Queries on bottom halves
Larry Chen
lchen at suse.com
Mon Sep 3 00:23:34 EDT 2018
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
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