Re: [announce, patch, RFC] "big IRQ lock" removal, IRQ cleanups.

[george anzinger <george@mvista.com>]

Ingo Molnar wrote:
> 
> the following patch, against 2.5.26:
> 
>   http://redhat.com/~mingo/remove-irqlock-patches/remove-irqlock-2.5.26-A2

I haven't applied the patch, but just looking at it, it
appears that you have removed the conditional call to
softirq on local_bh_enable().  Isn't this needed?

-g
> 
> is a work-in-progress massive cleanup of the IRQ subsystem. It's losely
> based on Linus' original idea and DaveM's original implementation, to fold
> our various irq, softirq and bh counters into the preemption counter.
> 
> with this approach it was possible:
> 
>  - to remove the 'big IRQ lock' on SMP - on which sti() and cli() relied.
> 
>  - to streamline/simplify arch/i386/kernel/irq.c significantly.
> 
>  - to simplify the softirq code.
> 
>  - to remove the preemption count increase/decrease code from the lowlevel
>    IRQ assembly code.
> 
>  - to speed up schedule() a bit.
> 
> sti() and cli() is gone forever, there is no more globally synchronizing
> irq-disabling capability. All code that relied on sti() and cli() and
> restore_flags() must use other locking mechanisms from now on (spinlocks
> and __cli()/__sti()).
> 
> obviously this patch breaks massive amounts of code, so only limited
> .configs are working at the moment, such as:
> 
>   http://redhat.com/~mingo/remove-irqlock-patches/config
> 
> otherwise the patch was developed and tested on SMP systems, and while the
> code is still a bit rough in places, the base IRQ code appears to be
> pretty robust and clean.
> 
> while it boots already so the worst is over, there is lots of work left:
> eg. to fix the serial layer to not use cli()/sti() and bhs ...
> 
> RMK, is there any chance to get your new serial layer into 2.5 sometime
> soon? ['soon' as in 'tomorrow' :-) ] That is perhaps one of the biggest
> kernel subsystems that make use of cli()/sti() currently. The rest is
> drivers mostly, which is still not unsignificant, but perhaps a bit easier
> to manage.
> 
>         Ingo
> 
> -
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-- 
George Anzinger   george@mvista.com
High-res-timers: 
http://sourceforge.net/projects/high-res-timers/
Real time sched:  http://sourceforge.net/projects/rtsched/
Preemption patch:
http://www.kernel.org/pub/linux/kernel/people/rml