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SubjectRe: Pathological case identified from contest
Quoting Andrew Morton <akpm@digeo.com>:

> Con Kolivas wrote:
> >
> > I found a pathological case in 2.5 while running contest with process_load
> > recently after checking the results which showed a bad result for
> 2.5.43-mm1:
> >
> > 2.5.43-mm1 101.38 72% 42 31%
> > 2.5.43-mm1 102.90 75% 34 28%
> > 2.5.43-mm1 504.12 14% 603 85%
> > 2.5.43-mm1 96.73 77% 34 26%
> >
> > This was very strange so I looked into it further
> >
> > The default for process_load is this command:
> >
> > process_load --processes $nproc --recordsize 8192 --injections 2
> >
> > where $nproc=4*num_cpus
> >
> > When I changed recordsize to 16384, many of the 2.5 kernels started
> exhibiting
> > the same behaviour. While the machine was apparently still alive and would
> > respond to my request to abort, the kernel compile would all but stop
> while
> > process_load just continued without allowing anything to happen from
> kernel
> > compilation for up to 5 minutes at a time. This doesnt happen with any 2.4
> kernels.
> >
>
> Well it doesn't happen on my test machine (UP or SMP). I tried
> various recordsizes. It's probably related to HZ, memory bandwidth
> and the precise timing at which things happen.
>
> The test describes itself thusly:
>
> * This test generates a load which simulates a process-loaded system.
> *
> * The test creates a ring of processes, each connected to its predecessor
> * and successor by a pipe. After the ring is created, the parent process
> * injects some dummy data records into the ring and then joins. The
> * processes pass the data records around the ring until they are killed.
> *
>
> It'll be starvation in the CPU scheduler I expect. For some reason
> the ring of piping processes is just never giving a timeslice to
> anything else. Or maybe something to do with the exceptional
> wakeup strategy which pipes use.
>
> Don't now, sorry. One for the kernel/*.c guys.

Ok well I've done some profiling as suggested by wli and it shows pretty much
what I find in the results - it gets stuck while doing process_load and never
moves on.

recordsize 8192 kern profile:
c01223ac 76997 4.48583 do_anonymous_page
c0188694 135835 7.91373 __generic_copy_from_user
c0188610 345071 20.1038 __generic_copy_to_user
c0105298 801429 46.6911 poll_idle
sysprofile:
00000000 160258 5.03854 (no symbol) /lib/i686/libc-2.2.5.so
c0188610 345071 10.8491 __generic_copy_to_user
/home/con/kernel/linux-2.5.43/vmlinux
c0105298 801429 25.1971 poll_idle
/home/con/kernel/linux-2.5.43/vmlinux
00000000 1132668 35.6113 (no symbol)
/usr/lib/gcc-lib/i686-pc-linux-gnu/2.95.3/cc1

Normal run consistent with doing kernel compilation most of the time.

recordsize 16384 kernprofile:
c0111ef4 403545 4.3407 do_schedule
c0105298 558704 6.00965 poll_idle
c0188694 2571995 27.6655 __generic_copy_from_user
c0188610 4489796 48.2941 __generic_copy_to_user
sysprofile:
c0111ef4 403545 4.24896 do_schedule
/home/con/kernel/linux-2.5.43/vmlinux
c0105298 558704 5.88264 poll_idle
/home/con/kernel/linux-2.5.43/vmlinux
c0188694 2571995 27.0807 __generic_copy_from_user
/home/con/kernel/linux-2.5.43/vmlinux
c0188610 4489796 47.2734 __generic_copy_to_user
/home/con/kernel/linux-2.5.43/vmlinux

I had to abort the run with recordsize 16384 but you can see it's just stuck in
process_load copying data between forked processes.

Can someone else on lkml decipher why it gets stuck here?

Con
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