Re: [PATCH] Avoid moving tasks when a schedule can be made.

[Nick Piggin <nickpiggin@yahoo.com.au>]

Ingo Molnar wrote:
> * Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> 
> 
>>>>So it is not a nice thing to tinker with unless there is good reason.
>>>
>>>unbound latencies with hardirqs off are obviously a good reason - but i 
>>>agree that the solution is not good enough, yet.
>>
>>Ah, so this is an RT tree thing where the scheduler lock turns off 
>>"hard irqs"? [...]
> 
> 
> no, this is about the mainline kernel turning off hardirqs for a long 
> time. (i used the hardirqs-off terminology instead of irqs-off to 
> differentiate it from softirqs-off a'ka local_bh_disable(). It's a 
> side-effect of working on the lock validator i guess ;).
> 

OK.

> 
>>[...] As opposed to something like the rwsem lock that only turns off 
>>your "soft irqs" (sorry, I'm not with the terminlogy)?
> 
> 
> rwsems/rwlocks are not an issue in -rt because they have different 
> semantics there - and thus readers cannot amass. I do think rwsems and 
> rwlocks have pretty nasty characteristics [non-latency ones] for the 
> mainline kernel's use too, but that's not being argued here ;)
> 

But all I'm saying is that while there are equivalent magnitudes of
interrupts off regions in mainline, there is little point introducing
a hack like this to "solve" one of them.

Sure, rework one of them nicely to solve the issue for that case, or
hack _all_ of them (not for mainline but for your specific RT system).
But hacking just one does not make any sense.

Just because this one path actually _triggered_ due to a silly
benchmark doesn't make it any more valid. I think it shouldn't be too
hard to cause similar or worse latencies with the mmap_sem rwsem using
the same number of threads.

If anyone is running hackbench 20 on their sound mixer, then they
deserve to have overruns.

-- 
SUSE Labs, Novell Inc.
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