Looking for comments on Bottom-Half/Tasklet/SoftIRQ

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?

Also, could someone give me an example of a service which is a bottom half/
tasklet/SoftIRQ?

Thanks in advance,
Chuck
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Quoted 4 lines · [email protected] 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)

Quoted 2 lines

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

Quoted 10 lines

Thanks in advance,
Chuck
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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?

Quoted 6 lines · [email protected] wrote:

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.

That is 100% correct.

Quoted 3 lines

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

Actually very few systems in the kernel has been rewritten to use tasklets instead og BH's.

But as they are very simillar to BH's, you should be able to use the same thinking, its just a new API.

Take a look at include/linux/interrupt.h
(or http://lxr.linux.no/source/include/linux/interrupt.h, an invaluable source when coding for linux).

Regards
Anders Fugmann

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

Quoted 38 lines · Anders Peter Fugmann

Quoted 4 lines · [email protected] 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)

Quoted 2 lines

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

Quoted 1 line

A simple example is the ../kernel/timer.c code. The "run_task_list()"
function is called from a tasklet. "do_timer()" is called from
interrupt and "mark_bh(TIMER_BH)" puts the tasklet in the queue. "timer_bh()" (old names die hard) is the tasklet.

George
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Quoted 8 lines · [email protected] wrote:

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.

Quoted 45 lines

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

Thanks much,
Chuck

Quoted 38 lines · Anders Peter Fugmann

Quoted 4 lines · [email protected] 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)

Quoted 2 lines

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

Quoted 1 line

A simple example is the ../kernel/timer.c code. The "run_task_list()" function is called from a tasklet. "do_timer()" is called from interrupt and "mark_bh(TIMER_BH)" puts the tasklet in the queue. "timer_bh()" (old names die hard) is the tasklet.

George