monitor_list_append(Monitor *mon)
>