linux-kernel - [PATCH AUTOSEL for 3.18 28/63] powerpc: Avoid taking a data miss on every userspace instruction miss

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Date:   Sat, 3 Mar 2018 22:33:26 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     Anton Blanchard <anton@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 3.18 28/63] powerpc: Avoid taking a data miss on
 every userspace instruction miss

From: Anton Blanchard <anton@...ba.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@...ba.org>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.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 f06b56baf0b3..9b47840be82e 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -294,7 +294,7 @@ int __kprobes 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