Queries on bottom halves

[Larry Chen]

Hi Abhinav,

On 09/02/2018 11:28 PM, Abhinav Misra wrote:
> Hi Larry,
> 
> Based on your answers below are my further queries.
> 
> 1. *Does softirq and tasklet will always runs in ksoftirqd thread context ?*
> As it is mentioned in the LKD (by robert love Pg-138) that there are 
> multiple places where pending softirq's
> are checked. Out of that one is in return from hardware interrupt code 
> path i.e in do_irq function.
> 
> If that is the case then it will be running in the irq context with just 
                            ~~ --->> Sorry, does "it" mean softirq or 
somethine else??
do_irq just wake up softirq thread. That does not mean softirq runs in 
irq context.

> the interrupts enabled.

> Now I know LKD is old and based on linux kernel 2.6. *Is that above 
> scenario is changed in new version of kernel ?*
> 
> Even I tried one example mentioned in LDD by Jerry cooperstein and 
> printing the pid of current task (current->pid) in the
> tasklet which is getting scheduled by the shared interrupt from n/w 
> card. Every time it is printing the pid of the ksoftirqd thread.
> 
> So does that mean, now in latest kernel, softirq and tasklet will always 
> run in context of ksoftirqd thread ?

Yes.

> 2. If we can sleep or use blocking calls in softirq's and tasklets then 
> *what is the difference between softirq/tasklet and workqueue's ?

That's another question, workqueue and tasklet is two machnisms provided 
for async operations. Generally I think workqueue is more powerful and 
flexible. You can get more info from kernel docs.


> *Because in old kernel, the main difference between softirq/tasklet and 
> workqueue's (Wq's) is that Wq's runs in process context
> and hence sleep is allowed while the same is not the case with other 
> counterparts.
I refered to kernel 2.6 version, softirq and tasklet are still almost 
the same machnism with the latest version.

Here is an explanation

https://www.safaribooksonline.com/library/view/understanding-the-linux/0596005652/ch04s07.html

> 
> But if in new kernel this implementation is changed then why we need so 
> many options to defer the work as all of them are basically getting 
> executed in almost the same way.*In that case code running softirq, 
> tasklet, workqueue and kernel thread are all same ?*
> If this is true then why we just remove all these these option and keep 
> one or two alternative.
> 
Emmm... tasklet and workqueue have something in common, but I think 
tasklet can satisfy simple scenarios, while workqueue, I think is more 
powerfull and flexible, does well in more complicated scenarios.

Maybe when you make clear what's their difference, you'll understand more.

BR,
Larry