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To: Anton Blanchard <anton@ozlabs.org>, benh@kernel.crashing.org,
 paulus@samba.org
From: Michael Ellerman <patch-notifications@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Subject: Re: powerpc: Avoid taking a data miss on every userspace instruction
 miss
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Date: Thu,  6 Apr 2017 23:06:03 +1000 (AEST)
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On Mon, 2017-04-03 at 06:41:02 UTC, Anton Blanchard wrote:
> From: Anton Blanchard <anton@samba.org>
> 
> Early on in do_page_fault() we call store_updates_sp(), regardless of
> the type of exception. For an instruction miss this doesn't make
> sense, because we only use this information to detect if a data miss
> is the result of a stack expansion instruction or not.
> 
> Worse still, it results in a data miss within every userspace
> instruction miss handler, because we try and load the very instruction
> we are about to install a pte for!
> 
> A simple exec microbenchmark runs 6% faster on POWER8 with this fix:
> 
>  #include <stdlib.h>
>  #include <stdio.h>
>  #include <unistd.h>
> 
> int main(int argc, char *argv[])
> {
> 	unsigned long left = atol(argv[1]);
> 	char leftstr[16];
> 
> 	if (left-- == 0)
> 		return 0;
> 
> 	sprintf(leftstr, "%ld", left);
> 	execlp(argv[0], argv[0], leftstr, NULL);
> 	perror("exec failed\n");
> 
> 	return 0;
> }
> 
> Pass the number of iterations on the command line (eg 10000) and time
> how long it takes to execute.
> 
> Signed-off-by: Anton Blanchard <anton@samba.org>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/a7a9dcd882a67b68568868b988289f

cheers