Re: futex priority based wakeup

["Nguyen Nguyen" <nnguyen@cs.umn.edu>]

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I have seen something similar before.  Our fix was to use
pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED) so child threads
wouldn't inherit attribute from parent.  Hope it helps.


On 9/11/07, Ilya Lipovsky <lipovsky@cs.bu.edu> wrote:
>
> Hmm. Just for kicks - inside the important thread could you add:
>
> int curpolicy;
> struct sched_param sp;
> pthread_getschedparam (pthread_self (), &curpolicy, &sp)
> printf("important's policy is %d and priority is %d\n", curpolicy,
> sp.__sched_priority);
>
> before the very first futex syscall and after your "printf("important got
> futex!\n");" line.
>
> Do similar for the unimportant thread, and see if you get anything weird -
> e.g. priorities come out to be the same for threads.
>
>
> -----Original Message-----
> From: linuxppc-embedded-bounces+lipovsky=cs.bu.edu@ozlabs.org
> [mailto:linuxppc-embedded-bounces+lipovsky=cs.bu.edu@ozlabs.org] On Behalf
> Of Benedict, Michael
> Sent: Monday, September 10, 2007 5:41 PM
> To: linuxppc-embedded@ozlabs.org
> Subject: RE: futex priority based wakeup
>
> Ilya Lipovsky wrote:
> > Your code looks correct to me, so if the kernel developers
> > did their job
> > correctly, the only potentially weak link is glibc.
> >
>
> Well, either the kernel developers didn't do their job, or I am missing
> something.  The following also fails, and it should be bypassing glibc:
>
> #define _XOPEN_SOURCE 600
>
> #include <linux/futex.h>
> #include <sys/time.h>
> #include <asm/atomic.h>
>
> #include <sys/syscall.h>
> #include <unistd.h>
>
> #include <sched.h>
> #include <stdint.h>
> #include <stdio.h>
> #include <stdlib.h>
>
> int myfutex = 0;
>
> void *important(void *ign)
> {
>         sleep(1);
>         printf("important waiting for futex\n");
>         fflush(stdout);
>         if(syscall(SYS_futex, &myfutex, FUTEX_WAIT, 0, NULL)) {
>                 perror("futex");
>                 exit(1);
>         } else {
>                 printf("important got futex!\n");
>                 fflush(stdout);
>                 syscall(SYS_futex, &myfutex, FUTEX_WAKE, 1, NULL);
> }
>
>         return NULL;
> }
>
>
> void *unimportant(void *ign)
> {
>         printf("unimportant waiting for futex\n");
>         fflush(stdout);
>         if(syscall(SYS_futex, &myfutex, FUTEX_WAIT, 0, NULL)) {
>                 perror("futex");
>                 exit(1);
>         } else {
>                 printf("unimportant got futex!\n");
>                 fflush(stdout);
>                 syscall(SYS_futex, &myfutex, FUTEX_WAKE, 1, NULL);
> }
>
>         return NULL;
> }
>
> int main()
> {
>         struct sched_param p;
>         pthread_attr_t attr;
>         pthread_t i, u;
>
>         p.__sched_priority = sched_get_priority_min(SCHED_FIFO);
>         if(-1 == p.__sched_priority) {
>                 perror("sched_get_priority_min");
>                 return 1;
>         }
>         pthread_attr_init(&attr);
>         pthread_attr_setschedpolicy(&attr, SCHED_FIFO);
>         pthread_attr_setschedparam(&attr, &p);
>         pthread_create(&u, &attr, unimportant, NULL);
>
>         p.__sched_priority = sched_get_priority_max(SCHED_FIFO);
>         pthread_attr_setschedparam(&attr, &p);
>         pthread_create(&i, &attr, important, NULL);
>
>         sleep(5);
>         printf("futex FUTEX_WAKE\n");
>         fflush(stdout);
>         syscall(SYS_futex, &myfutex, FUTEX_WAKE, 1, NULL);
>
>         pthread_join(u, NULL);
>         pthread_join(i, NULL);
>
>         return 0;
> }
>
> Which produces:
> unimportant waiting for futex
> important waiting for futex
> futex FUTEX_WAKE
> unimportant got futex!
> important got futex!
>
>
> Could someone with 2.6.22 please verify?
>
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>

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