linux-kernel - Re: [PATCH 2/5] kernel/watchdog_hld: clarify the condition in hardlockup_detector_event

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Date:   Wed, 15 Sep 2021 15:45:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wang Qing <wangqing@...o.com>,
        Santosh Sivaraj <santosh@...six.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 2/5] kernel/watchdog_hld: clarify the condition in
 hardlockup_detector_event_create()

On Wed, Sep 15, 2021 at 11:51:00AM +0800, Pingfan Liu wrote:
> hardlockup_detector_event_create() indirectly calls
> kmem_cache_alloc_node(), which is blockable.
> 
> So here, the really planned context is is_percpu_thread().
> 
> Signed-off-by: Pingfan Liu <kernelfans@...il.com>
> Cc: Petr Mladek <pmladek@...e.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Wang Qing <wangqing@...o.com>
> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> Cc: Santosh Sivaraj <santosh@...six.org>
> Cc: Sumit Garg <sumit.garg@...aro.org>
> Cc: Will Deacon <will@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> To: linux-kernel@...r.kernel.org
> ---
>  kernel/watchdog_hld.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
> index 247bf0b1582c..6876e796dbf5 100644
> --- a/kernel/watchdog_hld.c
> +++ b/kernel/watchdog_hld.c
> @@ -165,10 +165,13 @@ static void watchdog_overflow_callback(struct perf_event *event,
>  
>  static int hardlockup_detector_event_create(void)
>  {
> -	unsigned int cpu = smp_processor_id();
> +	unsigned int cpu;
>  	struct perf_event_attr *wd_attr;
>  	struct perf_event *evt;
>  
> +	/* This function plans to execute in cpu bound kthread */
> +	BUG_ON(!is_percpu_thread());
> +	cpu = raw_smp_processor_id();
>  	wd_attr = &wd_hw_attr;
>  	wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);

This patch makes no sense.