linux-kernel - RE: [RFC PATCH 2/2] resctrl2: Arch x86 modules for most of the legacy control/monitor functions

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Date:   Wed, 5 Jul 2023 04:46:03 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Peter Newman <peternewman@...gle.com>
CC:     James Morse <james.morse@....com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        Drew Fustini <dfustini@...libre.com>,
        Babu Moger <Babu.Moger@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        "carl@...amperecomputing.com" <carl@...amperecomputing.com>,
        "lcherian@...vell.com" <lcherian@...vell.com>,
        "bobo.shaobowang@...wei.com" <bobo.shaobowang@...wei.com>,
        "tan.shaopeng@...itsu.com" <tan.shaopeng@...itsu.com>,
        "xingxin.hx@...nanolis.org" <xingxin.hx@...nanolis.org>,
        "baolin.wang@...ux.alibaba.com" <baolin.wang@...ux.alibaba.com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>,
        "Pitre, Nicolas" <npitre@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        "aricciardi@...libre.com" <aricciardi@...libre.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: [RFC PATCH 2/2] resctrl2: Arch x86 modules for most of the legacy
 control/monitor functions

>> +       if (rr->event <= EV_LOC) {
>> +               spin_lock_irqsave(&m->msr_lock, flags);
>
> Will there ultimately be any locking at the filesystem layer? I recall
> from feedback on my change adding a spinlock here[1] before that the
> filesystem-layer locking took care of this.

Yes. Working on that now. The spinlock will go away when everything is protected.
by resctrl_mutex.

> > +       if (event <= EV_LOC)
> > +               smp_call_function_any(&d->cpu_mask, __rdt_rmid_read, &rr, 1);
> > +       else
> > +               __rdt_rmid_read(&rr);
>
> I like that the driver is responsible for deciding where IPIs need to
> be sent, but it looks like the consequence is that RDT-level code
> wants to add in the child monitors' event counts once executing within
> the correct domain. The one-per-domain IPI assumption from the current
> resctrl code being wrong is probably harder to overcome than needing
> to figure out what additional RMIDs to read, but I'd really need to
> know the synchronization requirements for __rdt_rmid_read() to inspect
> the monitoring group hierarchy.

There are likely some gaps in current code. But I think they should be
fixable.

> Would you continue to promise that the FS structure won't change
> during a monitor read?  To us, the biggest priority for
> parallelization is reading all the domain-group combinations in the
> system, because we have a lot of them and want the tightest possible
> snapshot of bandwidth usage, broken down by group.

The mbm_poll() code that makes sure that counters don't wrap is
doing all the expensive wrmsr(QM_EVTSEL);rdmsr(QM_COUNT)
once per second to give you the data you want. But existing resctrl
filesystem doesn't let you do a bulk read. I have some ideas on how
to provide something better. One question: do you really need that
snapshot to be system-wide? Or can you live with separate L3-scoped
snapshots that aren't tightly synchronized with each other?

-Tony