linux-kernel - [PATCH 2/3] drivers/net/b44: Align pwol_mask to unsigned long for better performance

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Date:   Mon, 16 Sep 2019 15:39:57 -0700
From:   Tony Luck <tony.luck@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "H Peter Anvin" <hpa@...or.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Radim Krcmar" <rkrcmar@...hat.com>,
        "Sai Praneeth Prakhya" <sai.praneeth.prakhya@...el.com>,
        "Ravi V Shankar" <ravi.v.shankar@...el.com>,
        "linux-kernel" <linux-kernel@...r.kernel.org>,
        "x86" <x86@...nel.org>
Subject: [PATCH 2/3] drivers/net/b44: Align pwol_mask to unsigned long for better performance

From: Peter Zijlstra <peterz@...radead.org>

A bit in pwol_mask is set in b44_magic_pattern() by atomic set_bit().
But since pwol_mask is local and never exposed to concurrency, there is
no need to set bit in pwol_mask atomically.

set_bit() sets the bit in a single unsigned long location. Because
pwol_mask may not be aligned to unsigned long, the location may cross two
cache lines. On x86, accessing two cache lines in locked instruction in
set_bit() is called split locked access and can cause overall performance
degradation.

So use non atomic __set_bit() to set pwol_mask bits. __set_bit() won't hit
split lock issue on x86.

Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
Signed-off-by: Tony Luck <tony.luck@...el.com>
---
 drivers/net/ethernet/broadcom/b44.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/broadcom/b44.c b/drivers/net/ethernet/broadcom/b44.c
index 97ab0dd25552..5738ab963dfb 100644
--- a/drivers/net/ethernet/broadcom/b44.c
+++ b/drivers/net/ethernet/broadcom/b44.c
@@ -1520,7 +1520,7 @@ static int b44_magic_pattern(u8 *macaddr, u8 *ppattern, u8 *pmask, int offset)
 
 	memset(ppattern + offset, 0xff, magicsync);
 	for (j = 0; j < magicsync; j++)
-		set_bit(len++, (unsigned long *) pmask);
+		__set_bit(len++, (unsigned long *)pmask);
 
 	for (j = 0; j < B44_MAX_PATTERNS; j++) {
 		if ((B44_PATTERN_SIZE - len) >= ETH_ALEN)
@@ -1532,7 +1532,7 @@ static int b44_magic_pattern(u8 *macaddr, u8 *ppattern, u8 *pmask, int offset)
 		for (k = 0; k< ethaddr_bytes; k++) {
 			ppattern[offset + magicsync +
 				(j * ETH_ALEN) + k] = macaddr[k];
-			set_bit(len++, (unsigned long *) pmask);
+			__set_bit(len++, (unsigned long *)pmask);
 		}
 	}
 	return len - 1;
-- 
2.20.1