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SubjectBug report: smp affinity patch
Date
Hi everyone,
On occasion, the smp affinity patch can leave one or more runnable
processes in such a state that the scheduler never selects them for
execution. The reason this occurs is unknown. This note reports
the symptoms and how the problem may be replicated:

I am using the smp affinity patch from Robert Love, which provides
a /proc/pid/affinity interface to the user. I presume the problem
is also present in the Ingo Molnar patch, since it is so similar
in implementation, although I have not tested it.

I ran across this problem when I wrote a shell script that
implemented cpu shielding. The shielding script modifies the
affinities of (most) all proceses in the system -- for each process,
it either forces that process to run only on the shielded cpu, or
forces it to avoid the shielded cpu altogether. The only processes
to which neither are applied are the ksoftirqd_CPUxx daemons, each of
which has to remain on the cpu originally attached to.

Joe

---------------------------------------

Environment:
linux-2.4.17.tar.gz
+ patch-2.4.18-rc2.gz
+ cpu-affinity-rml-2.4.16-1.patch (from www.tech9.net/rml/linux)
PC, Pentium III, dual cpu's, dual IO APIC's, scsi, console via com1.

Test Shell Script used (filename `shield'):

#!/bin/bash
# shell script that reserves some cpu to some small
# set of procs: accomplished by tweaking the affinity
# masks of all procs -- either by removing that cpu from
# those pids which are not to use it, or by setting the
# affinity to only that cpu, for those procs that are
# to be attached to the shielded cpu..
#
# usage: shield unshieldmask shieldmask pid pid ...
# example: shield 2 1 1027 1028
# meaning: pids 1027,1028 are to run on cpu0; every
# other procs is to run on cpu 1.
# example: shield 3 3
# meaning: make every cpu available to all procs.
# note: procs 3 & 4 (ksoftirqd_CPU[0-1]) are not and
# must not have their affinities changed by this script.

unshieldmask=${1:-e}
shift
shieldmask=${1:-1}
shift
cd /proc
for i in $(/bin/ls -d [0-9]*); do
if [ -d $i ]; then
case $i in
3|4) ;;
${1:-no}) echo $shieldmask >$i/affinity ;;
${2:-no}) echo $shieldmask >$i/affinity ;;
${3:-no}) echo $shieldmask >$i/affinity ;;
${4:-no}) echo $shieldmask >$i/affinity ;;
*) echo $unshieldmask >$i/affinity ;;
esac
fi
done

Test initialization sequence:

in window #1:
top -d1 -i
in window #2:
echo 'main() {for(;;);}' >l.c && make l
./l &
[1] 1087
./l &
[2] 1088
./l &
[3] 1089
./l &
[4] 1090

Test sequence and results:

In the below tests, to `Stall' means that the scheduler fails to
give a runnable process any time.

Notation:
1088 Stalls - pid 1088 stalls. viewable in the top(1) window
as the bottom `running' proc, but has 0% cpu
utilization.

top Stalls - the top window stops updating. Due to
top itself being a victim of the scheduling
bug. to see: run another top in another window.


result command line executed
------------------ ---------------------
ok shield 1 2
ok shield 1 2
ok shield 3 1
ok shield 3 1 1087
ok shield 2 1 1087
ok shield 3 3
top Stalls shield 2 1 1090
ok shield 3 3
top Stalls shield 2 1 1090
ok shield 3 3
top Stalls shield 2 1 1090
ok shield 3 3
1088 Stalls shield 2 1 1090
ok shield 3 3
ok shield 2 1
ok shield 1 2
ok shield 2 1
ok shield 1 2
ok shield 2 1
1090 Stalls shield 1 2 1090
1090 Stalls shield 1 2 1090
top Stalls plus shield 2 1 1090
1087, sendmail,
crond,init,lots
of kjournals, and
syslogd
ok shield 3 3
top + shell window shield 2 1 1090
Stalls
ok shield 3 3 (executed in another window)
1087 Stalls shield 1 2 1090


Sample good and bad top(1) Displays:

------------------------------------------------------------------- good

9:18pm up 38 min, 3 users, load average: 4.00, 4.31, 4.69
48 processes: 43 sleeping, 5 running, 0 zombie, 0 stopped
CPU0 states: 100.0% user, 0.0% system, 0.0% nice, 0.0% idle
CPU1 states: 99.0% user, 1.0% system, 0.0% nice, 0.0% idle
Mem: 513160K av, 44600K used, 468560K free, 0K shrd, 10156K buff
Swap: 1052216K av, 0K used, 1052216K free 17420K cached

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1088 root 20 0 268 268 220 R 52.0 0.0 15:28 l
1089 root 20 0 268 268 220 R 50.0 0.0 15:54 l
1087 root 18 0 268 268 220 R 48.0 0.0 13:38 l
1090 root 18 0 268 268 220 R 48.0 0.0 22:44 l
1027 root 10 0 1060 1060 856 R 1.0 0.2 0:13 top

------------------------------------------------------------------- bad

9:20pm up 40 min, 3 users, load average: 6.12, 5.19, 4.97
49 processes: 43 sleeping, 6 running, 0 zombie, 0 stopped
CPU0 states: 97.0% user, 3.0% system, 0.0% nice, 0.0% idle
CPU1 states: 100.0% user, 0.0% system, 0.0% nice, 0.0% idle
Mem: 513160K av, 44940K used, 468220K free, 0K shrd, 10392K buff
Swap: 1052216K av, 0K used, 1052216K free 17420K cached

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1090 root 16 0 268 268 220 R 99.9 0.0 24:24 l
1089 root 16 0 268 268 220 R 64.9 0.0 16:58 l
1088 root 9 0 268 268 220 R 33.9 0.0 16:31 l
978 root 9 0 664 664 552 R 0.0 0.1 0:00 in.telnetd
1027 root 9 0 1060 1060 856 R 0.0 0.2 0:14 top
1087 root 9 0 268 268 220 R 0.0 0.0 14:05 l

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