Date: Wed, 15 May 2024 10:23:02 -0700
From: Tony Luck <tony.luck@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: "Yu, Fenghua" <fenghua.yu@...el.com>,
	"Wieczor-Retman, Maciej" <maciej.wieczor-retman@...el.com>,
	Peter Newman <peternewman@...gle.com>,
	James Morse <james.morse@....com>, Babu Moger <babu.moger@....com>,
	Drew Fustini <dfustini@...libre.com>,
	Dave Martin <Dave.Martin@....com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: Re: [PATCH v17 7/9] x86/resctrl: Add new monitor files for Sub-NUMA
 cluster (SNC) monitoring

On Wed, May 15, 2024 at 09:47:28AM -0700, Reinette Chatre wrote:
> Hi Tony,
> 
> On 5/14/2024 2:53 PM, Tony Luck wrote:
> > On Tue, May 14, 2024 at 01:30:05PM -0700, Reinette Chatre wrote:
> >> Hi Tony,
> >>
> >> On 5/14/2024 11:26 AM, Luck, Tony wrote:
> >>>> On 5/13/2024 5:21 PM, Tony Luck wrote:
> >>>>> On Mon, May 13, 2024 at 11:53:17AM -0700, Reinette Chatre wrote:
> >>>>>> On 5/13/2024 10:05 AM, Tony Luck wrote:
> >>>>>>> On Fri, May 10, 2024 at 02:24:13PM -0700, Reinette Chatre wrote:
> >>>>>>> Thanks for the review. Detailed comments below. But overall I'm
> >>>>>>> going to split patch 7 into a bunch of smaller changes, each with
> >>>>>>> a better commit message.
> >>>>>>>
> >>>>>>>> On 5/3/2024 1:33 PM, Tony Luck wrote:
> >>>>>>>>
> >>>>>>>> (Could you please start the changelog with some context?)
> >>>>>>>>
> >>>>>>>>> Add a field to the rdt_resource structure to track whether monitoring
> >>>>>>>>> resources are tracked by hardware at a different scope (NODE) from
> >>>>>>>>> the legacy L3 scope.
> >>>>>>>>
> >>>>>>>> This seems to describe @mon_scope that was introduced in patch #3?
> >>>>>>>
> >>>>>>> Not really. Patch #3 made the change so that control an monitor
> >>>>>>> functions can have different scope. That's still needed as with SNC
> >>>>>>> enabled the underlying data collection is at the node level for
> >>>>>>> monitoring, while control stays at the L3 cache scope.
> >>>>>>>
> >>>>>>> This new field describes the legacy scope of monitoring, so that
> >>>>>>> resctrl can provide correctly scoped monitor files for legacy
> >>>>>>> applications that aren't aware of SNC. So I'm using this both
> >>>>>>> to indicate when SNC is enabled (with mon_scope != mon_display_scope)
> >>>>>>> or disabled (when they are the same).
> >>>>>>
> >>>>>> This seems to enforce the idea that these new additions aim to be
> >>>>>> generic on the surface but the only goal is to support SNC.
> >>>>>
> >>>>> If you have some more ideas on how to make this more generic and
> >>>>> less SNC specific I'm all ears.
> >>>>
> >>>> It may not end up being totally generic. It should not pretend to be
> >>>> when it is not. It makes the flows difficult to follow when there are
> >>>> these unexpected checks/quirks in what claims to be core code.
> >>>
> >>> Do you want some sort of warning comments in pieces of code
> >>> that are SNC specific?
> >>
> >> I cannot think now where warnings will be appropriate but if you
> >> find instances then please do. To start the quirks can at least be
> >> documented. For example, "Only user of <feature> is SNC, which does
> >> not require <custom> so simplify by <describe shortcut> ..."
> > 
> > The main spot that triggered this line of discussion was changing the
> > sanity check that operations to read monitors is being done from a
> > CPU within the right domain. I've added a short comment on the new
> > check:
> > 
> > -       if (!cpumask_test_cpu(smp_processor_id(), &d->hdr.cpu_mask))
> > +       /* Event counts can only be read from a CPU on the same L3 cache */
> > +       if (d->display_id != get_cpu_cacheinfo_id(smp_processor_id(), r->mon_display_scope))
> >                 return -EINVAL;
> > 
> > But my change embeds the assumption that monitor events are L3 scoped.
> > 
> > Should it be something like this (to keep the non-SNC case generic):
> > 
> > 	if (r->mon_scope == r->mon_display_scope) {
> > 		if (!cpumask_test_cpu(smp_processor_id(), &d->hdr.cpu_mask))
> > 			return -EINVAL;
> 
> Yes, keeping this check looks good to me ...
> 
> > 	} else {
> > 		/*
> > 		 * SNC: OK to read events on any CPU sharing same L3
> > 		 * cache instance.
> > 		 */
> > 		 if (d->display_id != get_cpu_cacheinfo_id(smp_processor_id(), r->mon_display_scope))
> > 		 	return -EINVAL;
> > 	}
> 
> ... while I remain unsure about where "display_id" fits in.

See below.

> > 
> >>
> >>>
> >>>>
> >>>>>>>>>         }
> >>>>>>>>> +
> >>>>>>>>> +       return 0;
> >>>>>>>>> +}
> >>>>>>>>> +
> >>>>>>>>> +static int mkdir_mondata_subdir(struct kernfs_node *parent_kn,
> >>>>>>>>> +                               struct rdt_mon_domain *d,
> >>>>>>>>> +                               struct rdt_resource *r, struct rdtgroup *prgrp)
> >>>>>>>>> +{
> >>>>>>>>> +       struct kernfs_node *kn, *ckn;
> >>>>>>>>> +       char name[32];
> >>>>>>>>> +       bool do_sum;
> >>>>>>>>> +       int ret;
> >>>>>>>>> +
> >>>>>>>>> +       do_sum = r->mon_scope != r->mon_display_scope;
> >>>>>>>>> +       sprintf(name, "mon_%s_%02d", r->name, d->display_id);
> >>>>>>>>> +       kn = kernfs_find_and_get_ns(parent_kn, name, NULL);
> >>>>>>>>> +       if (!kn) {
> >>>>>>>>> +               /* create the directory */
> >>>>>>>>> +               kn = kernfs_create_dir(parent_kn, name, parent_kn->mode, prgrp);
> >>>>>>>>> +               if (IS_ERR(kn))
> >>>>>>>>> +                       return PTR_ERR(kn);
> >>>>>>>>> +
> >>>>>>>>> +               ret = rdtgroup_kn_set_ugid(kn);
> >>>>>>>>> +               if (ret)
> >>>>>>>>> +                       goto out_destroy;
> >>>>>>>>> +               ret = mon_add_all_files(kn, d, r, prgrp, do_sum);
> >>>>>>>>
> >>>>>>>> This does not look right. If I understand correctly the private data
> >>>>>>>> of these event files will have whichever mon domain came up first as
> >>>>>>>> its domain id. That seems completely arbitrary and does not reflect
> >>>>>>>> accurate state for this file. Since "do_sum" is essentially a "flag"
> >>>>>>>> on how this file can be treated, can its "dom_id" not rather be
> >>>>>>>> the "monitor scope domain id"? Could that not help to eliminate
> >>>>>>>
> >>>>>>> You are correct that this should be the "monitor scope domain id" rather
> >>>>>>> than the first SNC domain that appears. I'll change to use that. I don't
> >>>>>>> think it helps in removing the per-domain display_id.
> >>>>>>
> >>>>>> Wouldn't the file metadata then be the "display_id"?
> >>>>>
> >>>>> Yes. The metadata is the display_id for files that need to sum across
> >>>>> SNC nodes, but the domain id for ones where no summation is needed.
> >>>>
> >>>> Right ... and there is a "sum" flag to tell which is which?
> >>>
> >>> Yes. sum==0 means the domid field is the one and only domain to
> >>> report for this resctrl monitor file. sum==1 means the domid field is
> >>> the display_id - all domains with this display_id must be summed to
> >>> provide the result to present to the user.
> >>>
> >>> I've tried to capture that in the kerneldoc comment for struct mon_event.
> >>> Here's what I'm planning to include in v18 (Outlook will probably mangle
> >>> the formatting ... just imagine that the text lines up neatly):
> >>>
> >>> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> >>> index 49440f194253..3411557d761a 100644
> >>> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> >>> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> >>> @@ -132,14 +132,19 @@ struct mon_evt {
> >>>   *                     as kernfs private data
> >>>   * @rid:               Resource id associated with the event file
> >>>   * @evtid:             Event id associated with the event file
> >>> - * @domid:             The domain to which the event file belongs
> >>> + * @sum:               Set when event must be summed across multiple
> >>> + *                     domains.
> >>> + * @domid:             When @sum is zero this is the domain to which
> >>> + *                     the event file belongs. When sum is one this
> >>> + *                     is the display_id of all domains to be summed
> >>
> >> Here is where I would like to understand why it cannot just be
> >> "When sum is one this is the domain id of the scope at which (for which?)
> >> the events must be summed." Although, you already mentioned this will be
> >> clear in next posting.
> >>
> >>>   * @u:                 Name of the bit fields struct
> >>>   */
> >>>  union mon_data_bits {
> >>>         void *priv;
> >>>         struct {
> >>>                 unsigned int rid                : 10;
> >>> -               enum resctrl_event_id evtid     : 8;
> >>> +               enum resctrl_event_id evtid     : 7;
> >>> +               unsigned int sum                : 1;
> >>>                 unsigned int domid              : 14;
> >>>         } u;
> >>>  };
> >>>
> >>> -Tony
> > 
> > Maybe an example might help. Assume an SNC system with two sockets,
> > three SNC nodes per socket, only supporting monitoring. The only domain
> > list created by resctrl is the mon_domains list on the RDT_RESOURCE_L3
> > resource. And it looks like this (with "disply_list" abbreviated to
> > "dspl" to keep the picture small):
> > 
> > 
> >        <------ SNC NODES ON SOCKET 0 ----->   <------ SNC NODES ON SOCKET 1 ------>
> > ----> +----------+ +----------+ +----------+ +----------+ +----------+ +----------+
> >       | id = 0   | | id = 1   | | id = 2   | | id = 3   | | id = 4   | | id = 5   |
> >       |          | |          | |          | |          | |          | |          |
> >       | dspl = 0 | | dspl = 0 | | dspl = 0 | | dspl = 1 | | dspl = 1 | | dspl = 1 |
> >       |          | |          | |          | |          | |          | |          |
> >       +----------+ +----------+ +----------+ +----------+ +----------+ +----------+
> > 
> > Reading the per-SNC node monitor values looks just the same as the
> > non-SNC case. The struct rmid_read passed across the smp_call*() has
> > the resource, domain, event, and reading the counters is essentially
> > unchanged.
> > 
> > Reading a file to sum event counts for SNC nodes on socket 1 needs to
> > find each of the "struct rdt_mon_domain" that are part of socket 1.
> > I'm doing that with meta data in the file that says sum=1 (need to add
> > up something) and domid=1 (the things to be added are those with
> > display_id = 1). So the code reads:
> > 
> > 	list_for_each_entry(d, &rr->r->mon_domains, hdr.list) {
> > 		if (d->display_id == rr->d->display_id) {
> > 			... call stuff to read and sum for domain "d"
> > 		}
> > 	}
> > 
> > The display_id is "the domain id of the scope at which (for which?)
> > the events must be summed." in your text above.
> 
> My point remains that it is not clear (to me) why it is required to
> carry the display_id around.
> 
>  	list_for_each_entry(d, &rr->r->mon_domains, hdr.list) {
> 		/* determine @id of @d at rr->r->mon_display_scope */
>  		if (id == domid) {
>  			... call stuff to read and sum for domain "d"
>  		}
>  	}

That "determine @id of @d at rr->r->mon_display_scope" is:

	display_id = get_domain_id_from_scope(cpumask_first(rr->d->hdr.cpu_mask), rr->r->mon_display_scope);
	if (display_id < 0) {
		take some error action
	}

So it certainly isn't *required* to carry display_id around. But doing
so makes the code simpler. I could bury the long line into a helper
macro/function. But I can't bury the error check.

I'd also need to change get_domain_id_from_scope() from "static" to
global so it can be used in other files besides core.c

Note that there are several places where I need to use display_id,
computing it at run time in each place, but it seems so much easier to
do it once at domain creation time.

> 
> Reinette

-Tony

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