Question about switch_mm function

[Sreejith M M]

On Wed, Mar 25, 2015 at 9:30 PM, Rajat Sharma <fs.rajat at gmail.com> wrote:
>
> On Mar 25, 2015 6:33 AM, "Sreejith M M" <sreejith.mm at gmail.com> wrote:
>>
>>
>>
>> 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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>>
>
> This code is handling context switch from a kernel thread back to user mode
> thread so TLB entries are invalid translation for user mode thread and do
> not correspond to user process pgd. It is Master kernel page table
> translation as a result of kernel thread execution.
>
> -Rajat
Hi Rajat,

If that is the case, why this code is put under CONFIG_SMP switch?


-- 
Regards,
Sreejith