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Date: Tue, 15 Mar 2016 01:09:21 +0200
From: Madars Vitolins <m@...odev.com>
To: Jason Baron <jbaron@...mai.com>,
"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, mingo@...nel.org,
peterz@...radead.org, viro@....linux.org.uk, normalperson@...t.net,
corbet@....net, luto@...capital.net, torvalds@...ux-foundation.org,
hagen@...u.net, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH] epoll: add exclusive wakeups flag
Hi Jason and Michael,
Hmm... I tried to play with those pipe samples bellow, but even with
sleep I got that all process wakeups (maybe I miss something too), also
tried with EPOLLIN.
On same bases I created sample with Posix Queues with EPOLLIN |
EPOLLEXCLUSIVE and the goods news are that it works correctly.
file q.c:
==================
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/epoll.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <errno.h>
#include <mqueue.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
#define usageErr(msg, progName) \
do { fprintf(stderr, "Usage: "); \
fprintf(stderr, msg, progName); \
exit(EXIT_FAILURE); } while (0)
#ifndef EPOLLEXCLUSIVE
#define EPOLLEXCLUSIVE (1 << 28)
#endif
#define MAX_SIZE 10
int
main (int argc, char *argv[])
{
int epfd, nready;
struct epoll_event ev, rev;
mqd_t fd;
struct mq_attr attr;
char buffer[MAX_SIZE + 1];
int cnum;
/* initialize the queue attributes */
attr.mq_flags = 0;
attr.mq_maxmsg = 5;
attr.mq_msgsize = MAX_SIZE;
attr.mq_curmsgs = 0;
/* cleanup for multiple runs... */
mq_unlink ("/TESTQ");
/* create the message queue */
fd =
mq_open ("/TESTQ", O_CREAT | O_RDWR | O_NONBLOCK, S_IWUSR | S_IRUSR,
&attr);
if (fd == -1)
errExit ("open");
for (cnum = 0; cnum < 3; cnum++)
{
switch (fork ())
{
case -1:
errExit ("fork");
case 0: /* Child */
epfd = epoll_create (2);
if (epfd == -1)
errExit ("epoll_create");
ev.events = EPOLLIN | EPOLLEXCLUSIVE;
if (epoll_ctl (epfd, EPOLL_CTL_ADD, fd, &ev) == -1)
errExit ("epoll_ctl");
printf ("About to wait...\n");
nready = epoll_wait (epfd, &rev, 1, -1);
if (nready == -1)
errExit ("epoll-wait");
printf ("Child %d: epoll_wait() returned %d\n", cnum, nready);
exit (EXIT_SUCCESS);
default:
break;
}
}
sleep (1);
/* send a msq to Q */
memset (buffer, 0, MAX_SIZE);
if (0 > mq_send (fd, buffer, MAX_SIZE, 0))
errExit ("mq_send");
printf ("msg sent ok...\n");
wait (NULL);
wait (NULL);
wait (NULL);
exit (EXIT_SUCCESS);
}
==================
$ gcc q.c -lrt
$ ./a.out
About to wait...
About to wait...
About to wait...
msg sent ok...
Child 2: epoll_wait() returned 1
^C
$
Best regards,
Madars
Jason Baron @ 2016-03-15 00:35 rakstīja:
> Hi Michael,
>
> On 03/14/2016 05:03 PM, Michael Kerrisk (man-pages) wrote:
>> Hi Jason,
>>
>> On 03/15/2016 09:01 AM, Michael Kerrisk (man-pages) wrote:
>>> Hi Jason,
>>>
>>> On 03/15/2016 08:32 AM, Jason Baron wrote:
>>>>
>>>>
>>>> On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote:
>>>>> [Restoring CC, which I see I accidentally dropped, one iteration
>>>>> back.]
>>
>> [...]
>>
>>>>> Returning to the second sentence in this description:
>>>>>
>>>>> When a wakeup event occurs and multiple epoll file
>>>>> descrip‐
>>>>> tors are attached to the same target file using
>>>>> EPOLLEXCLU‐
>>>>> SIVE, one or more of the epoll file descriptors
>>>>> will
>>>>> receive an event with epoll_wait(2).
>>>>>
>>>>> There is a point that is unclear to me: what does "target file"
>>>>> refer to?
>>>>> Is it an open file description (aka open file table entry) or an
>>>>> inode?
>>>>> I suspect the former, but it was not clear in your original text.
>>>>>
>>>>
>>>> So from epoll's perspective, the wakeups are associated with a 'wait
>>>> queue'. So if the open() and subsequent EPOLL_CTL_ADD (which is done
>>>> via
>>>> file->poll()) results in adding to the same 'wait queue' then we
>>>> will
>>>> get 'exclusive' wakeup behavior.
>>>>
>>>> So in general, I think the answer here is that its associated with
>>>> the
>>>> inode (I coudn't say with 100% certainty without really looking at
>>>> all
>>>> file->poll() implementations). Certainly, with the 'FIFO' example
>>>> below,
>>>> the two scenarios will have the same behavior with respect to
>>>> EPOLLEXCLUSIVE.
>>
>> So, I was actually a little surprised by this, and went away and
>> tested
>> this point. It appears to me that that the two scenarios described
>> below
>> do NOT have the same behavior with respect to EPOLLEXCLUSIVE. See
>> below.
>>
>>> So, in both scenarios, *one or more* processes will get a wakeup?
>>> (I'll try to add something to the text to clarify the detail we're
>>> discussing.)
>>>
>>>> Also, the 'non-exclusive' mode would be subject to the same question
>>>> of
>>>> which wait queue is the epfd is associated with...
>>>
>>> I'm not sure of the point you are trying to make here?
>>>
>>> Cheers,
>>>
>>> Michael
>>>
>>>
>>>>> To make this point even clearer, here are two scenarios I'm
>>>>> thinking of.
>>>>> In each case, we're talking of monitoring the read end of a FIFO.
>>>>>
>>>>> ===
>>>>>
>>>>> Scenario 1:
>>>>>
>>>>> We have three processes each of which
>>>>> 1. Creates an epoll instance
>>>>> 2. Opens the read end of the FIFO
>>>>> 3. Adds the read end of the FIFO to the epoll instance, specifying
>>>>> EPOLLEXCLUSIVE
>>>>>
>>>>> When input becomes available on the FIFO, how many processes
>>>>> get a wakeup?
>>
>> When I test this scenario, all three processes get a wakeup.
>>
>>>>> ===
>>>>>
>>>>> Scenario 3
>>>>>
>>>>> A parent process opens the read end of a FIFO and then calls
>>>>> fork() three times to create three children. Each child then:
>>>>>
>>>>> 1. Creates an epoll instance
>>>>> 2. Adds the read end of the FIFO to the epoll instance, specifying
>>>>> EPOLLEXCLUSIVE
>>>>>
>>>>> When input becomes available on the FIFO, how many processes
>>>>> get a wakeup?
>>
>> When I test this scenario, one process gets a wakeup.
>>
>> In other words, "target file" appears to mean open file description
>> (aka open file table entry), not inode.
>>
>> This is actually what I suspected might be the case, but now I am
>> puzzled. Given what I've discovered and what you suggest are the
>> semantics, is the implementation correct? (I suspect that it is,
>> but it is at odds with your statement above. My test programs are
>> inline below.
>>
>> Cheers,
>>
>> Michael
>>
>
> Thanks for the test cases. So in your first test case, you are exiting
> immediately after the epoll_wait() returns. So this is actually causing
> the next wakeup. And then the 2nd thread returns from epoll_wait() and
> this causes the 3rd wakeup.
>
> So the wakeups are actually not happening from the write directly, but
> instead from the readers doing a close(). If you do some sort of sleep
> after the epoll_wait() you can confirm the behavior. So I believe this
> is working as expected.
>
> Thanks,
>
> -Jason
>
>
>> ============
>>
>> /* t_EPOLLEXCLUSIVE_multipen.c
>>
>> Licensed under GNU GPLv2 or later.
>> */
>> #include <sys/epoll.h>
>> #include <sys/stat.h>
>> #include <fcntl.h>
>> #include <sys/types.h>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <unistd.h>
>> #include <string.h>
>>
>> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
>> } while (0)
>>
>> #define usageErr(msg, progName) \
>> do { fprintf(stderr, "Usage: "); \
>> fprintf(stderr, msg, progName); \
>> exit(EXIT_FAILURE); } while (0)
>>
>> #ifndef EPOLLEXCLUSIVE
>> #define EPOLLEXCLUSIVE (1 << 28)
>> #endif
>>
>> int
>> main(int argc, char *argv[])
>> {
>> int fd, epfd, nready;
>> struct epoll_event ev, rev;
>>
>> if (argc != 2 || strcmp(argv[1], "--help") == 0)
>> usageErr("%s <FIFO>n", argv[0]);
>>
>> epfd = epoll_create(2);
>> if (epfd == -1)
>> errExit("epoll_create");
>>
>> fd = open(argv[1], O_RDONLY);
>> if (fd == -1)
>> errExit("open");
>> printf("Opened %s\n", argv[1]);
>>
>> ev.events = EPOLLIN | EPOLLEXCLUSIVE;
>> if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1)
>> errExit("epoll_ctl");
>>
>> nready = epoll_wait(epfd, &rev, 1, -1);
>> if (nready == -1)
>> errExit("epoll-wait");
>> printf("epoll_wait() returned %d\n", nready);
>>
>> exit(EXIT_SUCCESS);
>> }
>>
>> ===============
>>
>> /* t_EPOLLEXCLUSIVE_fork.c
>>
>> Licensed under GNU GPLv2 or later.
>> */
>>
>> #include <sys/epoll.h>
>> #include <sys/stat.h>
>> #include <fcntl.h>
>> #include <sys/types.h>
>> #include <sys/wait.h>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <unistd.h>
>> #include <string.h>
>>
>> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
>> } while (0)
>>
>> #define usageErr(msg, progName) \
>> do { fprintf(stderr, "Usage: "); \
>> fprintf(stderr, msg, progName); \
>> exit(EXIT_FAILURE); } while (0)
>>
>> #ifndef EPOLLEXCLUSIVE
>> #define EPOLLEXCLUSIVE (1 << 28)
>> #endif
>>
>> int
>> main(int argc, char *argv[])
>> {
>> int fd, epfd, nready;
>> struct epoll_event ev, rev;
>> int cnum;
>>
>> if (argc != 2 || strcmp(argv[1], "--help") == 0)
>> usageErr("%s <FIFO>n", argv[0]);
>>
>> fd = open(argv[1], O_RDONLY);
>> if (fd == -1)
>> errExit("open");
>> printf("Opened %s\n", argv[1]);
>>
>> for (cnum = 0; cnum < 3; cnum++) {
>> switch (fork()) {
>> case -1:
>> errExit("fork");
>>
>> case 0: /* Child */
>> epfd = epoll_create(2);
>> if (epfd == -1)
>> errExit("epoll_create");
>>
>> ev.events = EPOLLIN | EPOLLEXCLUSIVE;
>> if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1)
>> errExit("epoll_ctl");
>>
>> nready = epoll_wait(epfd, &rev, 1, -1);
>> if (nready == -1)
>> errExit("epoll-wait");
>> printf("Child %d: epoll_wait() returned %d\n", cnum,
>> nready);
>> exit(EXIT_SUCCESS);
>>
>> default:
>> break;
>> }
>> }
>>
>> wait(NULL);
>> wait(NULL);
>> wait(NULL);
>>
>> exit(EXIT_SUCCESS);
>> }
>>
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