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

[Christophe LEROY]

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@xxxxxx> 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@xxxxxx>
> > ---
> >  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