Re: [PATCH v4 2/2] powerpc/hv-24x7: Add sysfs files inside hv-24x7 device to show cpumask

[Madhavan Srinivasan <maddy@linux.ibm.com>]

On 7/8/20 2:29 PM, Kajol Jain wrote:
> Patch here adds a cpumask attr to hv_24x7 pmu along with ABI documentation.
>
> Primary use to expose the cpumask is for the perf tool which has the
> capability to parse the driver sysfs folder and understand the
> cpumask file. Having cpumask file will reduce the number of perf command
> line parameters (will avoid "-C" option in the perf tool
> command line). It can also notify the user which is
> the current cpu used to retrieve the counter data.
>
> command:# cat /sys/devices/hv_24x7/cpumask
> 0
>
> Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
> Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> ---
>   .../sysfs-bus-event_source-devices-hv_24x7    |  7 ++++
>   arch/powerpc/perf/hv-24x7.c                   | 33 ++++++++++++++++++-
>   2 files changed, 39 insertions(+), 1 deletion(-)
>
> 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..ee89d0e94602 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 retrieve 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 93b4700dcf8c..3f769bb2d06a 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,
>   };
>
> @@ -1684,7 +1706,7 @@ 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;
>
> @@ -1731,6 +1753,15 @@ static int hv_24x7_init(void)
>   	if (r)
>   		return r;
>
> +	/*
> +	 * 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;
> +

We can avoid this complex stuff right.
Now that if the cpuhotplug init fail, we fail the pmu
registration right, with that, we dont need this dance.

Cant we just add the cpumask_attr_group right next to if_group
in "attr_group"?

Maddy

>   	r = perf_pmu_register(&h_24x7_pmu, h_24x7_pmu.name, -1);
>   	if (r)
>   		return r;