linux-kernel - RE: [PATCH v8 5/7] x86/resctrl: Add "mba_MBps_event" file to ctrl

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Message-ID: <SJ1PR11MB6083ACCB3B5D602CDA9F2269FC5A2@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date: Wed, 13 Nov 2024 00:53:46 +0000
From: "Luck, Tony" <tony.luck@...el.com>
To: "Chatre, Reinette" <reinette.chatre@...el.com>, "Yu, Fenghua"
	<fenghua.yu@...el.com>, Peter Newman <peternewman@...gle.com>, "Jonathan
 Corbet" <corbet@....net>, Shuah Khan <skhan@...uxfoundation.org>,
	"x86@...nel.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-kernel@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: RE: [PATCH v8 5/7] x86/resctrl: Add "mba_MBps_event" file to ctrl_mon
 directories

> >>> +static int set_mba_sc(bool mba_sc)
> >>> +{
> >>> +   struct rftype *rft;
> >>> +
> >>> +   rft = rdtgroup_get_rftype_by_name("mba_MBps_event");
> >>> +   if (rft)
> >>> +           rft->fflags = enable ? RFTYPE_CTRL_BASE : 0;
> >>
> >> I think this sets this file to be created for all CTRL groups, even when not supporting
> >> monitoring?

I think I misunderstood you. I though you said the these mba_MBps_event files
would be created even if monitoring is not supported,

But now I wonder what you mean by "all CTRL groups".

> > No. The calling sequence is:
> >
> > rdt_get_tree()
> >     rdt_enable_ctx()
> >
> >         if (ctx->enable_mba_mbps) {
> >                 ret = set_mba_sc(true);
> >                 if (ret)
> >                         goto out_cdpl3;
> >         }
> >
> > So set_mba_sc() is only called when the mba_MBps mount option has been used. So
> > mba_mbps_event_init() isn't called.
> >
> > Note that on unmount of the resctrl file system rdt_kill_sb() calls rdt_disable_ctx()
> > which calls set_mba_sc(false) which will clear rft->fflags on systems which support
> > mba_MBps.
>
> It sounds as though you are saying that setting the wrong flags are ok as long as it is
> done in some specific calling sequence. Is this correct? Relying on calling sequence
> instead of correct flags requires more in-depth knowledge of code flows and makes code
> harder to maintain.
> Why not just make this "RFTYPE_CTRL_BASE | RFTYPE_MON_BASE" to make it clear that the file
> applies to CTRL_MON groups? What am I missing?

The directories which need this new file are the same ones that get a "schemata" file.

The initialization of fflags for the schemata file just uses RFTYPE_CTRL_BASE:

        {
                .name           = "schemata",
                .mode           = 0644,
                .kf_ops         = &rdtgroup_kf_single_ops,
                .write          = rdtgroup_schemata_write,
                .seq_show       = rdtgroup_schemata_show,
                .fflags         = RFTYPE_CTRL_BASE,
        },

I don't see any files using .fflags = "RFTYPE_CTRL_BASE | RFTYPE_MON_BASE"

-Tony