linux-kernel - RE: [PATCH v9 02/17] drivers/net/b44: Align pwol_mask to unsigned long for better performance

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Date:   Mon, 24 Jun 2019 15:12:05 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Fenghua Yu' <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Christopherson Sean J <sean.j.christopherson@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Xiaoyao Li " <xiaoyao.li@...el.com>,
        "Sai Praneeth Prakhya" <sai.praneeth.prakhya@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>
CC:     linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [PATCH v9 02/17] drivers/net/b44: Align pwol_mask to unsigned
 long for better performance

From: Fenghua Yu
> Sent: 18 June 2019 23:41
> 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>
> ---
>  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);

Is this code expected to do anything sensible on BE systems?
Casting the bitmask[] argument to any of the set_bit() functions is dubious at best.

	David

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