How to renice a group of processes?

lb build can build live system image, and it invokes many processes while building the image. If I start it as below: # nice -n 19 ionice -n 7 -c 3 lb build all the children processes get the same nice level: PID USER IORR IOWR IO IO PRI NI VIRT RES SHR S CPU% MEM% TIME+ Command 26196 morfik 0 0 0 B4 20 0 23888 672 668 S 0.0 0.1 0:00.24 │ ├─ -bash 30926 root 0 0 0 B4 20 0 53668 536 536 S 0.0 0.1 0:00.02 │ │ └─ su - 31008 root 0 0 0 B4 20 0 34656 6260 1144 S 0.0 0.6 0:02.60 │ │ └─ -su 11784 root 0 0 0 ?? 39 19 4496 796 524 S 0.0 0.1 0:00.01 │ │ └─ /bin/sh /usr/lib/live/build/build 11797 root 0 0 0 ?? 39 19 4328 612 512 S 0.0 0.1 0:00.00 │ │ └─ /bin/sh ./auto/build 11806 root 0 0 0 ?? 39 19 7028 604 504 S 0.0 0.1 0:00.00 │ │ ├─ tee build.log 11798 root 0 0 0 ?? 39 19 4496 824 532 S 0.0 0.1 0:00.01 │ │ └─ /bin/sh /usr/lib/live/build/build noauto 11915 root 0 0 0 ?? 39 19 4496 836 544 S 0.0 0.1 0:00.01 │ │ └─ /bin/sh /usr/lib/live/build/bootstrap 12753 root 0 0 0 ?? 39 19 4496 856 548 S 0.0 0.1 0:00.02 │ │ └─ /bin/sh /usr/lib/live/build/bootstra 12867 root 0 0 0 ?? 39 19 62980 3508 2132 S 11.2 0.3 0:07.00 │ │ └─ aptitude --assume-yes update 12943 root 0 0 0 ?? 39 19 24296 1916 1568 S 0.0 0.2 0:00.14 │ │ ├─ /usr/lib/apt/methods/xz 12927 root 0 0 0 ?? 39 19 53280 30688 30172 R 86.5 3.0 0:28.65 │ │ ├─ /usr/lib/apt/methods/rred 12891 root 0 0 0 ?? 39 19 24304 1784 1440 S 0.0 0.2 0:00.00 │ │ ├─ /usr/lib/apt/methods/gpgv 12889 root 0 0 0 ?? 39 19 24292 1624 1384 S 0.0 0.2 0:00.00 │ │ ├─ /usr/lib/apt/methods/copy 12887 root 0 0 0 ?? 39 19 32860 1956 1560 S 0.0 0.2 0:00.17 │ │ ├─ /usr/lib/apt/methods/http 12886 root 0 0 0 ?? 39 19 24292 1696 1444 S 0.0 0.2 0:00.00 │ │ └─ /usr/lib/apt/methods/fil But I forgot to add this nice -n 19 ionice -n 7 -c 3 and ran just lb build. So, I tried to renice the parent process: # renice -n 19 -p 6187 But this doesn't renice the other processes. So, it looks like this: 26196 morfik 0 0 0 B4 20 0 23888 668 664 S 0.0 0.1 0:00.24 │ ├─ -bash 30926 root 0 0 0 B4 20 0 53668 528 528 S 0.0 0.1 0:00.02 │ │ └─ su - 31008 root 0 0 0 B4 20 0 34656 5952 1224 S 0.0 0.6 0:02.62 │ │ └─ -su 6187 root 0 0 0 B7 39 19 4496 800 524 S 0.0 0.1 0:00.00 │ │ └─ /bin/sh /usr/lib/live/build/build 6349 root 0 0 0 B4 20 0 4328 612 512 S 0.0 0.1 0:00.00 │ │ └─ /bin/sh ./auto/build 6351 root 0 0 0 B4 20 0 7028 592 488 S 0.0 0.1 0:00.00 │ │ ├─ tee build.log 6350 root 0 0 0 B4 20 0 4496 828 532 S 0.0 0.1 0:00.01 │ │ └─ /bin/sh /usr/lib/live/build/build noauto 6445 root 0 0 0 B4 20 0 4496 840 548 S 0.0 0.1 0:00.00 │ │ └─ /bin/sh /usr/lib/live/build/bootstrap 7580 root 0 0 0 B4 20 0 4496 856 552 S 0.0 0.1 0:00.02 │ │ └─ /bin/sh /usr/lib/live/build/bootstra 7692 root 0 0 0 B4 20 0 62924 5236 3848 S 15.5 0.5 0:03.78 │ │ └─ aptitude --assume-yes update 7932 root 0 0 0 B4 20 0 54776 16480 15916 R 84.1 1.6 0:16.60 │ │ ├─ /usr/lib/apt/methods/rred 7912 root 0 0 0 B4 20 0 24296 2036 1648 S 0.0 0.2 0:01.62 │ │ ├─ /usr/lib/apt/methods/gzip 7733 root 0 0 0 B4 20 0 27948 5552 1632 S 0.0 0.5 0:02.85 │ │ ├─ /usr/lib/apt/methods/bzip2 7711 root 0 0 0 B4 20 0 24304 1780 1436 S 0.0 0.2 0:00.00 │ │ ├─ /usr/lib/apt/methods/gpgv 7709 root 0 0 0 B4 20 0 30800 2200 1812 S 0.0 0.2 0:01.01 │ │ └─ /usr/lib/apt/methods/http I could renice all the processes manually but they are changing. **EDIT#1** The following command: # nice -n 19 ionice -n 7 -c 3 lb build sets also: > ionice - sets or gets process io scheduling class and priority. ... > > -c, --class scheduling class name or number > 0: none, 1: realtime, 2: best-effort, 3: idle > > -n, --classdata scheduling class data > 0-7 for realtime and best-effort classes How to set -c and -n option of ionice to the processes?