When policy->max is changed via _PPC or sysfs and is more than the max non turbo frequency, it does not really change resulting performance in some processors. When policy->max results in a P-State ratio more than the turbo activation ratio, then processor can choose any P-State up to max turbo. So the user or _PPC setting has no value, but this can cause undesirable side effects like: - Showing reduced max percentage in Intel P-State sysfs - It can cause reduced max performance under certain boundary conditions: The requested max scaling frequency either via _PPC or via cpufreq-sysfs, will be converted into a fixed floating point max percent scale. In majority of the cases this will result in correct max. But not 100% of the time. If the _PPC is requested at a point where the calculation lead to a lower max, this can result in a lower P-State then expected and it will impact performance. Example of this condition using a Broadwell laptop with config TDP. ACPI _PSS table from a Broadwell laptop 2301000 2300000 2200000 2000000 1900000 1800000 1700000 1500000 1400000 1300000 1100000 1000000 900000 800000 600000 500000 The actual results by disabling config TDP so that we can get what is requested on or below 2300000Khz. scaling_max_freq Max Requested P-State Resultant scaling max ---------------------------------------- ---------------------- 2400000 18 2900000 (max turbo) 2300000 17 2300000 (max physical non turbo) 2200000 15 2100000 2100000 15 2100000 2000000 13 1900000 1900000 13 1900000 1800000 12 1800000 1700000 11 1700000 1600000 10 1600000 1500000 f 1500000 1400000 e 1400000 1300000 d 1300000 1200000 c 1200000 1100000 a 1000000 1000000 a 1000000 900000 9 900000 800000 8 800000 700000 7 700000 600000 6 600000 500000 5 500000 ------------------------------------------------------------------ Now set the config TDP level 1 ratio as 0x0b (equivalent to 1100000KHz) in BIOS (not every system will let you adjust this). The turbo activation ratio will be set to one less than that, which will be 0x0a (So any request above 1000000KHz should result in turbo region assuming no thermal limits). Here _PPC will request max to 1100000KHz (which basically should still result in turbo as this is more than the turbo activation ratio up to max allowable turbo frequency), but actual calculation resulted in a max ceiling P-State which is 0x0a. So under any load condition, this driver will not request turbo P-States. This will be a huge performance hit. When config TDP feature is ON, if the _PPC points to a frequency above turbo activation ratio, the performance can still reach max turbo. In this case we don't need to treat this as the reduced frequency in set_policy callback. In this change when config TDP is active (by checking if the physical max non turbo ratio is more than the current max non turbo ratio), any request above current max non turbo is treated as full performance. Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com> [ rjw : Minor cleanups ] Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

author	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>	2016-04-27 15:48:07 -0700
committer	Rafael J. Wysocki <rafael.j.wysocki@intel.com>	2016-04-28 01:01:39 +0200
commit	3be9200d512bfa8d6292f5dce6c02aa151821e09 (patch)
tree	b5cb800b3468816fa62b67a26e0603d2ad822b64
parent	9522a2ff9cde26ef48c30e0c9ca9ae4dfb669764 (diff)

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