From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v4 02/17] drivers/net/b44: Align pwol_mask to unsigned long for better performance Date: Mon, 4 Mar 2019 11:00:22 +0100 Message-ID: <20190304100022.GA32477@hirez.programming.kicks-ass.net> References: <1551494711-213533-1-git-send-email-fenghua.yu@intel.com> <1551494711-213533-3-git-send-email-fenghua.yu@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , H Peter Anvin , Paolo Bonzini , Dave Hansen , Ashok Raj , Ravi V Shankar , Xiaoyao Li , linux-kernel , x86 , kvm@vger.kernel.org To: Fenghua Yu Return-path: Content-Disposition: inline In-Reply-To: <1551494711-213533-3-git-send-email-fenghua.yu@intel.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Fri, Mar 01, 2019 at 06:44:56PM -0800, Fenghua Yu wrote: > A bit in pwol_mask is set in b44_magic_pattern automatically by set_bit. > set_bit sets the bit in a single unsigned long location. Since pwol_mask > may not be aligned to unsigned long, the location may cross two cache > lines and accessing the location degradates performance. On x86, accessing > two cache lines in locked instruction in set_bit is called split lock and > can cause overall performance degradation. > > To avoid to impact performance by accessing two cache lines in set_bit, > align pwol_mask to unsigned long. > > Signed-off-by: Fenghua Yu > --- > drivers/net/ethernet/broadcom/b44.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/net/ethernet/broadcom/b44.c b/drivers/net/ethernet/broadcom/b44.c > index 97ab0dd25552..bc544b6b9c3a 100644 > --- a/drivers/net/ethernet/broadcom/b44.c > +++ b/drivers/net/ethernet/broadcom/b44.c > @@ -1547,7 +1547,8 @@ static void b44_setup_pseudo_magicp(struct b44 *bp) > u32 val; > int plen0, plen1, plen2; > u8 *pwol_pattern; > - u8 pwol_mask[B44_PMASK_SIZE]; > + /* Align to unsigned long for better performance in set_bit() */ > + u8 pwol_mask[B44_PMASK_SIZE] __aligned(sizeof(unsigned long)); > > pwol_pattern = kzalloc(B44_PATTERN_SIZE, GFP_KERNEL); > if (!pwol_pattern) That is truly horrid code. But afaict pwol_mask is local and never exposed to concurrency, so _why_ does it need atomic bitset in the first place? Would not the below be a _much_ better solution? diff --git a/drivers/net/ethernet/broadcom/b44.c b/drivers/net/ethernet/broadcom/b44.c index 97ab0dd25552..0b4226b406b1 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;