Re: [RFC PATCH] powerpc/book3s64/radix: Upgrade va tlbie to PID tlbie if we cross PMD_SIZE

[Michael Ellerman <mpe@ellerman.id.au>]

"Puvichakravarthy Ramachandran" <puvichakravarthy@in.ibm.com> writes:
>> With shared mapping, even though we are unmapping a large range, the kernel
>> will force a TLB flush with ptl lock held to avoid the race mentioned in
>> commit 1cf35d47712d ("mm: split 'tlb_flush_mmu()' into tlb flushing and memory freeing parts")
>> This results in the kernel issuing a high number of TLB flushes even for a large
>> range. This can be improved by making sure the kernel switch to pid based flush if the
>> kernel is unmapping a 2M range.
>> 
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> ---
>>  arch/powerpc/mm/book3s64/radix_tlb.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>> 
>> diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c > b/arch/powerpc/mm/book3s64/radix_tlb.c
>> index aefc100d79a7..21d0f098e43b 100644
>> --- a/arch/powerpc/mm/book3s64/radix_tlb.c
>> +++ b/arch/powerpc/mm/book3s64/radix_tlb.c
>> @@ -1106,7 +1106,7 @@ EXPORT_SYMBOL(radix__flush_tlb_kernel_range);
>>   * invalidating a full PID, so it has a far lower threshold to change > from
>>   * individual page flushes to full-pid flushes.
>>   */
>> -static unsigned long tlb_single_page_flush_ceiling __read_mostly = 33;
>> +static unsigned long tlb_single_page_flush_ceiling __read_mostly = 32;
>>  static unsigned long tlb_local_single_page_flush_ceiling __read_mostly > = POWER9_TLB_SETS_RADIX * 2;
>> 
>>  static inline void __radix__flush_tlb_range(struct mm_struct *mm,
>> @@ -1133,7 +1133,7 @@ static inline void __radix__flush_tlb_range(struct > mm_struct *mm,
>>       if (fullmm)
>>               flush_pid = true;
>>       else if (type == FLUSH_TYPE_GLOBAL)
>> -             flush_pid = nr_pages > tlb_single_page_flush_ceiling;
>> +             flush_pid = nr_pages >= tlb_single_page_flush_ceiling;
>>       else
>>               flush_pid = nr_pages > tlb_local_single_page_flush_ceiling;
>
> Additional details on the test environment. This was tested on a 2 Node/8 
> socket Power10 system.
> The LPAR had 105 cores and the LPAR spanned across all the sockets. 
>
> # perf stat -I 1000 -a -e cycles,instructions -e 
> "{cpu/config=0x030008,name=PM_EXEC_STALL/}" -e 
> "{cpu/config=0x02E01C,name=PM_EXEC_STALL_TLBIE/}" ./tlbie -i 10 -c 1  -t 1
>  Rate of work: = 176 
> #           time             counts unit events
>      1.029206442         4198594519      cycles  
>      1.029206442         2458254252      instructions              # 0.59 insn per cycle 
>      1.029206442         3004031488      PM_EXEC_STALL   
>      1.029206442         1798186036      PM_EXEC_STALL_TLBIE   
>  Rate of work: = 181 
>      2.054288539         4183883450      cycles  
>      2.054288539         2472178171      instructions              # 0.59 insn per cycle 
>      2.054288539         3014609313      PM_EXEC_STALL   
>      2.054288539         1797851642      PM_EXEC_STALL_TLBIE   
>  Rate of work: = 180 
>      3.078306883         4171250717      cycles  
>      3.078306883         2468341094      instructions              # 0.59 insn per cycle 
>      3.078306883         2993036205      PM_EXEC_STALL   
>      3.078306883         1798181890      PM_EXEC_STALL_TLBIE   
> .
> . 
>
> # cat /sys/kernel/debug/powerpc/tlb_single_page_flush_ceiling
> 34
>
> # echo 32 > /sys/kernel/debug/powerpc/tlb_single_page_flush_ceiling
>
> # perf stat -I 1000 -a -e cycles,instructions -e 
> "{cpu/config=0x030008,name=PM_EXEC_STALL/}" -e 
> "{cpu/config=0x02E01C,name=PM_EXEC_STALL_TLBIE/}" ./tlbie -i 10 -c 1  -t 1
>  Rate of work: = 313 
> #           time             counts unit events
>      1.030310506         4206071143      cycles  
>      1.030310506         4314716958      instructions              # 1.03 insn per cycle 
>      1.030310506         2157762167      PM_EXEC_STALL   
>      1.030310506          110825573      PM_EXEC_STALL_TLBIE   
>  Rate of work: = 322 
>      2.056034068         4331745630      cycles  
>      2.056034068         4531658304      instructions              # 1.05 insn per cycle 
>      2.056034068         2288971361      PM_EXEC_STALL   
>      2.056034068          111267927      PM_EXEC_STALL_TLBIE   
>  Rate of work: = 321 
>      3.081216434         4327050349      cycles  
>      3.081216434         4379679508      instructions              # 1.01 insn per cycle 
>      3.081216434         2252602550      PM_EXEC_STALL   
>      3.081216434          110974887      PM_EXEC_STALL_TLBIE   


What is the tlbie test actually doing?

Does it do anything to measure the cost of refilling after the full mm flush?

cheers