Re: Looking for comments on Bottom-Half/Tasklet/SoftIRQ

[chuckw@ieee.org]

Thanks.

	So, Bottom halves don't need to be re-entrant as do tasklets.  SoftIRQ's
need to be re-entrant.  The advantage of tasklets is that each tasklet can
be farmed out to different CPU's AND they don't need to be re-entrant 
because only one instance is allowed at a time.  I think I got it.

	Could you direct me to some code in the kernel which uses tasklets
so I can see the inner workings?

Thanks much,
Chuck

On Sun, Aug 19, 2001 at 06:59:22PM +0200, Anders Peter Fugmann wrote:
> 
> chuckw@ieee.org wrote:
> > Greetings,
> > 	I was reading the unreliable guide to kernel hacking and was looking for
> > a little clarification on something.  2 Bottom halves cannot run at the same
> > time, why? 
> 
> Per linux definition of bottom halves, there can only run one buttom 
> half at one system wide. But dont use those - They are old and waists 
> resources. Try tasklets instead. Multible tasklets can run in parrallel 
> (but not the same tasklet)
> 
> > 	Also, could someone give me an example of a service which is a bottom half/
> > tasklet/SoftIRQ?
> Simple.
> 
> Imagine some hardware that generates interrupts.
> Now we want to write a driver that keeps the hardware busy, so we 
> implement a top half handler (IRQ-handler), and let it retrieve som data 
> from the hardware. Instead of processing it right away, we shedule a 
> tasklet to do that job. This way we can handle more interrupts/sec from 
> the card, and the hardware is kept busy.
> 
> 
> To summerize.
> Buttom halves are the strictest (only one at a time.)
> Takslets can run in parralel, but still no need to worry about reentrant 
> code.
> SoftIrq give no guarrentee at all, and should be used with great care
> (code need to be reentrant).
> 
> Also try to readLinux device drivers by  A. Rubini:
> http://www.xml.com/ldd/chapter/book/index.html
> 
> Hope it helps.
> Anders Fugmann
> 
> > 
> > Thanks in advance,
> > Chuck
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