Re: [PATCH RFC 0/6] epoll: Introduce new syscall "epoll_mod_wait"

[Fam Zheng <famz-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Tue, 01/20 14:40, Andy Lutomirski wrote:
> On Tue, Jan 20, 2015 at 1:57 AM, Fam Zheng <famz-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> > This adds a new system call, epoll_mod_wait. It's described as below:
> >
> > NAME
> >        epoll_mod_wait - modify and wait for I/O events on an epoll file
> >                         descriptor
> >
> > SYNOPSIS
> >
> >        int epoll_mod_wait(int epfd, int flags,
> >                           int ncmds, struct epoll_mod_cmd *cmds,
> >                           struct epoll_wait_spec *spec);
> >
> > DESCRIPTION
> >
> >        The epoll_mod_wait() system call can be seen as an enhanced combination
> >        of several epoll_ctl(2) calls, which are followed by an epoll_pwait(2)
> >        call. It is superior in two cases:
> >
> >        1) When epoll_ctl(2) are followed by epoll_wait(2), using epoll_mod_wait
> >        will save context switches between user mode and kernel mode;
> >
> >        2) When you need higher precision than microsecond for wait timeout.
> >
> >        The epoll_ctl(2) operations are embedded into this call by with ncmds
> >        and cmds. The latter is an array of command structs:
> >
> >            struct epoll_mod_cmd {
> >
> >                   /* Reserved flags for future extension, must be 0 for now. */
> >                   int flags;
> >
> >                   /* The same as epoll_ctl() op parameter. */
> >                   int op;
> >
> >                   /* The same as epoll_ctl() fd parameter. */
> >                   int fd;
> >
> >                   /* The same as the "events" field in struct epoll_event. */
> >                   uint32_t events;
> >
> >                   /* The same as the "data" field in struct epoll_event. */
> >                   uint64_t data;
> >
> >                   /* Output field, will be set to the return code once this
> >                    * command is executed by kernel */
> >                   int error;
> >            };
> 
> I would add an extra u32 at the end so that the structure size will be
> a multiple of 8 bytes on all platforms.

OK, makes sense.

> 
> >
> >        There is no guartantee that all the commands are executed in order. Only
> >        if all the commands are successfully executed (all the error fields are
> >        set to 0), events are polled.
> 
> If this doesn't happen, what error is returned?

The last error in executing commands.

> 
> >            struct epoll_wait_spec {
> >
> >                   /* The same as "maxevents" in epoll_pwait() */
> >                   int maxevents;
> >
> >                   /* The same as "events" in epoll_pwait() */
> >                   struct epoll_event *events;
> >
> >                   /* Which clock to use for timeout */
> >                   int clockid;
> >
> >                   /* Maximum time to wait if there is no event */
> >                   struct timespec timeout;
> >
> >                   /* The same as "sigmask" in epoll_pwait() */
> >                   sigset_t *sigmask;
> >
> >                   /* The same as "sigsetsize" in epoll_pwait() */
> >                   size_t sigsetsize;
> >            } EPOLL_PACKED;
> 
> I think the convention is to align the structure's fields manually
> rather than declaring it to be packed.

OK.

> 
> >
> > RETURN VALUE
> >
> >        When any error occurs, epoll_mod_wait() returns -1 and errno is set
> >        appropriately. All the "error" fields in cmds are unchanged before they
> >        are executed, and if any cmds are executed, the "error" fields are set
> >        to a return code accordingly. See also epoll_ctl for more details of the
> >        return code.
> 
> Does this mean that callers should initialize the error fields to an
> impossible value first so they can tell which commands were executed?

Yes.

> 
> >
> >        When successful, epoll_mod_wait() returns the number of file
> >        descriptors ready for the requested I/O, or zero if no file descriptor
> >        became ready during the requested timeout milliseconds.
> >
> >        If spec is NULL, it returns 0 if all the commands are successful, and -1
> >        if an error occured.
> >
> > ERRORS
> >
> >        These errors apply on either the return value of epoll_mod_wait or error
> >        status for each command, respectively.
> 
> Please clarify which errors are returned overall and which are per-command.

OK.

Fam