kern/129164: Wrong priority value for normal processes

[Unga]

--- On Tue, 11/25/08, Unga <unga888 at yahoo.com> wrote:

> The priority value for root and other normal processes is
> 65504 (rtp.prio) where zero (0) is expected.
> 
> I checked the program flow from /usr/src/usr.bin/su/su.c to
> /usr/src/lib/libutil/login_class.c and it looks
> setusercontext() is setting the priority zero (0) right but
> the moment it come out from the setusercontext() call in
> su.c, the priority has already turn to 65504.
> 
> Maximum priority value for normal priority processes can
> take is 20, not 65504. Normal priority processes are
> expected to run at priority zero (0) as it is specified in
> /etc/login.conf under login class "default". 
> 

I have further checked the rtprio(2) system call for how it set and read priorities.

Setting Priority:
rtprio(RTP_SET, 0, &rtp)

rtprio() => rtprio_thread() => rtp_to_pri()

rtp_to_pri() calculates newpri as:
newpri = PRI_MIN_TIMESHARE + rtp->prio;

PRI_MIN_TIMESHARE is for normal priority, its PRI_MIN_REALTIME for realtime priority etc.

Now rtp_to_pri() calls sched_class() to set the priority class. It sets td->td_pri_class to the priority class given.

Then rtp_to_pri() calls sched_user_prio() to set the priority. It sets following fields to the priority calculated (newpri):
td->td_base_user_pri
td->td_user_pri

Then rtp_to_pri() calls sched_prio(). It sets following field to the priority calculated (newpri):
td->td_base_pri

The sched_prio() calls sched_thread_priority() which sets td->td_priority to the priority calculated (newpri).

Of course not all td->td_* fields are set in one go. Some are set conditionally. But the td->td_base_user_pri is always set.


Reading Priority:
rtprio(RTP_LOOKUP, 0, &rtp)

rtprio() => rtprio_thread() => pri_to_rtp()

At pri_to_rtp(), rtp->type and rtp->prio are set as follows:
rtp->type = td->td_pri_class;
rtp->prio = td->td_base_user_pri - PRI_MIN_TIMESHARE;

That is, rtprio(2) system call sets the td->td_base_user_pri when request to set priority, and when request to read the priority, it reads the td->td_base_user_pri.

In another word, for rtprio(2) to function properly the td->td_base_user_pri should not be changed.

As the rtp->prio is unsigned short, for rtp->prio to become a huge number (65504), td->td_base_user_pri should be less than PRI_MIN_TIMESHARE.


This shows the actual problem is in the scheduler. In this case, sched_ule.

I presume the sched_ule should not touch the td->td_base_user_pri. Instead, probably, it should use td->td_priority for its internal purposes. 

Appreciate if Jeffrey Roberson <jeff at freebsd.org> could shed more light on this issue.

Since I'm not conversant with the sched_ule, I may not be able to develop a fix for sched_ule. Appreciate either Jeffrey or somebody else could look into a fix for sched_ule. I can certainly help in apply a patch and test.

Best regards
Unga