Subject: Re: mpc880 linux-2.6.32 slow running processes

From: michael <[email-masked]>
Date: Thu, 06 Jan 2011 13:52:24 +0100

Hi

On 01/06/2011 12:42 AM, Scott Wood wrote:
> On Wed, 5 Jan 2011 19:23:33 +0100
> michael<[email-masked]>  wrote:
>
>> diff --git a/arch/powerpc/include/asm/pte-8xx.h b/arch/powerpc/include/asm/pte-8xx.h
>> index dd5ea95..cb67076 100644
>> --- a/arch/powerpc/include/asm/pte-8xx.h
>> +++ b/arch/powerpc/include/asm/pte-8xx.h
>> @@ -32,7 +32,7 @@
>>   #define _PAGE_FILE	0x0002	/* when !present: nonlinear file mapping */
>>   #define _PAGE_NO_CACHE	0x0002	/* I: cache inhibit */
>>   #define _PAGE_SHARED	0x0004	/* No ASID (context) compare */
>> -#define _PAGE_SPECIAL	0x0008	/* SW entry, forced to 0 by the TLB miss */
>> +#define _PAGE_SPECIAL	0x0000	/* SW entry, forced to 0 by the TLB miss */
> What do you think is going wrong with the special bit on 8xx?
>
> Or might the change to set_pte_filter() alone be what fixed the problem?
>
Only the set_pte_filter doesn't fix the problem. The slow-down depends on
the __HAVE_ARCH_PTE_SPECIAL related code, but 2 months ago I didn't find the reason
and now I don't have the architecture. I will do some tests when I will came back in Italy.
>> diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
>> index 5304093..1da03a8 100644
>> --- a/arch/powerpc/mm/pgtable.c
>> +++ b/arch/powerpc/mm/pgtable.c
>> @@ -173,21 +173,29 @@ static pte_t set_pte_filter(pte_t pte, unsigned long addr)
>>   	pte = __pte(pte_val(pte)&  ~_PAGE_HPTEFLAGS);
>>   	if (pte_looks_normal(pte)&&  !(cpu_has_feature(CPU_FTR_COHERENT_ICACHE) ||
>>   				       cpu_has_feature(CPU_FTR_NOEXECUTE))) {
>> -		struct page *pg = maybe_pte_to_page(pte);
>> -		if (!pg)
>> +		unsigned long pfn = pte_pfn(pte);
>> +		struct page *pg;
>> +
>> +		if (unlikely(!pfn_valid(pfn)))
>>   			return pte;
>> -		if (!test_bit(PG_arch_1,&pg->flags)) {
>> +
>> +		pg = pfn_to_page(pfn);
>>   #ifdef CONFIG_8xx
>> -			/* On 8xx, cache control instructions (particularly
>> -			 * "dcbst" from flush_dcache_icache) fault as write
>> -			 * operation if there is an unpopulated TLB entry
>> -			 * for the address in question. To workaround that,
>> -			 * we invalidate the TLB here, thus avoiding dcbst
>> -			 * misbehaviour.
>> -			 */
>> -			/* 8xx doesn't care about PID, size or ind args */
>> -			_tlbil_va(addr, 0, 0, 0);
>> +		/* On 8xx, cache control instructions (particularly
>> +		 * "dcbst" from flush_dcache_icache) fault as write
>> +		 * operation if there is an unpopulated TLB entry
>> +		 * for the address in question. To workaround that,
>> +		 * we invalidate the TLB here, thus avoiding dcbst
>> +		 * misbehaviour.
>> +		 */
>> +		/* 8xx doesn't care about PID, size or ind args */
>> +		_tlbil_va(addr, 0, 0, 0);
>>   #endif /* CONFIG_8xx */
>> +
>> +		if (!pg)
>> +			return pte;
>> +
>> +		if (!PageReserved(pg)&&  !test_bit(PG_arch_1,&pg->flags)) {
>>   			flush_dcache_icache_page(pg);
>>   			set_bit(PG_arch_1,&pg->flags);
>>   		}
> Rex, do you recall under what specific circumstances the _tlbil_va is
> needed?  Is it possible that it will be caused by a dcbst in other
> contexts that are not dependent on the state of PG_arch_1?
>
> -Scott
>
Michael