netdev - Re: [RFC] csum experts, csum_replace2() is too expensive

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Date:	Fri, 21 Mar 2014 06:32:58 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Patrick McHardy <kaber@...sh.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"H.K. Jerry Chu" <hkchu@...gle.com>,
	Michael Dalton <mwdalton@...gle.com>,
	netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] csum experts, csum_replace2() is too expensive

On Fri, 2014-03-21 at 05:50 -0700, Eric Dumazet wrote:

> Or the fact that we mix 16 bit stores and 32bit loads ?
> 
> iph->tot_len = newlen;
> iph->check = 0;
> iph->check = ip_fast_csum(iph, 5);

Yep definitely. Using 16 bit loads in ip_fast_csum() totally removes the
stall. I no longer see inet_gro_complete() in perf top...

+       if (__builtin_constant_p(ihl) && ihl == 5) {
+               asm("  movw (%[iph]), %w[sum]\n"        /* ihl/version/tos */
+                   "  addw 2(%[iph]), %w[sum]\n"       /* tot_len      */
+                   "  adcw 8(%[iph]), %w[sum]\n"       /* ttl/protocol */
+                   "  adcw 10(%[iph]), %w[sum]\n"      /* check        */
+                   "  adcl 4(%[iph]), %[sum]\n"        /* id/frag_off  */
+                   "  adcl 12(%[iph]), %[sum]\n"       /* saddr        */
+                   "  adcl 16(%[iph]), %[sum]\n"       /* daddr        */
+                   "  adcl $0, %[sum]\n"
+                   : [sum] "=r" (sum)
+                   : [iph] "r" (iph)
+                   );
+               return csum_fold(sum);


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