Re: [Xen-devel] [PATCH V2] Switch from select() to poll() in xenconsoled's IO loop.

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

> +static int initialize_pollfd_arrays(void)
> +{
> +       fds = (struct pollfd *)
> +               malloc(sizeof(struct pollfd) * DEFAULT_ARRAY_SIZE);
> +       if (!fds)
> +               goto fail;
> +       fd_to_pollfd = (struct pollfd **)
> +               malloc(sizeof(struct pollfd *) * DEFAULT_ARRAY_SIZE);
> +       if (!fd_to_pollfd)
> +               goto fail;
> +       memset(fds, 0, sizeof(struct pollfd) * DEFAULT_ARRAY_SIZE);
> +       memset(fd_to_pollfd, 0, sizeof(struct pollfd *) * DEFAULT_ARRAY_SIZE);
> +       current_array_size = DEFAULT_ARRAY_SIZE;
> +       return 0;
> +fail:
> +       free(fds);
> +       free(fd_to_pollfd);
> +       return -ENOMEM;
> +}
> +
> +static void destroy_pollfd_arrays(void)
> +{
> +       free(fds);
> +       free(fd_to_pollfd);
> +       current_array_size = 0;
> +}
> +
> +static void set_fds(int fd, short events)
> +{
> +       if (current_array_size < fd+1) {
> +               struct pollfd  *p1 = NULL;
> +               struct pollfd **p2 = NULL;
> +               unsigned int newsize = current_array_size;
> +
> +               do { newsize += GROWTH_LENGTH; } while (newsize < fd+1);

Steal #define ROUNDUP from tools/libxc/xc_linux_osdep.c.

> +
> +               p1 = realloc(fds, sizeof(struct pollfd)*newsize);
> +               if (!p1)
> +                       goto fail;
> +               fds = p1;
> +
> +               p2 = realloc(fd_to_pollfd, sizeof(struct pollfd *)*newsize);

realloc(NULL, ...) is the same as malloc() so I think you can initialise
current_array_size to 0 and the various pointers to NULL and avoid the
need for initialize_pollfd_arrays -- i.e. it will just grow from 0 on
the first use here.

>         for (;;) {
>                 struct domain *d, *n;
> -               int max_fd = -1;
> -               struct timeval timeout;
> +               int poll_timeout; /* timeout in milliseconds */
>                 struct timespec ts;
>                 long long now, next_timeout = 0;
> 
> -               FD_ZERO(&readfds);
> -               FD_ZERO(&writefds);
> +               reset_fds();
> 
> -               FD_SET(xs_fileno(xs), &readfds);
> -               max_fd = MAX(xs_fileno(xs), max_fd);
> +               set_fds(xs_fileno(xs), POLLIN);

Do you know, or can you track, the maximum fd over time?

If you can then you could likely make use of automatic stack allocations
(struct pollfd fds[max_fd]) and therefore avoid the pain of manual
memory management.

Not sure what the semantics of those are inside a for loop where max_fd
can change but worst case you could put the content of the loop into a
function.

Ian.


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