linux-kernel - Re: [PATCH] Inhibit NMI watchdog when Alt-SysRq-T operation is underway.

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Message-Id: <20070713164504.66b72e74.akpm@linux-foundation.org>
Date:	Fri, 13 Jul 2007 16:45:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	darnok@....org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Inhibit NMI watchdog when Alt-SysRq-T operation is
 underway.

On Mon, 9 Jul 2007 11:53:02 -0400
darnok@....org wrote:

>  static void print_trace_address(void *data, unsigned long addr)
>  {
> +	static int i = 0;		
> +	if (i && ((i % 8) == 0)) 
> +		touch_nmi_watchdog();
> +	i++;
>  	printk_address(addr);
>  }

I doubt if the "% 8" thing is really needed?  printk_address() is pretty
slow and touch_nmi_watchdog is _reasonably_ fast.  It could be made heaps
faster by:

From: Andrew Morton <akpm@...ux-foundation.org>

Avoid dirtying remote cpu's memory if it already has the correct value.

Cc: Andi Kleen <ak@...e.de>
Cc: Konrad Rzeszutek <konrad@...nok.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 arch/i386/kernel/nmi.c |    8 +++++---
 x86_64/kernel/nmi.c    |    0 
 2 files changed, 5 insertions(+), 3 deletions(-)

diff -puN arch/i386/kernel/nmi.c~i386-speedup-touch_nmi_watchdog arch/i386/kernel/nmi.c
--- a/arch/i386/kernel/nmi.c~i386-speedup-touch_nmi_watchdog
+++ a/arch/i386/kernel/nmi.c
@@ -298,7 +298,7 @@ static unsigned int
 	last_irq_sums [NR_CPUS],
 	alert_counter [NR_CPUS];

-void touch_nmi_watchdog (void)
+void touch_nmi_watchdog(void)
 {
 	if (nmi_watchdog > 0) {
 		unsigned cpu;
@@ -307,8 +307,10 @@ void touch_nmi_watchdog (void)
 		 * Just reset the alert counters, (other CPUs might be
 		 * spinning on locks we hold):
 		 */
-		for_each_present_cpu (cpu)
-			alert_counter[cpu] = 0;
+		for_each_present_cpu(cpu) {
+			if (alert_counter[cpu])
+				alert_counter[cpu] = 0;
+		}
 	}

 	/*

So I'd be inclined to simplify your patch to a bare

From: Konrad Rzeszutek <konrad@...nok.org>

On large memory configuration with not so fast CPUs the NMI watchdog is
triggered when memory addresses are being gathered and printed.  The code
paths for Alt-SysRq-t are sprinkled with touch_nmi_watchdog in various
places but not in this routine (or in the loop that utilizes this
function).  The patch has been tested for regression on large CPU+memory
configuration (128 logical CPUs + 224 GB) and 1,2,4,16-CPU sockets with
various memory sizes (1,2,4,6,20).

Cc: Andi Kleen <ak@...e.de>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 arch/x86_64/kernel/traps.c |    1 +
 1 files changed, 1 insertion(+)

diff -puN arch/x86_64/kernel/traps.c~inhibit-nmi-watchdog-when-alt-sysrq-t-operation-is-underway arch/x86_64/kernel/traps.c
--- a/arch/x86_64/kernel/traps.c~inhibit-nmi-watchdog-when-alt-sysrq-t-operation-is-underway
+++ a/arch/x86_64/kernel/traps.c
@@ -397,6 +397,7 @@ static int print_trace_stack(void *data,

 static void print_trace_address(void *data, unsigned long addr)
 {
+	touch_nmi_watchdog();
 	printk_address(addr);
 }

_

OK?
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