softIRQ execution when CONFIG_PREEMPT is disabled

[Tony Mathew]

Hi All,

If my linux kernel is configured with CONFIG_PREEMPT not mentioned or CONFIG_PREEMPT=n , can a softIRQ execution be preempted by hardIRQ?

Regards,
Tony

[Anil Kumar Pugalia]

By hardIRQ, if you mean hardware interrupts, they typically preempt everything else in the system, except few cases in RT mode.

Regards
Anil

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Tony Mathew wrote on 15/08/23 6:35 am:

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[Aijaz Baig]

Maybe I'm mistaken, but don't softIRQs execute outside of process context but below hardware interrupt context? So essentially at the same privilege level of system calls?

How is that related to pre-emption? Can someone please clear it out to me?

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[Anil Kumar Pugalia]

softIRQs are neither process context nor hardware interrupt context. They have their own context.

Regards
Anil

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Aijaz Baig wrote on 15/08/23 8:32 pm:

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[Aijaz Baig]

Can you expand on softIRQs  'having their own context'? 

On Linux I see each core having a kernel thread called 'ksoftirqd'. What's this one for?

Thanks for your help 

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[Anil Kumar Pugalia]

Check the 14th slide in https://www.slideshare.net/anil_pugalia/interrupts-4638852

ksoftirqd is not ksoftirq: https://man.cx/ksoftirqd(9)

Regards
Anil

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Aijaz Baig wrote on 30/08/23 1:02 pm:

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