[PATCH AUTOSEL for 4.9 100/219] powerpc: Avoid taking a data miss on every userspace instruction miss

Sasha Levin
2018-03-03T23:49:00Z

From: Anton Blanchard <anton@samba.org>

[ Upstream commit a7a9dcd882a67b68568868b988289fce5ffd8419 ]

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>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
arch/powerpc/mm/fault.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index d0b137d96df1..9376e8e53bfa 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -294,7 +294,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
* can result in fault, which will cause a deadlock when called with
* mmap_sem held
*/
- if (user_mode(regs))
+ if (!is_exec && user_mode(regs))
store_update_sp = store_updates_sp(regs);

if (user_mode(regs))
--
2.14.1