Hello Fam Zheng,
On 01/21/2015 05:59 AM, Fam Zheng wrote:
On Tue, 01/20 13:50, Michael Kerrisk (man-pages) wrote:
quoted
Hello Fam Zheng,
On 01/20/2015 10:57 AM, Fam Zheng wrote:
quoted
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.
Okay.
[...]
quoted
quoted
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.
Transactional would be more comfortable for user-space, and while I
can see that it would be complex to implement, perhaps the greater
point might be that the implementation is CPU expensive.
But even with that, the error reporting on
which command failed is a complication.
My suggestions below could make the error reporting much simpler...
quoted
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].
Ahh yes, I certainly should not have forgotten that. But that's easily solved.
Do as for pselect6():
strcut sigargs {
const sigset_t *ss;
size_t ss_len; /* Size (in bytes) of object pointed
to by 'ss' */
}
epoll_pwait1(int epfd, struct epoll_event *events, int maxevents,
struct timespec *timeout, int clock_id, int flags,
int timeout,
const struct sigargs *sargs);
I don't see epoll_mod_wait as a *significantly more* complicated interface
compared to epoll_ctl_batch and epoll_pwait1 above.
My biggest problem with epoll_ctl_wait() is the complexity of error
handling. epoll_ctl_batch and epoll_pwait1 would simplify that, as I
note below.
Aside from that, I do think that epoll_ctl_wait() passes a certain
threshold of complexity that warrants good justification, which
I haven't really seen yet.
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. :]
But it also has the downside that the same discussions
may be repeated.
quoted
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.
I'm puzzled by that response. Surely you *must* guarantee it.
If there's no defined order, then if the batch includes multiple
commands that operate on the same FD, the result is undefined
unless you provide that guarantee. (Unless, of course, you want to
explicitly specify that using the same FD multiple times in the
batch gives undefined behavior.)
quoted
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.
See my comment in the earlier mail. If you split this into two
APIs, and epoll_ctl_batch() is guaranteed to execute 'cmds' in order,
then the return value of epoll_ctl_batch() could be used to tell
user space how many commands succeeded. Much simpler!
quoted
quoted
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 :-{.
quoted
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 <redacted>
---
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;
quoted
+ 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);
quoted
+ 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 (/
quoted
+ 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().)
You missed responding to this point; I think it matters.
(There were also some other points to consider in my reply
to your 0/6 mail.)
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
On Wed, 01/21 08:52, Michael Kerrisk (man-pages) wrote:
Hello Fam Zheng,
On 01/21/2015 05:59 AM, Fam Zheng wrote:
quoted
On Tue, 01/20 13:50, Michael Kerrisk (man-pages) wrote:
quoted
Hello Fam Zheng,
On 01/20/2015 10:57 AM, Fam Zheng wrote:
quoted
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.
Okay.
[...]
quoted
quoted
quoted
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.
Transactional would be more comfortable for user-space, and while I
can see that it would be complex to implement, perhaps the greater
point might be that the implementation is CPU expensive.
Good point.
quoted
But even with that, the error reporting on
which command failed is a complication.
My suggestions below could make the error reporting much simpler...
quoted
quoted
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].
Ahh yes, I certainly should not have forgotten that. But that's easily solved.
Do as for pselect6():
strcut sigargs {
const sigset_t *ss;
size_t ss_len; /* Size (in bytes) of object pointed
to by 'ss' */
}
epoll_pwait1(int epfd, struct epoll_event *events, int maxevents,
struct timespec *timeout, int clock_id, int flags,
int timeout,
const struct sigargs *sargs);
quoted
I don't see epoll_mod_wait as a *significantly more* complicated interface
compared to epoll_ctl_batch and epoll_pwait1 above.
My biggest problem with epoll_ctl_wait() is the complexity of error
handling. epoll_ctl_batch and epoll_pwait1 would simplify that, as I
note below.
Aside from that, I do think that epoll_ctl_wait() passes a certain
threshold of complexity that warrants good justification, which
I haven't really seen yet.
OK, see below.
quoted
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. :]
But it also has the downside that the same discussions
may be repeated.
Yes, that's why I definitely do it for any version that is > v1.
quoted
quoted
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.
I'm puzzled by that response. Surely you *must* guarantee it.
If there's no defined order, then if the batch includes multiple
commands that operate on the same FD, the result is undefined
unless you provide that guarantee. (Unless, of course, you want to
explicitly specify that using the same FD multiple times in the
batch gives undefined behavior.)
OK. Then let's guarentee it.
quoted
quoted
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.
See my comment in the earlier mail. If you split this into two
APIs, and epoll_ctl_batch() is guaranteed to execute 'cmds' in order,
then the return value of epoll_ctl_batch() could be used to tell
user space how many commands succeeded. Much simpler!
Yes it is much simpler. However the reason to add batching in the first place is
to make epoll faster, by reducing syscalls. Splitting makes the result
sub-optimal: we still need at least 2 calls instead of 1. Each one of the three
proposed new call *is* a step forward, but I don't think we will have everything
solved even by implementing them all. Compromise needed between performance or
complexity.
My take for simplicity will be leaving epoll_ctl as-is, and my take for
performance will be epoll_pwait1. And I don't really like putting my time on
epoll_ctl_batch, thinking it as a ambivalent compromise in between.
Andy, will you be OK with the above epoll_pwait1? Or do you have more
justifications for epoll_mod_and_wait?
quoted
quoted
quoted
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 :-{.
quoted
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 <redacted>
---
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;
quoted
+ 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);
quoted
+ 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 (/
quoted
+ 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().)
You missed responding to this point; I think it matters.
(There were also some other points to consider in my reply
to your 0/6 mail.)
My bad, something I typed when replying went into /dev/null for unknown reasons,
what I had was:
This should be easy to solve: glibc can be responsible for building spec, and
applications will do something like:
epoll_mod_wait(epfd, ncmds, cmds, maxevents, events, clockid,
timeout, sigmask);
Thanks,
Fam
On 21/01/2015 08:52, Michael Kerrisk (man-pages) wrote:
quoted
quoted
The problem is that there is no room for flags field in epoll_pwait1, which is
asked for, in previous discussion thread [1].
Ahh yes, I certainly should not have forgotten that. But that's easily solved.
Do as for pselect6():
strcut sigargs {
const sigset_t *ss;
size_t ss_len; /* Size (in bytes) of object pointed
to by 'ss' */
}
epoll_pwait1(int epfd, struct epoll_event *events, int maxevents,
struct timespec *timeout, int clock_id, int flags,
int timeout,
const struct sigargs *sargs);
Alternatively, place the clock_id in the lower 16 bits of flags.
MAX_CLOCKS is 16 right now, so there's room.
Paolo