Re: [PATCH v2 09/18] x86/resctrl: Allow resctrl_arch_rmid_read() to sleep

[James Morse <james.morse@arm.com>]

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