Re: an issue with 'xm save'

[Zhenzhong Duan <zhenzhong.duan@oracle.com>]

On 2012-09-26 20:35, Konrad Rzeszutek Wilk wrote:
> On Wed, Sep 26, 2012 at 04:48:42PM +0800, Zhenzhong Duan wrote:
>> Konrad Rzeszutek Wilk wrote:
>>> On Fri, Sep 21, 2012 at 05:41:27PM +0800, Zhenzhong Duan wrote:
>>>> Hi maintainers,
>>>>
>>>> I found there is an issue when 'xm save' a pvm guest. See below:
>>>>
>>>> When I do save then restore once, CPU(%) in xentop showed around 99%.
>>>> When I do that second time, CPU(%) showed 199%
>>>>
>>>> top in dom0 showed:
>>>>      PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>>>>     20946 root      18  -2 10984 1284  964 S 19.8  0.3   0:48.93 block
>>>>     4939 root      18  -2 10984 1288  964 S 19.5  0.3   1:34.68 block
>>>>
>>>> I could kill the block process, then all look normal again.
>>> What is the 'block' process? If you attach 'perf' to it do you get an idea
>>> of what it is spinning at?
>> It's /etc/xen/scripts/block
>> I add 'set -x' to /etc/xen/scripts/block, found it blocked at claim_lock.
>> When domU was created first time, claim_lock/release_lock finished quickly,
>> when 'xm save' was called, claim_lock spin in its own while loop.
>> I can ensure no other domU create/save/etc happen when I test.
> OK, so how come you have two block processes? Is it b/c you have two
> disks attached to the guest? The are multiple claim_lock in the shell
> script - do you know where each of two threads are spinning? Are they
> spinning on the same function?
In above test, I run save/restore twice, so two block processes.
In other test, run save/restore once, there is only one block process.
After do 'xm save', I see block process spin at line 328:
321   remove)
322     case $t in
323       phy)
324         exit 0
325         ;;
326
327       file)
328         claim_lock "block"
329         node=$(xenstore_read "$XENBUS_PATH/node")
330         losetup -d "$node"
331         release_lock "block"
332         exit 0
333         ;;

thanks
zduan