Re: [patch] voluntary-preempt-2.6.8.1-P1

[Hans Reiser]

Ingo Molnar wrote:

> * Thomas Charbonnel <thomas@xxxxxxxxxx> wrote:
>
>  
>
> > Tested P1. The biggest offender on my system is still reiserfs'
> > search_by_key. The trace is made of a very long loop of :
> > 0.016ms (+0.000ms): reiserfs_in_journal (scan_bitmap_block)
> > 0.016ms (+0.000ms): find_next_zero_bit (scan_bitmap_block)
> > (...)
> > 0.977ms (+0.000ms): reiserfs_in_journal (scan_bitmap_block)
> > 0.977ms (+0.000ms): find_next_zero_bit (scan_bitmap_block)
> > with interrupts showing up in the trace from time to time. Do you have
> > plans to fix this, or should I switch to ext3 ?
> >    
>
> i took a quick look and the reiserfs locking rules in that place do not
> seem to be easily fixable - this is the tree-lookup code which i suspect
> cannot be preempted. The reiser journalling code also makes use of the
> big kernel lock. I'd suggest reporting this to the reiserfs folks
the fix to reiserfs doing single threaded balancing/searching is called 
reiser4;-)  It was not a trivial fix.  It is however released.

> and
> (if it's not too much effort to migrate) use ext3 meanwhile.
>
>  
>
> > but the weird thing is that the latency sum goes way above those 100us,
> > the culprit being do_IRQ, regularly chewing up to 1ms !
> >    
>
>  
>
> > 0.011ms (+0.000ms): enable_8259A_irq (startup_8259A_irq)
> > 0.954ms (+0.943ms): do_IRQ (common_interrupt)
> >    
>
> weird. This has the looks of a preempt-timing bug (we get a timer IRQ
> every 1 msec) - but there should be no preempt-timing when we are in the
> idle task (swapper).
>
>  
>
> > http://www.undata.org/~thomas/swapper.trace
> >    
>
> i'll upload -P2 in a couple of minutes, it will trace the code that
> do_IRQ() interrupted too - that would be quite useful in your case.
>
> 	Ingo
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