Re: scheduling anomaly on uml (was: -rt doesn't compile for UML)

[Miklos Szeredi <miklos@szeredi.hu>]

> > I can't say I'm understading these traces very well, but here's a 
> > snippet that looks a bit strange.  I'm running 'while true; do date; 
> > done' in parallel with the dd.
> > 
> > For some time it is doing 100% CPU as expected, then it goes into a 
> > second or so of mosty idle (afaics), and then returns to the normal 
> > pattern again.
> 
> try:
> 
>   echo 1 > /proc/sys/kernel/stackframe_tracing
> 
> to get symbolic stack backdumps for the wakeup points, and add 
> trace_special_sym() calls to generate extra stackdump entries at 
> arbitrary places. schedule() does not have it right now - it might make 
> sense to add it.

OK, this helped.

It looks like the delays are caused by the page allocator, and not
UML.  Which is good news, because I'm supposed to understand memory
management better than scheduling.

Thanks,
Miklos



    bash-504   0.... 6312803us!: schedule <kswapd0-33> (-5 0)
    bash-504   0.... 6313820us : userspace()<-fork_handler()<-<2f0a3670>()<-(    0)()
    bash-504   0.... 6313821us+: (    0)()<-(    0)()<-(    0)()<-(    0)()
    date-5726  0.... 6315334us+: pipe_read()<-do_sync_read()<-vfs_read()<-sys_read()
    date-5726  0.... 6315336us : handle_syscall()<-handle_trap()<-userspace()<-fork_handler()
    bash-504   0.... 6315340us!: schedule <date-5726> (0 0)
    bash-504   0.... 6315731us+: io_schedule_timeout()<-congestion_wait()<-throttle_vm_writeout()<-shrink_zone()
    bash-504   0.... 6315733us : shrink_zones()<-try_to_free_pages()<-__alloc_pages()<-do_wp_page()
  <idle>-0     0.... 6315738us!: schedule <bash-504> (0 20)
  <idle>-0     0.Nh. 6476223us+: __trace_start_sched_wakeup <bash-504> (120 -1)
  <idle>-0     0.Nh. 6476228us+: __trace_start_sched_wakeup <kswapd0-33> (115 -1)
  <idle>-0     0.Nh. 6476232us+: __trace_start_sched_wakeup <dd-5155> (120 -1)
  <idle>-0     0.... 6476256us+: cpu_idle()<-rest_init()<-start_kernel()<-start_kernel_proc()