RE: [PATCH v2] watchdog: Prefer use "ref-cycles" for NMI watchdog

[Li, Xin3]

> NMI watchdog permanently consumes one hardware counters per CPU on the
> system. For systems that use many hardware counters, this causes more
> aggressive time multiplexing of perf events.
> 
> OTOH, some CPUs (mostly Intel) support "ref-cycles" event, which is rarely
> used. Try use "ref-cycles" for the watchdog, so that one more hardware
> counter is available to the user. If the CPU doesn't support "ref-cycles",
> fall back to "cycles".
> 
> The downside of this change is that users of "ref-cycles" need to disable
> nmi_watchdog.

>From the discussion in v1, the users don't have to disable the NMI watchdog
*permanently*, right?

> 
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Song Liu <song@xxxxxxxxxx>
> 
> ---
> 
> Changes in v2:
> 1. Do not send warning when failed to create ref-cycles event.
> ---
>  kernel/watchdog_hld.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
> index 247bf0b1582c..a1d2a43ea31f 100644
> --- a/kernel/watchdog_hld.c
> +++ b/kernel/watchdog_hld.c
> @@ -100,7 +100,7 @@ static inline bool watchdog_check_timestamp(void)
> 
>  static struct perf_event_attr wd_hw_attr = {
>  	.type		= PERF_TYPE_HARDWARE,
> -	.config		= PERF_COUNT_HW_CPU_CYCLES,
> +	.config		= PERF_COUNT_HW_REF_CPU_CYCLES,
>  	.size		= sizeof(struct perf_event_attr),
>  	.pinned		= 1,
>  	.disabled	= 1,
> @@ -163,7 +163,7 @@ static void watchdog_overflow_callback(struct perf_event
> *event,
>  	return;
>  }
> 
> -static int hardlockup_detector_event_create(void)
> +static int hardlockup_detector_event_create(bool send_warning)
>  {
>  	unsigned int cpu = smp_processor_id();
>  	struct perf_event_attr *wd_attr;
> @@ -176,8 +176,10 @@ static int hardlockup_detector_event_create(void)
>  	evt = perf_event_create_kernel_counter(wd_attr, cpu, NULL,
>  					       watchdog_overflow_callback, NULL);
>  	if (IS_ERR(evt)) {
> -		pr_debug("Perf event create on CPU %d failed with %ld\n", cpu,
> -			 PTR_ERR(evt));
> +		if (send_warning) {
> +			pr_debug("Perf event create on CPU %d failed with
> %ld\n", cpu,
> +				 PTR_ERR(evt));
> +		}
>  		return PTR_ERR(evt);
>  	}
>  	this_cpu_write(watchdog_ev, evt);
> @@ -189,7 +191,7 @@ static int hardlockup_detector_event_create(void)
>   */
>  void hardlockup_detector_perf_enable(void)
>  {
> -	if (hardlockup_detector_event_create())
> +	if (hardlockup_detector_event_create(true))
>  		return;
> 
>  	/* use original value for check */
> @@ -284,7 +286,13 @@ void __init hardlockup_detector_perf_restart(void)
>   */
>  int __init hardlockup_detector_perf_init(void)
>  {
> -	int ret = hardlockup_detector_event_create();
> +	int ret = hardlockup_detector_event_create(false);
> +
> +	if (ret) {
> +		/* Failed to create "ref-cycles", try "cycles" instead */
> +		wd_hw_attr.config = PERF_COUNT_HW_CPU_CYCLES;
> +		ret = hardlockup_detector_event_create(true);
> +	}
> 
>  	if (ret) {
>  		pr_info("Perf NMI watchdog permanently disabled\n");
> --
> 2.34.1