Messages in this thread | | | Date | Mon, 6 Mar 2023 11:33:54 +0000 | Subject | Re: [PATCH v2 09/18] x86/resctrl: Allow resctrl_arch_rmid_read() to sleep | From | James Morse <> |
| |
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
| |