Re: [PATCH v8 10/13] x86/resctrl: Add sysfs interface to write mbm_total_bytes_config

[James Morse <james.morse@arm.com>]

Hi Babu,

(Nit: all the 'sysfs' in the subjects should really be 'resctrl', but as they already have
'x86/resctrl', could you just remove the sysfs?
This patch would be "x86/resctrl: Add interface to write mbm_total_bytes_config")

On 04/11/2022 20:01, Babu Moger wrote:
> The current event configuration for mbm_total_bytes can be changed by
> the user by writing to the file
> /sys/fs/resctrl/info/L3_MON/mbm_total_bytes_config.
> 
> The event configuration settings are domain specific and will affect all
> the CPUs in the domain.
> 
> Following are the types of events supported:
> 
> ====  ===========================================================
> Bits   Description
> ====  ===========================================================
> 6      Dirty Victims from the QOS domain to all types of memory
> 5      Reads to slow memory in the non-local NUMA domain
> 4      Reads to slow memory in the local NUMA domain
> 3      Non-temporal writes to non-local NUMA domain
> 2      Non-temporal writes to local NUMA domain
> 1      Reads to memory in the non-local NUMA domain
> 0      Reads to memory in the local NUMA domain
> ====  ===========================================================
> 
> For example:
> To change the mbm_total_bytes to count only reads on domain 0, the bits
> 0, 1, 4 and 5 needs to be set, which is 110011b (in hex 0x33). Run the
> command.
> 	$echo  0=0x33 > /sys/fs/resctrl/info/L3_MON/mbm_total_bytes_config
> 
> To change the mbm_total_bytes to count all the slow memory reads on
> domain 1, the bits 4 and 5 needs to be set which is 110000b (in hex 0x30).
> Run the command.
> 	$echo  1=0x30 > /sys/fs/resctrl/info/L3_MON/mbm_total_bytes_config

> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 18f9588a41cf..0cdccb69386e 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -1505,6 +1505,133 @@ static int mbm_local_bytes_config_show(struct kernfs_open_file *of,
>  	return 0;
>  }
>  
> +static void mon_event_config_write(void *info)
> +{
> +	struct mon_config_info *mon_info = info;
> +	u32 index;
> +
> +	index = mon_event_config_index_get(mon_info->evtid);
> +	if (index >= MAX_CONFIG_EVENTS) {
> +		pr_warn_once("Invalid event id %d\n", mon_info->evtid);
> +		return;
> +	}
> +	wrmsr(MSR_IA32_EVT_CFG_BASE + index, mon_info->mon_config, 0);
> +}
> +
> +static int mbm_config_write(struct rdt_resource *r, struct rdt_domain *d,
> +			    u32 evtid, u32 val)
> +{
> +	struct mon_config_info mon_info = {0};
> +	int ret = 0;
> +
> +	rdt_last_cmd_clear();
> +
> +	/* mon_config cannot be more than the supported set of events */
> +	if (val > MAX_EVT_CONFIG_BITS) {
> +		rdt_last_cmd_puts("Invalid event configuration\n");
> +		return -EINVAL;
> +	}
> +
> +	/*
> +	 * Read the current config value first. If both are same then
> +	 * we don't need to write it again.
> +	 */
> +	mon_info.evtid = evtid;

> +	mondata_config_read(d, &mon_info);

This reads the MSR on this CPU, which gets the result for this domain...


> +	if (mon_info.mon_config == val)
> +		goto write_exit;
> +
> +	mon_info.mon_config = val;
> +
> +	/*
> +	 * Update MSR_IA32_EVT_CFG_BASE MSRs on all the CPUs in the
> +	 * domain. The MSRs offset from MSR MSR_IA32_EVT_CFG_BASE
> +	 * are scoped at the domain level. Writing any of these MSRs
> +	 * on one CPU is supposed to be observed by all CPUs in the
> +	 * domain. However, the hardware team recommends to update
> +	 * these MSRs on all the CPUs in the domain.
> +	 */

> +	on_each_cpu_mask(&d->cpu_mask, mon_event_config_write, &mon_info, 1);

... but here you IPI all the CPUs in the target domain to update them.

This means you unnecessarily IPI the CPUs in the target domain if they already had this
value, but the write syscall occurred on a domain that differs. This isn't what you
intended, but its benign.
More of a problem is: Won't this get skipped if the write syscall occurs on a domain that
happens to have the target configuration already?

Because you need the same value to be written on every CPU ... what happens to CPUs that
are offline when the configuration is changed? Do they keep their previous value, or does
it get reset?


I think this is best solved with a percpu variable for the current value of the MSR. You
can then read it for CPUs in a remote domain, and only issue IPIs to 'sync' the value if
needed. You can then re-use the sync call in resctrl_online_cpu() to set the MSR to
whatever value it should currently be.


> +
> +	/*
> +	 * When an Event Configuration is changed, the bandwidth counters
> +	 * for all RMIDs and Events will be cleared by the hardware. The
> +	 * hardware also sets MSR_IA32_QM_CTR.Unavailable (bit 62) for
> +	 * every RMID on the next read to any event for every RMID.
> +	 * Subsequent reads will have MSR_IA32_QM_CTR.Unavailable (bit 62)
> +	 * cleared while it is tracked by the hardware. Clear the
> +	 * mbm_local and mbm_total counts for all the RMIDs.
> +	 */
> +	memset(d->mbm_local, 0, sizeof(struct mbm_state) * r->num_rmid);
> +	memset(d->mbm_total, 0, sizeof(struct mbm_state) * r->num_rmid);
> +
> +write_exit:
> +	return ret;
> +}


> +static int mon_config_parse(struct rdt_resource *r, char *tok, u32 evtid)
> +{
> +	char *dom_str = NULL, *id_str;
> +	unsigned long dom_id, val;
> +	struct rdt_domain *d;
> +	int ret = 0;
> +
> +next:
> +	if (!tok || tok[0] == '\0')
> +		return 0;
> +
> +	/* Start processing the strings for each domain */
> +	dom_str = strim(strsep(&tok, ";"));
> +	id_str = strsep(&dom_str, "=");
> +
> +	if (!dom_str || kstrtoul(id_str, 10, &dom_id)) {
> +		rdt_last_cmd_puts("Missing '=' or non-numeric domain id\n");
> +		return -EINVAL;
> +	}
> +
> +	if (!dom_str || kstrtoul(dom_str, 16, &val)) {
> +		rdt_last_cmd_puts("Missing '=' or non-numeric event configuration value\n");
> +		return -EINVAL;
> +	}

This is parsing the same format strings as parse_line(). Is there any chance that code
could be re-used instead of duplicated? This way anything that is added to the format (or
bugs found!) only need supporting in once place.



> +	list_for_each_entry(d, &r->domains, list) {
> +		if (d->id == dom_id) {
> +			ret = mbm_config_write(r, d, evtid, val);
> +			if (ret)
> +				return -EINVAL;
> +			goto next;
> +		}
> +	}
> +
> +	return -EINVAL;
> +}


Thanks,

James