linux-kernel - Re: [PATCH v9] powerpc/mm: Only read faulting instruction when necessary in do_page

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Date:   Wed, 23 May 2018 19:36:59 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v9] powerpc/mm: Only read faulting instruction when
 necessary in do_page_fault()

On Wed, 23 May 2018 10:53:22 +0200 (CEST)
Christophe Leroy <christophe.leroy@....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 likely
> needed.
> 
> On an MPC885, with the same benchmark app as in the commit referred
> above, we see a reduction of about 3900 dTLB misses (approx 3%):
> 
> Before the patch:
>  Performance counter stats for './fault 500' (10 runs):
> 
>          683033312      cpu-cycles                                                    ( +-  0.03% )
>             134538      dTLB-load-misses                                              ( +-  0.03% )
>              46099      iTLB-load-misses                                              ( +-  0.02% )
>              19681      faults                                                        ( +-  0.02% )
> 
>        5.389747878 seconds time elapsed                                          ( +-  0.06% )
> 
> With the patch:
> 
>  Performance counter stats for './fault 500' (10 runs):
> 
>          682112862      cpu-cycles                                                    ( +-  0.03% )
>             130619      dTLB-load-misses                                              ( +-  0.03% )
>              46073      iTLB-load-misses                                              ( +-  0.05% )
>              19681      faults                                                        ( +-  0.01% )
> 
>        5.381342641 seconds time elapsed                                          ( +-  0.07% )
> 
> The proper work of the huge stack expansion was tested with the
> following app:
> 
> int main(int argc, char **argv)
> {
> 	char buf[1024 * 1025];
> 
> 	sprintf(buf, "Hello world !\n");
> 	printf(buf);
> 
> 	exit(0);
> }
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>

Reviewed-by: Nicholas Piggin <npiggin@...il.com>

Thanks,
Nick