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Date:   Thu, 10 Oct 2019 11:46:37 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Li RongQing <lirongqing@baidu.com>
Cc:     mingo@redhat.com, acme@kernel.org, mark.rutland@arm.com,
        alexander.shishkin@linux.intel.com, jolsa@redhat.com,
        namhyung@kernel.org, tglx@linutronix.de, bp@alien8.de,
        hpa@zytor.com, x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH][RFC] perf/x86: avoid false-positives hard lockup
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On Thu, Oct 10, 2019 at 04:41:38PM +0800, Li RongQing wrote:
> if perf counter is used as nmi watchdog, and twice nmi in soft
> watchdog sample period will trigger hard lockup
> 
> make sure left time is not less than soft watchdog period by
> compared with 3/5 period to skip forward, since soft watchdog
> sample period is 2/5 of watchdog_thresh, nmi watchdog sample
> period, computed by set_sample_period
> 
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
>  arch/x86/events/core.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 7b21455d7504..1f5309456d4c 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -1196,7 +1196,11 @@ int x86_perf_event_set_period(struct perf_event *event)
>  	/*
>  	 * If we are way outside a reasonable range then just skip forward:
>  	 */
> +#ifdef CONFIG_HARDLOCKUP_DETECTOR_PERF
> +	if (unlikely(left <= -(period * 3 / 5))) {
> +#else
>  	if (unlikely(left <= -period)) {
> +#endif

NAK. This is 100% the wrong place to do anything like that.