Re: Pathological case identified from contest

[Con Kolivas <conman@kolivas.net>]

Quoting Con Kolivas <conman@kolivas.net>:

> 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?

Well this has become more common with 2.5.43-mm2. I had to abort the
process_load run 3 times when benchmarking it. Going back to other kernels and
trying them it didnt happen so I dont think its my hardware failing or something
like that.

Con