linux-kernel - Re: [PATCH] x86/resctrl: Fix event counts regression in reused RMIDs

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Message-ID: <CALPaoCiB-vOuXJYkaLLsxSKHcjT55q1RSNBjhHUWmPL9rFdF8A@mail.gmail.com>
Date:   Thu, 8 Dec 2022 11:04:22 +0100
From:   Peter Newman <peternewman@...gle.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...el.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>,
        James Morse <james.morse@....com>,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        linux-kernel@...r.kernel.org, eranian@...gle.com,
        Babu Moger <Babu.Moger@....com>
Subject: Re: [PATCH] x86/resctrl: Fix event counts regression in reused RMIDs

Hi Reinette,

On Wed, Dec 7, 2022 at 8:48 PM Reinette Chatre
<reinette.chatre@...el.com> wrote:
>
> To get back to the original behavior before the refactoring it also seems
> that __mon_event_count() needs to return right after calling
> resctrl_arch_reset_rmid(). The only caller with rr->first set is when
> the mon directory is created and the returned values are not used,
> it is just run to get prev_msr set. This also avoids unnecessarily reading
> the counters twice.
>
> So, how about:
>
> static int __mon_event_count(u32 rmid, struct rmid_read *rr)
> {
>
> ...
> if (rr->first) {
> resctrl_arch_reset_rmid(rr->r, rr->d, rmid, rr->evtid);
> return 0;
> }
> ...
>
> }

Avoiding the double-read sounds good, but...

>
> Also ... there appears to be a leftover related snippet in __mon_event_count()
> that does not belong anymore and may still cause incorrect behavior:
>
> static int __mon_event_count(u32 rmid, struct rmid_read *rr)
> {
> ...
> if (rr->first) {
> memset(m, 0, sizeof(struct mbm_state));
> return 0;
> }
> ...
> }

I'm less sure about removing (or skipping) this. mbm_state::mbm_local
still seems to be used by the mba_sc code. That might be why James
left this code in.

I was sort of confused about the new role of mbm_state following the
refactoring when reviewing Babu's change. (which reminds me that I
should have CC'ed him on this patch)

-Peter