linux-kernel - Re: Poll about irqsafe_cpu

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Message-ID: <1300380801.6315.306.camel@edumazet-laptop>
Date:	Thu, 17 Mar 2011 17:53:21 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: Poll about irqsafe_cpu_add and others

Le jeudi 17 mars 2011 à 10:18 -0500, Christoph Lameter a écrit :
> On Thu, 17 Mar 2011, David Miller wrote:
> 
> >
> > I had been meaning to bring this up from another perspective.
> >
> > In networking, we often only ever access objects in base or
> > BH context.  Therefore in BH context cases we can do just
> > normal counter bumps without any of the special atomic or
> > IRQ disabling code at all.
> 
> We have the __ functions for that purpose. __this_cpu_inc f.e. falls back
> to a simply ++ operation if the arch cannot provide something better.
> irqsafe_xx are only used if the context does not provide any protection
> and if there is the potential of the counter being incremented from an
> interrupt context.
> 

What David and I have in mind is to use one array per mib instead of
two. This is an old idea.

https://patchwork.kernel.org/patch/15883/

When we know we run from BH context, we can use __this_cpu_inc(), but if
we dont know or run from user/process context, we would need irqsafe_inc
variant.

For x86 this maps to same single instruction, but for other arches, this
might be too expensive.

BTW, I think following patch is possible to save some text and useless
tests (on 64bit platform at least)

size vmlinux.old vmlinux

Thanks

[PATCH] snmp: SNMP_UPD_PO_STATS_BH() always called from softirq

We dont need to test if we run from softirq context.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
Cc: Christoph Lameter <cl@...ux-foundation.org>
---
 include/net/snmp.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/net/snmp.h b/include/net/snmp.h
index 762e2ab..be2424d 100644
--- a/include/net/snmp.h
+++ b/include/net/snmp.h
@@ -149,8 +149,8 @@ struct linux_xfrm_mib {
 	} while (0)
 #define SNMP_UPD_PO_STATS_BH(mib, basefield, addend)	\
 	do { \
-		__typeof__(*mib[0]) *ptr = \
-			__this_cpu_ptr((mib)[!in_softirq()]); \
+		__typeof__(*mib[0]) *ptr = __this_cpu_ptr((mib)[0]);	\
+									\
 		ptr->mibs[basefield##PKTS]++; \
 		ptr->mibs[basefield##OCTETS] += addend;\
 	} while (0)


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