Question about switch_mm function

[Sreejith M M]

On Wed, Jan 28, 2015 at 9:56 PM, Sreejith M M <sreejith.mm at gmail.com> wrote:

> Hi,
>
> I was trying to understand the difference in scheduling between
> processes and threads(belong to same process).
>
> I was thinking that, when kernel has to switch to a task which belong
> to the same process, it does not have to clear / replace page global
> directories and other memory related information.
>
> But in switch_mm function some code is put under CONFIG_SMP function.
> What is its signigicance? Code is
> below(
> http://lxr.free-electrons.com/source/arch/x86/include/asm/mmu_context.h#L37
> )
> .
> What I infer is that the code is doing flush tlb, reload page table
> directories etc in multiprocessor mode(obviously)  but I believe this
> code may never be executed .
>
> Can anyone help to understand what this part of the function supposed to
> do?
>
>  60 #ifdef CONFIG_SMP
>  61           else {
>  62                 this_cpu_write(cpu_tlbstate.state, TLBSTATE_OK);
>  63                 BUG_ON(this_cpu_read(cpu_tlbstate.active_mm) != next);
>  64
>  65                 if (!cpumask_test_cpu(cpu, mm_cpumask(next))) {
>  66                         /*
>  67                          * On established mms, the mm_cpumask is
> only changed
>  68                          * from irq context, from
> ptep_clear_flush() while in
>  69                          * lazy tlb mode, and here. Irqs are blocked
> during
>  70                          * schedule, protecting us from
> simultaneous changes.
>  71                          */
>  72                         cpumask_set_cpu(cpu, mm_cpumask(next));
>  73                         /*
>  74                          * We were in lazy tlb mode and leave_mm
> disabled
>  75                          * tlb flush IPI delivery. We must reload CR3
>  76                          * to make sure to use no freed page tables.
>  77                          */
>  78                         load_cr3(next->pgd);
>  79                         trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH,
> TLB_FLUSH_ALL);
>  80                         load_LDT_nolock(&next->context);
>  81                 }
>  82         }
>  83 #endif
>
>
> --
> Regards,
> Sreejith
>

Hi ,

can someone please give me any answers for this?

-- 
Regards,
Sreejith
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