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    SubjectRe: [PATCH v2 09/18] x86/resctrl: Allow resctrl_arch_rmid_read() to sleep
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    Hi Peter,

    On 23/01/2023 15:33, Peter Newman wrote:
    > On Fri, Jan 13, 2023 at 6:56 PM James Morse <james.morse@arm.com> wrote:
    >> MPAM's cache occupancy counters can take a little while to settle once
    >> the monitor has been configured. The maximum settling time is described
    >> to the driver via a firmware table. The value could be large enough
    >> that it makes sense to sleep.
    >
    > Would it be easier to return an error when reading the occupancy count
    > too soon after configuration? On Intel it is already normal for counter
    > reads to fail on newly-allocated RMIDs.

    For x86, you have as many counters as there are RMIDs, so there is no issue just accessing
    the counter.

    With MPAM there may be as few as 1 monitor for the CSU (cache storage utilisation)
    counter, which needs to be multiplexed between different PARTID to find the cache
    occupancy (This works for CSU because its a stable count, it doesn't work for the
    bandwidth monitors)
    On such a platform the monitor needs to be allocated and programmed before it reads a
    value for a particular PARTID/CLOSID. If you had two threads trying to read the same
    counter, they could interleave perfectly to prevent either thread managing to read a value.
    The 'not ready' time is advertised in a firmware table, and the driver will wait at most
    that long before giving up and returning an error.

    Clearly 1 monitor is a corner case, and I hope no-one ever builds that. But if there are
    fewer monitors than there are PARTID*PMG you get the same problem, (you just need more
    threads reading the counters)


    Thanks,

    James

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