Kernel 3.6.x RT_PREEMPT: Issues with sync / umount of hdd partitions

Kernel 3.6.x RT_PREEMPT: Issues with sync / umount of hdd partitions

[Koehrer Mathias (ETAS/ESS2)]

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Hi all,

Since changing to the 3.6.x series of RT_PREEMPT kernel (it was 2.6.33.x before) I have some strange issues with mounting/umounting/syncing of hard disk partitions.

What I do is, I format a partition of the hard disk, I mount this partition, extract an tgz image (about 750 MB) into that freshly formatted partition and umount it again:
      mkfs.ext3 ....
      mount ...
      tar xf ...
      umount  ...

Doing the umount causes occasionally kernel panics (unable to handle kernel paging request...  in ksoftirqd/0).
When I  add a "sync" before the umount, I never got the kernel panic.
However, I can observe that the time needed for the "sync" varies dramatically.
Very often it takes around 6-8 seconds, however sometimes it takes up to 53 seconds!
Apart from the test script that performs this actions, the PC is idle.

Kernel: 3.6.11.3-rt35, x86 (32 bit mode no PAE), 4 GB RAM, CPU core i7.
Kernel boot parameters:    root=/dev/sda6 vga=0x31a isolcpus=1-31 

I have attached the kernel config and the result of the test output (script.log).

Here is the script:
--------------------------- BEGIN SCRIPT -----------------------
#!/bin/bash
MNTDIR=/tmp/mnt.$$
mkdir -p $MNTDIR
DEV=/dev/sda5

# This is a tgz file with 750MB
FILE=/tmp/hugefile.tgz

title()
{
        echo "-----------------------"
        echo "$1"
}

mountCopy()
{
        title "mount"
        time mount $DEV $MNTDIR
        title "sync 1"
        time sync
        (
            cd $MNTDIR
            title "tar"
            time tar -xSzp --numeric-owner --atime-preserve  -f $FILE
        )
        title "sync 2"
        time sync
        title "umount"
        time umount $MNTDIR
}

main()
{
        for (( i=0 ; i<20 ; i++ )) ; do
                echo "****** $i ******"
                title "mkfs"
                time mkfs.ext3 -q $DEV
                mountCopy
        done
}

main 2>&1
--------------------------- END SCRIPT -----------------------

The times needed for the sync before umount:
grep -A 2 "sync 2" script.log| grep real
real    0m6.672s
real    0m52.831s
real    0m7.522s
real    0m8.543s
real    0m8.394s
real    0m6.160s
real    0m11.234s
real    0m51.943s
real    0m6.527s
real    0m53.752s
real    0m7.045s
real    0m9.985s
real    0m7.907s
real    0m47.593s
real    0m8.086s
real    0m6.806s
real    0m8.433s
real    0m8.490s
real    0m8.424s
real    0m47.382s

Is there any possibility to avoid this long durations of the "sync"? Is there any kernel option that could somehow fix this?

Any help is highly welcome!
Thanks a lot.

Best Regards

Mathias


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