linux-kernel - Re: [PATCH v1 8/9] x86/resctrl: Use mbm

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Date:   Tue, 6 Jun 2023 15:48:33 +0200
From:   Peter Newman <peternewman@...gle.com>
To:     Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>
Cc:     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 8/9] x86/resctrl: Use mbm_update() to push soft RMID counts

On Fri, Apr 21, 2023 at 4:18 PM Peter Newman <peternewman@...gle.com> wrote:
>
> __mon_event_count() only reads the current software count and does not
> cause CPUs in the domain to flush. For mbm_update() to be effective in
> preventing overflow in hardware counters with soft RMIDs, it needs to
> flush the domain CPUs so that all of the HW RMIDs are read.
>
> When RMIDs are soft, mbm_update() is intended to push bandwidth counts
> to the software counters rather than pulling the counts from hardware
> when userspace reads event counts, as this is a lot more efficient when
> the number of HW RMIDs is fixed.

The low frequency with which the overflow handler is run would
introduce too much error into bandwidth calculations and running it
more frequently regardless of whether event count reads are being
requested by the user is not a good use of CPU time.

mon_event_read() needs to pull fresh event count values from hardware.

> When RMIDs are soft, mbm_update() only calls mbm_flush_cpu_handler() on
> each CPU in the domain rather than reading all RMIDs.

I'll try going back to Stephane's original approach of rate-limiting
how often domain CPUs need to be flushed and allowing the user to
configure the time threshold. This will allow mbm_update() to read all
of the RMIDs without triggering lots of redundant IPIs. (redundant
because only the current RMID on each CPU can change when RMIDs are
soft)