From: Rafael J. Wysocki > Sent: 30 January 2020 14:47 > > In certain system configurations it may not be desirable to use some > C-states assumed to be available by intel_idle and the driver needs > to be prevented from using them even before the cpuidle sysfs > interface becomes accessible to user space. Currently, the only way > to achieve that is by setting the 'max_cstate' module parameter to a > value lower than the index of the shallowest of the C-states in > question, but that may be overly intrusive, because it effectively > makes all of the idle states deeper than the 'max_cstate' one go > away (and the C-state to avoid may be in the middle of the range > normally regarded as available). > > To allow that limitation to be overcome, introduce a new module > parameter called 'states_off' to represent a list of idle states to > be disabled by default in the form of a bitmask and update the > documentation to cover it. The problem I see is that there are (at least) 3 different ways of referring to the C-States: 1) The state names, C1, C1E, C3, C7 etc. I'm not sure these are visible outside intel_idle.c. 2) The maximum allowed latency in us. 3) The index into the cpu-dependant tables in intel_idle.c. Boot parameters that set 3 are completely hopeless for normal users. The C-state names might be - but they aren't documented. Unless you know exactly which cpu table is being used the only constraint a user can request is the latency. (I've had the misfortune to read intel_idle.c in the last week. Almost impenetrable TLA ridden uncommented code.) ... > + * The positions of the bits that are set in the two's complement representation > + * of this value are the indices of the idle states to be disabled by default > + * (as reflected by the names of the corresponding idle state directories in > + * sysfs, "state0", "state1" ... "state<i>" ..., where <i> is the index of the > + * given state). What has 'two's complement' got to do with anything? ... > +The value of the ``states_off`` module parameter (0 by default) represents a > +list of idle states to be disabled by default in the form of a bitmask. Namely, > +the positions of the bits that are set in the two's complement representation of > +that value are the indices of idle states to be disabled by default (as > +reflected by the names of the corresponding idle state directories in ``sysfs``, > +:file:`state0`, :file:`state1` ... :file:`state<i>` ..., where ``<i>`` is the > +index of the given idle state; see :ref:`idle-states-representation` in > +:doc:`cpuidle`). For example, if ``states_off`` is equal to 3, the driver will > +disable idle states 0 and 1 by default, and if it is equal to 8, idle state 3 > +will be disabled by default and so on (bit positions beyond the maximum idle > +state index are ignored). The idle states disabled this way can be enabled (on > +a per-CPU basis) from user space via ``sysfs``. A few line breaks would make that easier to read. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
