Re: [PATCH v2 2/2] powerpc/hv-24x7: Add sysfs files inside hv-24x7 device to show cpumask

[Gautham R Shenoy <ego@linux.vnet.ibm.com>]

Hi Kajol,

On Wed, Jun 24, 2020 at 03:47:54PM +0530, Kajol Jain wrote:
> Patch here adds a cpumask attr to hv_24x7 pmu along with ABI documentation.
> 
> command:# cat /sys/devices/hv_24x7/cpumask
> 0

Since this sysfs interface is read-only, and the user cannot change
the CPU which will be making the HCALLs to obtain the 24x7 counts,
does the user even need to know if currently CPU X is the one which is
going to make HCALLs to retrive the 24x7 counts ? Does it help in any
kind of trouble-shooting ?

It would have made sense if the interface was read-write, since a user
can set this to a CPU which is not running user applications. This
would help in minimising jitter on those active CPUs running the user
applications.


> 
> Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
> ---
>  .../sysfs-bus-event_source-devices-hv_24x7    |  7 ++++
>  arch/powerpc/perf/hv-24x7.c                   | 36 +++++++++++++++++--
>  2 files changed, 41 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-bus-event_source-devices-hv_24x7 b/Documentation/ABI/testing/sysfs-bus-event_source-devices-hv_24x7
> index e8698afcd952..f9dd3755b049 100644
> --- a/Documentation/ABI/testing/sysfs-bus-event_source-devices-hv_24x7
> +++ b/Documentation/ABI/testing/sysfs-bus-event_source-devices-hv_24x7
> @@ -43,6 +43,13 @@ Description:	read only
>  		This sysfs interface exposes the number of cores per chip
>  		present in the system.
> 
> +What:		/sys/devices/hv_24x7/cpumask
> +Date:		June 2020
> +Contact:	Linux on PowerPC Developer List <linuxppc-dev@lists.ozlabs.org>
> +Description:	read only
> +		This sysfs file exposes the cpumask which is designated to make
> +		HCALLs to retrive hv-24x7 pmu event counter data.
> +
>  What:		/sys/bus/event_source/devices/hv_24x7/event_descs/<event-name>
>  Date:		February 2014
>  Contact:	Linux on PowerPC Developer List <linuxppc-dev@lists.ozlabs.org>
> diff --git a/arch/powerpc/perf/hv-24x7.c b/arch/powerpc/perf/hv-24x7.c
> index ce4739e2b407..3c699612d29f 100644
> --- a/arch/powerpc/perf/hv-24x7.c
> +++ b/arch/powerpc/perf/hv-24x7.c
> @@ -448,6 +448,12 @@ static ssize_t device_show_string(struct device *dev,
>  	return sprintf(buf, "%s\n", (char *)d->var);
>  }
> 
> +static ssize_t cpumask_get_attr(struct device *dev,
> +				struct device_attribute *attr, char *buf)
> +{
> +	return cpumap_print_to_pagebuf(true, buf, &hv_24x7_cpumask);
> +}
> +
>  static ssize_t sockets_show(struct device *dev,
>  			    struct device_attribute *attr, char *buf)
>  {
> @@ -1116,6 +1122,17 @@ static DEVICE_ATTR_RO(sockets);
>  static DEVICE_ATTR_RO(chipspersocket);
>  static DEVICE_ATTR_RO(coresperchip);
> 
> +static DEVICE_ATTR(cpumask, S_IRUGO, cpumask_get_attr, NULL);
> +
> +static struct attribute *cpumask_attrs[] = {
> +	&dev_attr_cpumask.attr,
> +	NULL,
> +};
> +
> +static struct attribute_group cpumask_attr_group = {
> +	.attrs = cpumask_attrs,
> +};
> +
>  static struct bin_attribute *if_bin_attrs[] = {
>  	&bin_attr_catalog,
>  	NULL,
> @@ -1143,6 +1160,11 @@ static const struct attribute_group *attr_groups[] = {
>  	&event_desc_group,
>  	&event_long_desc_group,
>  	&if_group,
> +	/*
> +	 * This NULL is a placeholder for the cpumask attr which will update
> +	 * onlyif cpuhotplug registration is successful
> +	 */
> +	NULL,
>  	NULL,
>  };
> 
> @@ -1683,7 +1705,7 @@ static int hv_24x7_cpu_hotplug_init(void)
> 
>  static int hv_24x7_init(void)
>  {
> -	int r;
> +	int r, i = -1;
>  	unsigned long hret;
>  	struct hv_perf_caps caps;
> 
> @@ -1727,8 +1749,18 @@ static int hv_24x7_init(void)
> 
>  	/* init cpuhotplug */
>  	r = hv_24x7_cpu_hotplug_init();
> -	if (r)
> +	if (r) {
>  		pr_err("hv_24x7: CPU hotplug init failed\n");
> +	} else {
> +		/*
> +		 * Cpu hotplug init is successful, add the
> +		 * cpumask file as part of pmu attr group and
> +		 * assign it to very first NULL location.
> +		 */
> +		while (attr_groups[++i])
> +			/* nothing */;
> +		attr_groups[i] = &cpumask_attr_group;
> +	}
> 
>  	r = perf_pmu_register(&h_24x7_pmu, h_24x7_pmu.name, -1);
>  	if (r)
> -- 
> 2.18.2
>