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

David Laight
2019-06-24T15:12:15Z

From: Fenghua Yu
> Sent: 18 June 2019 23:41
> From: Peter Zijlstra <peterz@infradead.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@infradead.org>
> Signed-off-by: Fenghua Yu <fenghua.yu@intel.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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