Re: Process Scheduling Issue using sg/libata

[Tejun Heo <htejun@gmail.com>]

Fajun Chen wrote:
> I use sg/libata and ata pass through for read/writes. Linux 2.6.18-rc2
> and libata version 2.00 are loaded on ARM XScale board.  Under heavy
> cpu load (e.g. when blocks per transfer/sector count is set to 1),
> I've observed that the test application can suck cpu away for long
> time (more than 20 seconds) and other processes including high
> priority shell can not get the time slice to run.  What's interesting
> is that if the application is under heavy IO load (e.g. when blocks
> per transfer/sector count is set to 256),  the problem goes away. I
> also tested with open source code sg_utils and got the same result, so
> this is not a problem specific to my user-space application.
> 
> Since user preemption is checked when the kernel is about to return to
> user-space from a system call,  process scheduler should be invoked
> after each system call. Something seems to be broken here.  I found a
> similar issue below:
> http://marc.info/?l=linux-arm-kernel&m=103121214521819&w=2
> But that turns out to be an issue with MTD/JFFS2 drivers, which are
> not used in my system.
> 
> Has anyone experienced similar issues with sg/libata? Any information
> would be greatly appreciated.

That's one weird story.  Does kernel say anything during that 20 seconds?

-- 
tejun