Re: [PATCH RFC 5/6] epoll: Add implementation for epoll_mod_wait

[Fam Zheng]

On Tue, 01/20 13:50, Michael Kerrisk (man-pages) wrote:
> Hello Fam Zheng,
> 
> On 01/20/2015 10:57 AM, Fam Zheng wrote:
> > This syscall is a sequence of
> > 
> > 1) a number of epoll_ctl calls
> > 2) a epoll_pwait, with timeout enhancement.
> > 
> > The epoll_ctl operations are embeded so that application doesn't have to use
> > separate syscalls to insert/delete/update the fds before poll. It is more
> > efficient if the set of fds varies from one poll to another, which is the
> > common pattern for certain applications. 
> 
> Which applications? Could we have some specific examples? This is a 
> complex API, and it needs good justification.

OK, I'll explain more in v2.

> 
> > For example, depending on the input
> > buffer status, a data reading program may decide to temporarily not polling an
> > fd.
> > 
> > Because the enablement of batching in this interface, even that regular
> > epoll_ctl call sequence, which manipulates several fds, can be optimized to one
> > single epoll_ctl_wait (while specifying spec=NULL to skip the poll part).
>          ^^^^^^^^^^^^^^ should be epoll_mod_wait
> 
> I think you mean to say:
> 
>     The ability to batch multiple "epoll_ctl" operations into a single call
>     means that even when no wait events are requested (i.e., spec == NULL),
>     poll_mod_wait() provides a performance optimization over using multiple
>     epoll_ctl() calls.
> 
> Right? If yes, please amend the commit message, and this text should
> also make its way into the revised man page under a heading "NOTES".

OK.

> 
> > The only complexity is returning the result of each operation.  For each
> > epoll_mod_cmd in cmds, the field "error" is an output field that will be stored
> > the return code *iff* the command is executed (0 for success and -errno of the
> > equivalent epoll_ctl call), and will be left unchanged if the command is not
> > executed because some earlier error, for example due to failure of
> > copy_from_user to copy the array.
> > 
> > Applications can utilize this fact to do error handling: they could initialize
> > all the epoll_mod_wait.error to a positive value, which is by definition not a
> > possible output value from epoll_mod_wait. Then when the syscall returned, they
> > know whether or not the command is executed by comparing each error with the
> > init value, if they're different, they have the result of the command.
> > More roughly, they can put any non-zero and not distinguish "not run" from
> > failure.
> 
> The "cmds' are not executed in a specified order plus the need to
> initialize the 'errors' fields to a positive value feels a bit ugly.
> And indeed the whole "command list was only partially run" case
> is not pretty. Am I correct to understand that if an error is found
> during execution of one of the "epoll_ctl" commands in 'cmds' then
> the system call will return -1 with errno set, indicating an error,
> even though the epoll interest list may have changed because some
> of the earlier 'cmds' executed successfully? This all seems a bit of
> a headache for user space.

This is the trade-off for batching. The best we can do is probably make this
transactional: none or all of the commands succeeds. It will require a much
more complex implementation, though. But even with that, the error reporting on
which command failed is a complication.

> 
> I have a couple of questions:
> 
> Q1. I can see that batching "epoll_ctl" commands might be useful,
> since it results in fewer systems calls. But, does it really
> need to be bound together with the "epoll_pwait" functionality?
> (Perhaps this point was covered in previous discussions, but
> neither the message accompanying this patch nor the 0/6 man page
> provide a compelling rationale for the need to bind these two
> operations together.)
> 
> Yes, I realize you might save a system call, but it makes for a
> cumbersome API that has the above headache, and also forces the 
> need for double pointer indirection in the 'spec' argument (i.e., 
> spec is a pointer to an array of structures where each element
> in turn includes an 'events' pointer that points to another array).
> 
> Why not a simpler API with two syscalls such as:
> 
> epoll_ctl_batch(int epfd, int flags,
>                 int ncmds, struct epoll_mod_cmd *cmds);
> 
> epoll_pwait1(int epfd, struct epoll_event *events, int maxevents, 
>              struct timespec *timeout, int clock_id, 
>              const sigset_t *sigmask, size_t sigsetsize);

The problem is that there is no room for flags field in epoll_pwait1, which is
asked for, in previous discussion thread [1].

I don't see epoll_mod_wait as a *significantly more* complicated interface
compared to epoll_ctl_batch and epoll_pwait1 above. In epoll_mod_wait, if you
leave out ncmds and cmds, it is effectively a poll without batch; and if
leaving out spec, it is effectively a batch without poll.

The most important change here is the timeout. IMO I wouldn't mind leaving out
batching. Integrating it is requested by Andy:

[1]: http://thread.gmane.org/gmane.linux.kernel/1861430/focus=91591

which also made sense to me; I do have a patch in QEMU to call epoll_ctl for a
number of times right before epoll_wait.

[Sorry for not putting anything into cover letter changelog, but it is also
interesting to see people's reaction on the patch itself without bias of
others' opinions. This indeed brings in more points. :]

> 
> This gives us much of the benefit of reducing system calls, but 
> with greater simplicity. And epoll_ctl_batch() could simply return
> the number of 'cmds' that were successfully executed.)
> 
> Q2. In the man page in 0/6 you said that the 'cmds' were not 
> guaranteed to be executed in order. Why not? If you did provide
> such a guarantee, then, when using your current epoll_mod_wait(),
> user space could do the following:

I guess we can make a guarentee on that.

> 
> 1. Initialize the cmd.errors fields to zero.
> 2. Call epoll_ctl_mod()
> 3. Iterate through cmd.errors looking for the first nonzero 
>    field.

It's close, but zero is not good enough, if copy_from_user of cmds failed in
the first place. Impossible value, or error value, will be safer.

> 
> > Also, timeout parameter is enhanced: timespec is used, compared to the old ms
> > scalar. This provides higher precision. 
> 
> Yes, that change seemed inevitable. It slightly puzzled me at the time when
> Davide Libenzi added epoll_wait() that the timeout was milliseconds, even
> though pselect() already had demonstrated the need for higher precision.
> I should have called it out way back then :-{.
> 
> > The parameter field in struct
> > epoll_wait_spec, "clockid", also makes it possible for users to use a different
> > clock than the default when it makes more sense.
> > 
> > Signed-off-by: Fam Zheng <famz@xxxxxxxxxx>
> > ---
> >  fs/eventpoll.c           | 60 ++++++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/syscalls.h |  5 ++++
> >  2 files changed, 65 insertions(+)
> > 
> > diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> > index e7a116d..2cc22c9 100644
> > --- a/fs/eventpoll.c
> > +++ b/fs/eventpoll.c
> > @@ -2067,6 +2067,66 @@ SYSCALL_DEFINE6(epoll_pwait, int, epfd, struct epoll_event __user *, events,
> >  			      sigmask ? &ksigmask : NULL);
> >  }
> >  
> > +SYSCALL_DEFINE5(epoll_mod_wait, int, epfd, int, flags,
> > +		int, ncmds, struct epoll_mod_cmd __user *, cmds,
> > +		struct epoll_wait_spec __user *, spec)
> > +{
> > +	struct epoll_mod_cmd *kcmds = NULL;
> > +	int i, ret = 0;
> > +	int cmd_size = sizeof(struct epoll_mod_cmd) * ncmds;
> > +
> > +	if (flags)
> > +		return -EINVAL;
> > +	if (ncmds) {
> > +		if (!cmds)
> > +			return -EINVAL;
> > +		kcmds = kmalloc(cmd_size, GFP_KERNEL);
> > +		if (!kcmds)
> > +			return -ENOMEM;
> > +		if (copy_from_user(kcmds, cmds, cmd_size)) {
> > +			ret = -EFAULT;
> > +			goto out;
> > +		}
> > +	}
> > +	for (i = 0; i < ncmds; i++) {
> > +		struct epoll_event ev = (struct epoll_event) {
> > +			.events = kcmds[i].events,
> > +			.data = kcmds[i].data,
> > +		};
> > +		if (kcmds[i].flags) {
> > +			kcmds[i].error = ret = -EINVAL;
> 
> To make the 'ret' change a little more obvious, maybe it's better to write
> 
> 			ret = kcmds[i].error = -EINVAL;
> 
> > +			goto out;
> > +		}
> > +		kcmds[i].error = ret = ep_ctl_do(epfd, kcmds[i].op, kcmds[i].fd, ev);
> 
> Likewise:
> 		ret = kcmds[i].error = ep_ctl_do(epfd, kcmds[i].op, kcmds[i].fd, ev);
> 
> > +		if (ret)
> > +			goto out;
> > +	}
> > +	if (spec) {
> > +		sigset_t ksigmask;
> > +		struct epoll_wait_spec kspec;
> > +		ktime_t timeout;
> > +
> > +		if(copy_from_user(&kspec, spec, sizeof(struct epoll_wait_spec)))
> 
> Cosmetic point: s/if(/if (/
> 
> > +			return -EFAULT;
> > +		if (kspec.sigmask) {
> > +			if (kspec.sigsetsize != sizeof(sigset_t))
> > +				return -EINVAL;
> > +			if (copy_from_user(&ksigmask, kspec.sigmask, sizeof(ksigmask)))
> > +				return -EFAULT;
> > +		}
> > +		timeout = timespec_to_ktime(kspec.timeout);
> > +		ret = epoll_pwait_do(epfd, kspec.events, kspec.maxevents,
> > +				     kspec.clockid, timeout,
> > +				     kspec.sigmask ? &ksigmask : NULL);
> 
> If I understand correctly, the implementation means that the
> 'size_t sigsetsize' field will probably need to be exposed to 
> applications. In the existing epoll_pwait() call (as in  ppoll()
> and pselect()) the 'size_t sigsetsize' argument is hidden by glibc.
> However, unless we expect glibc to do some structure copying to/from
> a structure that hides this field, then we're going end up exposing
> 'size_t sigsetsize' to applications. (This could be avoided, if we
> split the API as I suggest above. glibc would do the same thing 
> in epoll_pwait1() that it currently does in epoll_pwait().)
> 
> Thanks,
> 
> Michael
> 
> -- 
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/
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