linux-kernel - Re: [PATCH v8 6/7] x86/resctrl: Add write option to "mba_MBps

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Message-ID: <6a677a4b-7163-cc2d-a615-6b8c499eb281@intel.com>
Date: Fri, 1 Nov 2024 16:26:59 -0700
From: Fenghua Yu <fenghua.yu@...el.com>
To: Tony Luck <tony.luck@...el.com>, Reinette Chatre
	<reinette.chatre@...el.com>, Peter Newman <peternewman@...gle.com>, "Jonathan
 Corbet" <corbet@....net>, Shuah Khan <skhan@...uxfoundation.org>,
	<x86@...nel.org>
CC: James Morse <james.morse@....com>, Jamie Iles <quic_jiles@...cinc.com>,
	Babu Moger <babu.moger@....com>, Randy Dunlap <rdunlap@...radead.org>,
	"Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>,
	<linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH v8 6/7] x86/resctrl: Add write option to "mba_MBps_event"
 file

Hi, Tony,

On 10/29/24 10:28, Tony Luck wrote:
> A user can choose any of the memory bandwidth monitoring events
> listed in /sys/fs/resctrl/info/L3_mon/mon_features independently
> for each ctrl_mon group by writing to the "mba_MBps_event" file.
> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>   arch/x86/kernel/cpu/resctrl/internal.h    |  2 +
>   arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 46 +++++++++++++++++++++++
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c    |  1 +
>   3 files changed, 49 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index 5f3438ca9e2b..35483c6615b6 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -609,6 +609,8 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,
>   				char *buf, size_t nbytes, loff_t off);
>   int rdtgroup_schemata_show(struct kernfs_open_file *of,
>   			   struct seq_file *s, void *v);
> +ssize_t rdtgroup_mba_mbps_event_write(struct kernfs_open_file *of,
> +				      char *buf, size_t nbytes, loff_t off);
>   int rdtgroup_mba_mbps_event_show(struct kernfs_open_file *of,
>   				 struct seq_file *s, void *v);
>   bool rdtgroup_cbm_overlaps(struct resctrl_schema *s, struct rdt_ctrl_domain *d,
> diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> index b9ba419e5c88..fc5585dc688f 100644
> --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
> @@ -518,6 +518,52 @@ static int smp_mon_event_count(void *arg)
>   	return 0;
>   }
>   
> +ssize_t rdtgroup_mba_mbps_event_write(struct kernfs_open_file *of,
> +				      char *buf, size_t nbytes, loff_t off)
> +{
> +	struct rdtgroup *rdtgrp;
> +	int ret = 0;
> +
> +	/* Valid input requires a trailing newline */
> +	if (nbytes == 0 || buf[nbytes - 1] != '\n')
> +		return -EINVAL;
> +	buf[nbytes - 1] = '\0';
> +
> +	rdtgrp = rdtgroup_kn_lock_live(of->kn);
> +	if (!rdtgrp) {
> +		rdtgroup_kn_unlock(of->kn);
> +		return -ENOENT;
> +	}
> +	rdt_last_cmd_clear();
> +
> +	if (!strcmp(buf, "mbm_local_bytes")) {
> +		if (is_mbm_local_enabled())
> +			rdtgrp->mba_mbps_event = QOS_L3_MBM_LOCAL_EVENT_ID;
> +		else
> +			ret = -ENXIO;
> +	} else if (!strcmp(buf, "mbm_total_bytes")) {
> +		if (is_mbm_total_enabled())
> +			rdtgrp->mba_mbps_event = QOS_L3_MBM_TOTAL_EVENT_ID;


User may think each time toggling the local/total event will effect MBA. 
And they may create usage case like frequently changing the events to 
maintain/adjust both total and local within bw boundary.

But toggling mba_mbps_event faster than 1sec doesn't have any effect on 
MBA SC because MBA SC is called every one second.

Maybe need to add a ratelimit of 1 second on calling this function? And 
adding info in the document that toggling speed should be slower than 1 
second?

> +		else
> +			ret = -ENXIO;
> +	} else {
> +		ret = -EINVAL;
> +	}
> +
> +	switch (ret) {
> +	case -ENXIO:
> +		rdt_last_cmd_printf("Unsupported event id '%s'\n", buf);
> +		break;
> +	case -EINVAL:
> +		rdt_last_cmd_printf("Unknown event id '%s'\n", buf);
> +		break;
> +	}
> +
> +	rdtgroup_kn_unlock(of->kn);
> +
> +	return ret ?: nbytes;
> +}
> +
>   int rdtgroup_mba_mbps_event_show(struct kernfs_open_file *of,
>   				 struct seq_file *s, void *v)
>   {
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 3ba81963e981..6fa501ef187f 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -1947,6 +1947,7 @@ static struct rftype res_common_files[] = {
>   		.name		= "mba_MBps_event",
>   		.mode		= 0644,
>   		.kf_ops		= &rdtgroup_kf_single_ops,
> +		.write		= rdtgroup_mba_mbps_event_write,
>   		.seq_show	= rdtgroup_mba_mbps_event_show,
>   	},
>   	{

Thanks.

-Fenghua