Re: [PATCH v2] powerpc/mm: Only read faulting instruction when necessary in do_page_fault()

[Christophe LEROY <christophe.leroy@c-s.fr>]

Le 01/05/2017 à 05:00, Nicholas Piggin a écrit :
> On Fri, 28 Apr 2017 08:13:01 +0200 (CEST)
> Christophe Leroy <christophe.leroy@c-s.fr> wrote:
>
>> Commit a7a9dcd882a67 ("powerpc: Avoid taking a data miss on every
>> userspace instruction miss") has shown that limiting the read of
>> faulting instruction to likely cases improves performance.
>>
>> This patch goes further into this direction by limiting the read
>> of the faulting instruction to the only cases where it is definitly
>> needed.
>>
>> On an MPC885, with the same benchmark app as in the commit referred
>> above, we see a reduction of 4000 dTLB misses (approx 3%):
>>
>> Before the patch:
>>  Performance counter stats for './fault 500' (10 runs):
>>
>>          720495838      cpu-cycles                                                    ( +-  0.04% )
>>             141769      dTLB-load-misses                                              ( +-  0.02% )
>>              52722      iTLB-load-misses                                              ( +-  0.01% )
>>              19611      faults                                                        ( +-  0.02% )
>>
>>        5.750535176 seconds time elapsed                                          ( +-  0.16% )
>>
>> With the patch:
>>  Performance counter stats for './fault 500' (10 runs):
>>
>>          717669123      cpu-cycles                                                    ( +-  0.02% )
>>             137344      dTLB-load-misses                                              ( +-  0.03% )
>>              52731      iTLB-load-misses                                              ( +-  0.01% )
>>              19614      faults                                                        ( +-  0.03% )
>>
>>        5.728423115 seconds time elapsed                                          ( +-  0.14% )
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>> ---
>>  v2: Changes 'if (cond1) if (cond2)' by 'if (cond1 && cond2)'
>>
>>  In case the instruction we read has value 0, store_update_sp() will
>>  return false, so it will bail out.
>>
>>  This patch applies after the serie "powerpc/mm: some cleanup of do_page_fault()"
>>
>>  arch/powerpc/mm/fault.c | 22 ++++++++++++----------
>>  1 file changed, 12 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index 400f2d0d42f8..2ec82a279d28 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -280,14 +280,6 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>>
>>  	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>>
>> -	/*
>> -	 * We want to do this outside mmap_sem, because reading code around nip
>> -	 * can result in fault, which will cause a deadlock when called with
>> -	 * mmap_sem held
>> -	 */
>> -	if (is_write && is_user)
>> -		__get_user(inst, (unsigned int __user *)regs->nip);
>> -
>>  	if (is_user)
>>  		flags |= FAULT_FLAG_USER;
>>
>> @@ -356,8 +348,18 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>>  		 * between the last mapped region and the stack will
>>  		 * expand the stack rather than segfaulting.
>>  		 */
>> -		if (address + 2048 < uregs->gpr[1] && !store_updates_sp(inst))
>> -			goto bad_area;
>> +		if (address + 2048 < uregs->gpr[1] && !inst) {
>> +			/*
>> +			 * We want to do this outside mmap_sem, because reading
>> +			 * code around nip can result in fault, which will cause
>> +			 * a deadlock when called with mmap_sem held
>> +			 */
>> +			up_read(&mm->mmap_sem);
>> +			__get_user(inst, (unsigned int __user *)regs->nip);
>> +			if (!store_updates_sp(inst))
>> +				goto bad_area_nosemaphore;
>> +			goto retry;
>> +		}
>
> Yes, nice patch. I wonder if you can do __get_user first as non-faulting to
> avoid retaking the mmap_sem and retrying? Along the lines of:
>
> +               nip = (unsigned int __user *)regs->nip;
> +               pagefault_disable();
> +               if (unlikely(__get_user_inatomic(inst, nip))) {
> +                       pagefault_enable();
> +                       up_read(&mm->mmap_sem);
> +                       if (get_user(inst, nip)) {
>                            ...
>                            goto retry;
>
> The user instruction should practically always have a Linux pte, so a
> fault there should be exceedingly rare, I think?

Thanks Nick.
I have submitted a new version of the patch taking your suggestion into 
accout.

Christophe