On Tue, 2012-10-23 at 18:30 +0100, Pawel Moll wrote:
>
> === Option 1: Trace event ===
>
> This seems to be the "cheapest" option. Simply defining a trace event
> that can be generated by a hwmon (or any other) driver makes the
> interesting data immediately available to any ftrace/perf user. Of
> course it doesn't really help with the cpufreq case, but seems to be
> a good place to start with.
>
> The question is how to define it... I've came up with two prototypes:
>
> = Generic hwmon trace event =
>
> This one allows any driver to generate a trace event whenever any
> "hwmon attribute" (measured value) gets updated. The rate at which the
> updates happen can be controlled by already existing "update_interval"
> attribute.
>
> 8<-------------------------------------------
> TRACE_EVENT(hwmon_attr_update,
> TP_PROTO(struct device *dev, struct attribute *attr, long long input),
> TP_ARGS(dev, attr, input),
>
> TP_STRUCT__entry(
> __string( dev, dev_name(dev))
> __string( attr, attr->name)
> __field( long long, input)
> ),
>
> TP_fast_assign(
> __assign_str(dev, dev_name(dev));
> __assign_str(attr, attr->name);
> __entry->input = input;
> ),
>
> TP_printk("%s %s %lld", __get_str(dev), __get_str(attr), __entry->input)
> );
> 8<-------------------------------------------
>
> It generates such ftrace message:
>
> <...>212.673126: hwmon_attr_update: hwmon4 temp1_input 34361
>
> One issue with this is that some external knowledge is required to
> relate a number to a processor core. Or maybe it's not an issue at all
> because it should be left for the user(space)?
If the external knowledge can be characterized in a userspace tool with
the given data here, I see no issues with this.
>
> = CPU power/energy/temperature trace event =
>
> This one is designed to emphasize the relation between the measured
> value (whether it is energy, temperature or any other physical
> phenomena, really) and CPUs, so it is quite specific (too specific?)
>
> 8<-------------------------------------------
> TRACE_EVENT(cpus_environment,
> TP_PROTO(const struct cpumask *cpus, long long value, char unit),
> TP_ARGS(cpus, value, unit),
>
> TP_STRUCT__entry(
> __array( unsigned char, cpus, sizeof(struct cpumask))
> __field( long long, value)
> __field( char, unit)
> ),
>
> TP_fast_assign(
> memcpy(__entry->cpus, cpus, sizeof(struct cpumask));
Copying the entire cpumask seems like overkill. Especially when you have
4096 CPU machines.
> __entry->value = value;
> __entry->unit = unit;
> ),
>
> TP_printk("cpus %s %lld[%c]",
> __print_cpumask((struct cpumask *)__entry->cpus),
> __entry->value, __entry->unit)
> );
> 8<-------------------------------------------
>
> And the equivalent ftrace message is:
>
> <...>127.063107: cpus_environment: cpus 0,1,2,3 34361[C]
>
> It's a cpumask, not just single cpu id, because the sensor may measure
> the value per set of CPUs, eg. a temperature of the whole silicon die
> (so all the cores) or an energy consumed by a subset of cores (this
> is my particular use case - two meters monitor a cluster of two
> processors and a cluster of three processors, all working as a SMP
> system).
>
> Of course the cpus __array could be actually a special __cpumask field
> type (I've just hacked the __print_cpumask so far). And I've just
> realised that the unit field should actually be a string to allow unit
> prefixes to be specified (the above should obviously be "34361[mC]"
> not "[C]"). Also - excuse the "cpus_environment" name - this was the
> best I was able to come up with at the time and I'm eager to accept
> any alternative suggestions :-)
Perhaps making a field that can be a subset of cpus may be better. That
way we don't waste the ring buffer with lots of zeros. I'm guessing that
it will only be a group of cpus, and not a scattered list? Of course,
I've seen boxes where the cpu numbers went from core to core. That is,
cpu 0 was on core 1, cpu 1 was on core 2, and then it would repeat.
cpu 8 was on core 1, cpu 9 was on core 2, etc.
But still, this could be compressed somehow.
I'll let others comment on the rest.
-- Steve
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