Question about switch_mm function
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
On Wed, Jan 28, 2015 at 9:56 PM, Sreejith M M <sreejith.mm@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
On Mar 25, 2015 6:33 AM, "Sreejith M M" <sreejith.mm@gmail.com> wrote:
On Wed, Jan 28, 2015 at 9:56 PM, Sreejith M M <sreejith.mm@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
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
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
On Wed, Mar 25, 2015 at 9:30 PM, Rajat Sharma <fs.rajat@gmail.com> wrote:
On Mar 25, 2015 6:33 AM, "Sreejith M M" <sreejith.mm@gmail.com> wrote:
On Wed, Jan 28, 2015 at 9:56 PM, Sreejith M M <sreejith.mm@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
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
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
On Wed, 25 Mar 2015 21:35:22 +0530, Sreejith M M said:
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?
Vastly simplified because I'm lazy :) If you look at the code, it's poking the status on *other* CPUs. That's why the cpumask() stuff. If you're on a single execution unit, you don't have to tell the other CPU about the change in state, because there isn't an other CPU.
On Wed, Mar 25, 2015 at 10:55 PM, <Valdis.Kletnieks@vt.edu> wrote:
On Wed, 25 Mar 2015 21:35:22 +0530, Sreejith M M said:
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?
Vastly simplified because I'm lazy :)
If you look at the code, it's poking the status on *other* CPUs. That's why the cpumask() stuff.
If you're on a single execution unit, you don't have to tell the other CPU about the change in state, because there isn't an other CPU.
can you come out of this lazy mode explain this a bit more because I am a newbie ?or tell me what else I should know before I have to understand this code -- Regards, Sreejith
participants (3)
-
Rajat Sharma -
Sreejith M M -
Valdis.Kletnieks＠vt.edu