linux-kernel - Re: [PATCH v1 3/9] x86/resctrl: Add resctrl_mbm_flush

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Date:   Thu, 1 Jun 2023 16:45:27 +0200
From:   Peter Newman <peternewman@...gle.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>, Babu Moger <babu.moger@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Stephane Eranian <eranian@...gle.com>,
        James Morse <james.morse@....com>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v1 3/9] x86/resctrl: Add resctrl_mbm_flush_cpu() to
 collect CPUs' MBM events

Hi Reinette,

On Thu, May 11, 2023 at 11:37 PM Reinette Chatre
<reinette.chatre@...el.com> wrote:
> On 4/21/2023 7:17 AM, Peter Newman wrote:
> > +     /* Count bandwidth after the first successful counter read. */
> > +     if (counter->initialized) {
> > +             /* Assume that mbm_update() will prevent double-overflows. */
> > +             if (val != counter->prev_bytes)
> > +                     atomic64_add(val - counter->prev_bytes,
> > +                                  &m->soft_rmid_bytes);
> > +     } else {
> > +             counter->initialized = true;
> > +     }
> > +
> > +     counter->prev_bytes = val;
>
> I notice a lot of similarities between the above and the software controller,
> see mbm_bw_count().

I see the "a=now(); a-b; b=a;" and the not handling overflow parts
being similar, but the use of the initialized flag seems quite
different from delta_comp.

Also mbm_state is on the arch-independent side and the new code is
going to the arch-dependent side, so it wouldn't be convenient to try
to use the mbm_bw structures for this.

>From this, I don't think trying to reuse this is worth it unless you
have other suggestions.

-Peter