HW perf. events arch implementation
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I have taken a look at the code in the x86, sparc and ppc implementations and tried to drum up an Alpha implementation for the EV67/7/79 cpus, but it ain't working and is producing obviously erroneous counts. Part of the problem is that I don't understand under what conditions, and with what assumptions, the performance event subsystem is calling into the architecture specific code. Is there any documentation available that describes the architecture specific interface?
The Alpha CPUs of interest have two 20-bit performance monitoring counters that can count cycles, instructions, Bcache misses and Mbox replays (but not all combinations of those). For round numbers consider a 1GHz CPU, with a theoretical maximal sustained throughput of four instructions per cycle, then a single performance counter could potentially generate 4000 interrupts per second to signal counter overflow when counting instructions.
The x86, sparc and PPC implementations seem to me to assume that calls to read back the counters occur more frequently than performance counter overflow interrupts, and that the highest bit of the counter can safely be used to detect overflow. (Am I correct?) That is likely not to be true of the Alpha because of the small width of the counter. Is there someone who would be happy to give me, a kernel newbie who probably doesn't even make the grade of neophyte, a bit of direction on this?
Also, the Alpha CPUs have an interesting mode whereby one programmes up one counter with a specified (or random) value that specifies a future instruction to profile. The CPU runs for that number of instructions/cycles, then a short monitoring window (of a few cycles) is opened about the profiled instruction and when completed an interrupt is generated. One can then read back a whole lot of information about the pipeline at the time of the profiled instruction. This can be used for statistical sampling. Does the performance events subsystem support monitoring with such a mode?
Cheers Michael. -- To unsubscribe from this list: send the line "unsubscribe linux-alpha" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html