Re: tasklets vs. workqueues
From: George Anzinger
Date: Wed Feb 25 2004 - 14:08:01 EST
Martin Diehl wrote:
[just saw this - CC-list dropped]
On Thu, 19 Feb 2004, George Anzinger wrote:
Being in process context, you can also change the priority and schedule policy
as needed to fit your application, while you are rather stuck with tasklets in
this regard.
How would one do that correctly? Something like
read_lock_irq(&tasklist_lock);
current->rt_priority = 0;
current->policy = SCHED_FIFO;
current->prio = MAX_USER_RT_PRIO - 1;
read_unlock_irq(&tasklist_lock);
used to fail for me with SMP kernel - looks like occasional triple-fault
leading to the box rebooting all of the sudden. Looking into
kernel/sched.c suggests we might need to acquire the runqueue lock as
well, but this is private there. - And anyway, AFAICS it's enclosed in
tasklist_lock. So given we are currently running when the above code gets
executed, taking the tasklist_lock should be sufficient - IMHO at least
but reality proved me wrong.
OTOH, directly calling sys_sched_setscheduler doesn't work (without some
set_fs-magic at least), because it is expecting its parameters from
userland.
Depends on where you want to be when you do it. From user land you would do
exactly what the attached program does. In SMP you would, likely, want to do
all the tasks in the work queue (one per cpu).
From the kernel, again calling setscheduler() is the way to go. I am not sure
what is in the community tree just now, but if I recall properly, the scheduler
itself does this so, one should be able to copy that code.
Ah, yes, there it is in migration_thread(). It calls setscheduler().
--
George Anzinger george@xxxxxxxxxx
High-res-timers: http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml
/*
rt - a utility to set the realtime priority and scheduling policy
*/
/* includes */
#include <stdio.h>
#include <stdlib.h>
#include <sched.h>
#include <unistd.h>
#define _GNU_LIBRARY__
#include <getopt.h>
/* defines */
#define RUNSTRING "usage: rt [-f] [-v] prio [--] runstring \n \
or: rt [-f] [-v] -p PID prio\\ \n\n \
where: prio specifies the realtime priority \n \
-f set scheduling policy to SCHED_FIFO \n \
-v turns on verbose mode. \n \
-p PID specifies an existing process to modify \n \
runstring is a process and parameters \n \
(use '--' if runstring contains options). \n"
#define POLICY(x) x ? x-1 ? "SCHED_RR" : "SCHED_FIFO" : "SCHED_OTHER"
/* prototypes */
void print_usage(char *[]);
/* globals */
int verbose=0; /* 0=none, !0=verbose */
main(int argc, char *argv[])
{
struct sched_param prio_struct;
int policy = -1;
int pid = 0;
int pidopt = 0;
int optprobs = 0; /* problems parsing? */
int c; /* generic single character */
/* "standard" option parsing... */
while ( (c=getopt(argc, argv, "+fp:v?")) != EOF)
{
switch (c) {
case 'f': /* set FIFO mode */
policy = SCHED_FIFO;
break;
case 'p': /* read PID */
sscanf(optarg,"%d",&pid);
pidopt=1;
break;
case 'v':
verbose=1; /* verbosity */
break;
case '?': /* help? */
printf("%s",RUNSTRING);
exit(0);
default: /* something went wrong */
optprobs=1; /* we'll deal with this problem later */
break;
}
}
if (optprobs) {
fprintf(stderr,RUNSTRING);
exit(1);
}
if((argc - optind) < 2-pidopt) {
print_usage(argv);
}
sscanf(argv[optind], "%d", &(prio_struct.sched_priority));
/* sanity checking... */
if ( (prio_struct.sched_priority != 0) && (policy < 0 ) ) {
policy=SCHED_RR;
if (verbose)
printf("Defaulting sched policy to %s.\n", POLICY(policy));
}
if ( (prio_struct.sched_priority == 0 ) && (policy != SCHED_OTHER) ) {
policy=SCHED_OTHER;
fprintf(stderr,"Priority of %d implies sched policy of %s.\n",
prio_struct.sched_priority, POLICY(policy));
}
policy = (prio_struct.sched_priority)? policy : SCHED_OTHER;
if( sched_setscheduler(pid,policy,&prio_struct)){
perror("Priority out of range");
print_usage(argv);
}
if ( pid ) exit(0);
argv+=optind; /* adjust argv to point to the runstring */
argv++;
execvp(argv[0],argv);
perror("exec failed..");
}
void print_usage(char * who[])
{
printf("%s",RUNSTRING);
exit (1);
}