Real time processes scheduling in Linux

Question

I have been learning some scheduling concepts. Currently my understanding so far is as below.

• There are real time processes and non real time processes.
• Non real time processes can have nice values for their priority in the range of -20 to +20. The higher positive value indicates that the process has lower priority.
• The real time processes will have a niceness value listed as - as explained in this answer here. This is mainly because the real time processes have higher priorities than the non real time processes and niceness value do not apply to them.
• Now, I can use chrt to see the real time attributes of a process.

For a real time process, the chrt gives output as,

chrt -p 5
pid 5's current scheduling policy: SCHED_FIFO
pid 5's current scheduling priority: 99

As we can see for process 5, the priority is 99 which is the highest. Also, the scheduling policy is SCHED_FIFO

Now, for a non real time process, the chrt gives output as,

chrt -p 22383
pid 22383's current scheduling policy: SCHED_OTHER
pid 22383's current scheduling priority: 0

As we can see for process 22383, the priority is 0 and the scheduling policy is SCHED_OTHER.

Questions

1. Is it possible for me to make any process as real time process?
2. Is it possible for me to set some other scheduling algorithm other than SCHED_OTHER for a non real time process?

3. From here, I also see that I could modify the attribute for a running process as,

   chrt -p prio pid
   

   Also, I see chrt -m gives me the list of scheduling algorithms. The command gives me the output as,

   SCHED_OTHER min/max priority    : 0/0
   SCHED_FIFO min/max priority     : 1/99
   SCHED_RR min/max priority       : 1/99
   SCHED_BATCH min/max priority    : 0/0
   SCHED_IDLE min/max priority     : 0/0
   

   Now, as suggested above, if I set chrt -p 55 22383 which algorithm will be used?

Solution

Question 1

It is possible for an user to use real time priority for a process as well. This configuration could be set from /etc/security/limits.conf file. I see the below contents in that file.

# /etc/security/limits.conf
#
#Each line describes a limit for a user in the form:
#
#<domain>        <type>  <item>  <value>

If we check the item section, we see the below entry which enables to set a real time priority for the users.

#        - rtprio - max realtime priority

Question 2 and Question 3

To set scheduling policy to SCHED_FIFO, enter:

chrt -f -p [1..99] {pid}

To set scheduling policy to SCHED_RR, enter:

chrt -r -p [1..99] {pid}

So to answer question 3, we should verify the scheduling algorithms available and the priorities using the chrt -m command and then use any scheduling algorithm that suits our need. To set different priorities, we could use the commands as above.