linux-kernel - Re: [PATCH] x86/resctrl: mba_MBps: Fall back to total b/w if local b/w unavailable

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Date:   Wed, 25 Oct 2023 12:38:50 -0700
From:   Tony Luck <tony.luck@...el.com>
To:     Peter Newman <peternewman@...gle.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>, x86@...nel.org,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        patches@...ts.linux.dev
Subject: Re: [PATCH] x86/resctrl: mba_MBps: Fall back to total b/w if local
 b/w unavailable

On Wed, Oct 25, 2023 at 02:46:53PM +0200, Peter Newman wrote:
> Hi Tony,
> 
> On Tue, Oct 24, 2023 at 8:16 PM Tony Luck <tony.luck@...el.com> wrote:
> > --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> > +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> > @@ -418,6 +418,14 @@ static int __mon_event_count(u32 rmid, struct rmid_read *rr)
> >         return 0;
> >  }
> >
> > +static struct mbm_state *get_mbm_data(struct rdt_domain *dom_mbm, int rmid)
> > +{
> > +       if (is_mbm_local_enabled())
> > +               return &dom_mbm->mbm_local[rmid];
> > +
> > +       return &dom_mbm->mbm_total[rmid];
> > +}
> 
> That looks very similar to the get_mbm_state() function I added to
> this same file recently:
> 
> https://lore.kernel.org/all/20221220164132.443083-2-peternewman%40google.com
> 
> I think the name you picked is misleadingly general. "local if
> available, otherwise total" seems to be a choice specific to the mbps
> controller. I think these functions should be reconciled a little
> better.
> 

Peter (and Babu, who made the same point about get_mbm_state().

Do you want to see your function extended to do the "pick an MBM event?"

I could add a s/w defined "event" to the enum resctrl_event_id and
extend get_mbm_state() like this:


static struct mbm_state *get_mbm_state(struct rdt_domain *d, u32 rmid,
				       enum resctrl_event_id evtid)
{
	switch (evtid) {
	case QOS_L3_MBM_TOTAL_EVENT_ID:
		return &d->mbm_total[rmid];
	case QOS_L3_MBM_LOCAL_EVENT_ID:
		return &d->mbm_local[rmid];
+	case QOS_L3_MBM_LOCAL_OR_TOTAL_EVENT_ID:
+		if (is_mbm_local_enabled())
+			return &d->mbm_local[rmid];
+		if (is_mbm_total_enabled())
+			return &d->mbm_total[rmid];
+		fallthrough;
	default:
		return NULL;
	}
}

Is this the direction you are thinking of?

Callers then look like:

static void mbm_bw_count(u32 rmid, struct rmid_read *rr)
{
	struct mbm_state *m = get_mbm_state(rr->d, rmid, QOS_L3_MBM_LOCAL_OR_TOTAL_EVENT_ID);
	u64 cur_bw, bytes, cur_bytes;

similar for the other three places where this is needed.

Any suggestions on how "QOS_L3_MBM_LOCAL_OR_TOTAL_EVENT_ID" could be
abbreviated, or just have some different, but descriptive, name?

-Tony