Hi James, On 8/16/24 11:56, James Morse wrote: > Hi Babu, > > On 06/08/2024 23:00, Babu Moger wrote: >> The mbm_mode displays list of monitor modes supported. >> >> The mbm_cntr_assign is one of the currently supported modes. It is also >> called ABMC (Assignable Bandwidth Monitoring Counters) feature. ABMC >> feature provides option to assign a hardware counter to an RMID and >> monitor the bandwidth as long as it is assigned. ABMC mode is enabled >> by default when supported. >> >> Legacy mode works without the assignment option. >> >> Provide an interface to display the monitor mode on the system. >> $cat /sys/fs/resctrl/info/L3_MON/mbm_mode >> [mbm_cntr_assign] >> legacy >> >> Switching the mbm_mode will reset all the mbm counters of all resctrl >> groups. > >> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst >> index 30586728a4cd..d4ec605b200a 100644 >> --- a/Documentation/arch/x86/resctrl.rst >> +++ b/Documentation/arch/x86/resctrl.rst >> @@ -257,6 +257,40 @@ with the following files: >> # cat /sys/fs/resctrl/info/L3_MON/mbm_local_bytes_config >> 0=0x30;1=0x30;3=0x15;4=0x15 >> >> +"mbm_mode": >> + Reports the list of assignable monitoring features supported. The >> + enclosed brackets indicate which feature is enabled. >> + :: >> + >> + cat /sys/fs/resctrl/info/L3_MON/mbm_mode >> + [mbm_cntr_assign] >> + legacy >> + >> + "mbm_cntr_assign": >> + AMD's ABMC feature is one of the mbm_cntr_assign mode supported. >> + The bandwidth monitoring feature on AMD system only guarantees >> + that RMIDs currently assigned to a processor will be tracked by >> + hardware. The counters of any other RMIDs which are no longer >> + being tracked will be reset to zero. The MBM event counters >> + return "Unavailable" for the RMIDs that are not tracked by >> + hardware. So, there can be only limited number of groups that can >> + give guaranteed monitoring numbers. With ever changing configurations >> + there is no way to definitely know which of these groups are being >> + tracked for certain point of time. Users do not have the option to >> + monitor a group or set of groups for certain period of time without >> + worrying about RMID being reset in between. >> + >> + The ABMC feature provides an option to the user to assign a hardware >> + counter to an RMID and monitor the bandwidth as long as it is assigned. >> + The assigned RMID will be tracked by the hardware until the user >> + unassigns it manually. There is no need to worry about counters being >> + reset during this period. > > While debugging my rebase of MPAM on top of this series, I've come back to this wording a > few times to try and work out what I should expect to see ... > > Is it possible to disentangle the AMD hardware feature description from the description of > the filesystem behaviour this enables? You are really describing what the hardware does if > you don't enable this mode... > > An incomplete suggestion of the shape would be something like: > > | In mbm_cntr_assign mode user-space is able to specify which control > | or monitor groups in resctrl should have a hardware counter assigned > | using the 'mbm_control' file. The number of hardware counters available > | is described in the 'num_mbm_cntrs' file. > | Changing this mode will cause all counters on a resource to reset. > | > | The feature is needed on platforms which support more control and monitor > | groups than hardware counters, meaning 'unassigned' control or monitor groups will > | report 'Unavailable' or not count all the traffic in an unpredictable way. > | > | Platforms with AMDs ABMC feature enable this mode by default so that counters > | remain assigned even when the corresponding RMID is not in use by any processor. > Looks good to me. > >> + "Legacy": > > Calling "enough hardware counters" 'legacy' is a bit curious.... 'default'? "Default" Sound good me to if no objections from others. > (but I haven't worked out the benefit of disabling this mode, so maybe it doesn't need a > name.) > >> + Legacy mode works without the assignment option. The monitoring works >> + as long as there are enough RMID counters available to support number >> + of monitoring groups. > > How can user-space tell this is the case? Could we be specific as to what 'works' means? > > Something like: > | By default resctrl assumes each control and monitor group has a hardware counter. > | Hardware without this property will still allow more control or monitor groups > | than 'num_mbm_cntrs' to be created. Reading the mbm files may report > | 'Unavailable' if there is no hardware resource assigned. Looks good. > > > N.B. I don't suggest referring to the num_rmid file in these as MPAM doesn't have an > equivalent property. > > > Thanks, > > James > -- Thanks Babu Moger
