linux-kernel - RE: [PATCH] x86/nmi: Fix nmi_handle duration miscalculation

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Date:   Thu, 20 Aug 2020 02:31:45 +0000
From:   "Zhou, Libing (NSB - CN/Hangzhou)" <libing.zhou@...ia-sbell.com>
To:     "peterz@...radead.org" <peterz@...radead.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "bp@...e.de" <bp@...e.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hpa@...or.com" <hpa@...or.com>
Subject: RE: [PATCH] x86/nmi: Fix nmi_handle duration miscalculation

Thanks for your comments, I will recommit patch soon.

-----Original Message-----
From: peterz@...radead.org <peterz@...radead.org> 
Sent: 2020年8月19日 16:07
To: Zhou, Libing (NSB - CN/Hangzhou) <libing.zhou@...ia-sbell.com>
Cc: tglx@...utronix.de; mingo@...hat.com; bp@...en8.de; bp@...e.de; x86@...nel.org; linux-kernel@...r.kernel.org; hpa@...or.com
Subject: Re: [PATCH] x86/nmi: Fix nmi_handle duration miscalculation

On Mon, Aug 17, 2020 at 05:04:41PM +0800, Libing Zhou wrote:
> In nmi_check_duration(), the 'whole_msecs' value should get from 
> 'duration' to reflect actual time duration, but not 
> 'action->max_duration'.

Fixes: 248ed51048c4 ("x86/nmi: Remove irq_work from the long duration NMI handler")

> Signed-off-by: Libing Zhou <libing.zhou@...ia-sbell.com>
> ---
>  arch/x86/kernel/nmi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c index 
> 4fc9954a9560..c51ee659e520 100644
> --- a/arch/x86/kernel/nmi.c
> +++ b/arch/x86/kernel/nmi.c
> @@ -102,7 +102,7 @@ fs_initcall(nmi_warning_debugfs);
>  
>  static void nmi_check_duration(struct nmiaction *action, u64 
> duration)  {
> -	u64 whole_msecs = READ_ONCE(action->max_duration);
> +	u64 whole_msecs = duration;
>  	int remainder_ns, decimal_msecs;
>  
>  	if (duration < nmi_longest_ns || duration < action->max_duration)

The, IMO, saner solution is:

---
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c index 4fc9954a9560..47381666d6a5 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -102,7 +102,6 @@ fs_initcall(nmi_warning_debugfs);
 
 static void nmi_check_duration(struct nmiaction *action, u64 duration)  {
-	u64 whole_msecs = READ_ONCE(action->max_duration);
 	int remainder_ns, decimal_msecs;
 
 	if (duration < nmi_longest_ns || duration < action->max_duration) @@ -110,12 +109,12 @@ static void nmi_check_duration(struct nmiaction *action, u64 duration)
 
 	action->max_duration = duration;
 
-	remainder_ns = do_div(whole_msecs, (1000 * 1000));
+	remainder_ns = do_div(duration, (1000 * 1000));
 	decimal_msecs = remainder_ns / 1000;
 
 	printk_ratelimited(KERN_INFO
 		"INFO: NMI handler (%ps) took too long to run: %lld.%03d msecs\n",
-		action->handler, whole_msecs, decimal_msecs);
+		action->handler, duration, decimal_msecs);
 }
 
 static int nmi_handle(unsigned int type, struct pt_regs *regs)