Re: [PATCH] powerpc: Avoid taking a data miss on every userspace instruction miss

[LEROY Christophe]

Anton Blanchard <an...@ozlabs.org> a écrit :

From: Anton Blanchard <an...@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 <an...@samba.org>
---
 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 fd6484fc2fa9..3a7d580fdc59 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c

@@ -287,7 +287,7 @@ 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))

Shouldn't it also check 'is_write' ?
If it is a store, is_write should be set, shouldn't it ?

Christophe

                store_update_sp = store_updates_sp(regs);

        if (user_mode(regs))
--
2.11.0