netdev - Re: [pchecks v1 2/4] Use raw cpu ops for calls that would trigger with checks

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Date:	Tue, 24 Sep 2013 09:32:50 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Tejun Heo <tj@...nel.org>, akpm@...uxfoundation.org,
	Steven Rostedt <srostedt@...hat.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>, netdev@...r.kernel.org
Subject: Re: [pchecks v1 2/4] Use raw cpu ops for calls that would trigger
 with checks


(netdev Cc:-ed)

* Christoph Lameter <cl@...ux.com> wrote:

> These location triggered during testing with KVM.
> 
> These are fetches without preemption off where we judged that
> to be more performance efficient or where other means of
> providing synchronization (BH handling) are available.

> Index: linux/include/net/snmp.h
> ===================================================================
> --- linux.orig/include/net/snmp.h	2013-09-12 13:26:29.216103951 -0500
> +++ linux/include/net/snmp.h	2013-09-12 13:26:29.208104037 -0500
> @@ -126,7 +126,7 @@ struct linux_xfrm_mib {
>  	extern __typeof__(type) __percpu *name[SNMP_ARRAY_SZ]
>  
>  #define SNMP_INC_STATS_BH(mib, field)	\
> -			__this_cpu_inc(mib[0]->mibs[field])
> +			raw_cpu_inc(mib[0]->mibs[field])
>  
>  #define SNMP_INC_STATS_USER(mib, field)	\
>  			this_cpu_inc(mib[0]->mibs[field])
> @@ -141,7 +141,7 @@ struct linux_xfrm_mib {
>  			this_cpu_dec(mib[0]->mibs[field])
>  
>  #define SNMP_ADD_STATS_BH(mib, field, addend)	\
> -			__this_cpu_add(mib[0]->mibs[field], addend)
> +			raw_cpu_add(mib[0]->mibs[field], addend)

Are the networking folks fine with allowing unafe operations of SNMP stats 
in preemptible sections, or should the kernel produce an optional warning 
message if CONFIG_PREEMPT_DEBUG=y and these ops are used in preemptible 
(non-bh, non-irq-handler, non-irqs-off, etc.) sections?

RAW_SNMP_*_STATS() ops could be used to annotate those places where that 
kind of usage is safe.

Thanks,

	Ingo
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