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From:   Peter Newman <peternewman@google.com>
Date:   Tue, 16 May 2023 16:27:46 +0200
Message-ID: <CALPaoCj4a=ePOfdd1CKHkOP1B9DZTGLCDHeNb0QLvCophX941g@mail.gmail.com>
Subject: Re: [PATCH v1 3/9] x86/resctrl: Add resctrl_mbm_flush_cpu() to
 collect CPUs' MBM events
To:     Reinette Chatre <reinette.chatre@intel.com>
Cc:     Fenghua Yu <fenghua.yu@intel.com>, Babu Moger <babu.moger@amd.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Stephane Eranian <eranian@google.com>,
        James Morse <james.morse@arm.com>,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org
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On Tue, May 16, 2023 at 4:18=E2=80=AFPM Peter Newman <peternewman@google.co=
m> wrote:
> According to cpu_hotplug.rst, the startup callbacks are called before
> a CPU is started and the teardown callbacks are called after the CPU
> has become dysfunctional, so it should always be safe for a CPU to
> access its own data, so all I need to do here is avoid walking domain
> lists in resctrl_mbm_flush_cpu().
>
> However, this also means that resctrl_{on,off}line_cpu() call
> clear_closid_rmid() on a different CPU, so whichever CPU executes
> these will zap its own pqr_state struct and PQR_ASSOC MSR.

Sorry, I read the wrong section. I was looking at PREPARE section
callbacks. ONLINE callbacks are called on the CPU, so calling
clear_closid_rmid() is fine.

It says the offline callback is called from the per-cpu hotplug
thread, so I'm not sure if that means another context switch is
possible after the teardown handler has run. To be safe, I can make
resctrl_mbm_flush_cpu() check to see if it still has its domain state.

-Peter