Hi Vignesh, Thank you. I understood from your description that SCHED_SOFTIRQ has no relation with scheduling directly but only deals with load balancing between groups. I have one more question regarding timer interrupts and its relation to schedule() function. As I said, my understanding was that schedule() function is invoked in 2 cases 1. When a process goes to sleep or waiting for a resource 2. For each timer tick Regarding point 2, I am still not clear how each timer tick invokes the scheduler.Please give any pointers On Wed, Jan 28, 2015 at 11:23 PM, Vignesh Radhakrishnan <vignesh1192@gmail.com> wrote:

Hey Sreejith,

softirq's are run as bottom half processing (after an interrupt is handled or when there is no work to be done - https://www.kernel.org/doc/htmldocs/kernel-hacking/basics-softirqs.html ) and sched_softirq is one such soft irq whose only function is confined to the routine run_rebalance_domain().

__schedule() is the main scheduling function that tries to pick the next task and then perform context switch and other associated scheduling functions. Since run_rebalance_domain() need not be run at that exact time frame, it is scheduled for later time when the cpu can take it up using softirq's. Therefore __schedule() need not be called in this softirq context as such because these are independent operations.

Hope this helps.

Thanks and regards, Vignesh Radhakrishnan

On Wed, Jan 28, 2015 at 10:25 PM, Sreejith M M <sreejith.mm@gmail.com> wrote:

...

Hi,

I was reading LKD by Robert Love. I got the following idea from the book.

Correct me if I am worng I was checking through source code and I found that on every timer interrupt, through sched/fair.c we are raising the SCHED_SOFTIRQ(). I was checking the relation between SCHED_SOFTIRQ and actual __schedule() function.

My assumption: schedule() function is the function which selects the processes which are ready to run in run queue. schedule() function is called in every timer tick.

What I was thinking is that schedule() will be called as part of handling SCHED_SOFTIRQ() . But in source code, SCHED_SOFTIRQ is handled through run_rebalance_domain() function (sched/fair.c) . I am unable to trace __schedule() from this function.

Am I missing anything or my assumptions are wrong? -- Regards, Sreejith

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-- Regards, Sreejith