On 2011.06.06 at 18:34 +0200, Vincent Guittot wrote: > On 6 June 2011 16:16, Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> wrote: > > On 2011.06.06 at 15:11 +0200, Vincent Guittot wrote: > >> On 6 June 2011 13:20, Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> wrote: > >> > On 2011.06.06 at 09:35 +0200, Vincent Guittot wrote: > >> >> On 2 June 2011 13:41, Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> wrote: > >> >> > On 2011.06.01 at 20:00 +0200, Markus Trippelsdorf wrote: > >> >> >> But I have found the root cause of symptoms described above by > >> >> >> bisection. It turned out that 2.6.39 is also affected, so I've bisected > >> >> >> down to 2.6.38. > >> >> >> This is the result: > >> >> >> > >> >> >> 5cb2c3bd0c5e0f3ced63f250ec2ad59d7c5c626a is the first bad commit > >> >> >> commit 5cb2c3bd0c5e0f3ced63f250ec2ad59d7c5c626a > >> >> >> Author: Vincent Guittot <vincent.guittot@xxxxxxxxxx> > >> >> >> Date: Mon Feb 7 17:14:25 2011 +0100 > >> >> >> > >> >> >> [CPUFREQ] calculate delay after dbs_check_cpu > >> >> >> > >> >> >> When I revert the above in 3.0-rc1 the CONFIG_NO_HZ=y symptoms vanish. > >> >> > > >> >> > >> >> The patch, you have mentioned, solves a problem when ondemand governor > >> >> goes from highest frequency to a lower one. Without the patch, the > >> >> governor uses the longest sampling period (sampling period * scaling > >> >> down factor) with a low frequency during the 1st period after > >> >> decreasing the frequency. This can lead to a large time frame > >> >> (sampling period * scaling down factor) with a low frequency but an > >> >> overloaded cpu. > >> > > >> > The problem with the patch is that it results in an ondemand behavior > >> > that almost totally ignores the middle frequencies (2100 and 2500 MHz in > >> > my case) with CONFIG_NO_HZ. If you also set the sampling_down_factor to > >> > something like >=100 then the CPU will spend much of the time at the top > >> > frequency even if there is no workload whatsoever. > >> > > >> > >> In fact, one main goal of the ondemand governor is to switch to max > >> frequency as soon as there is a cpu activity is detected to ensure the > >> responsiveness of the system. If your idle activity is made of burst > >> of cpu activity and your sampling period is small, your sytems will > >> switch between the highest and the lowest frequency. At the contrary, > >> the conservative governor modifies the frequency in a step by step > >> manner. > > > > Understood. But this a change in behavior due to your patch. > > > >> >> The other correction of the patch is linked to the powersave bias > >> >> mode. The governor didn't use the right period for the low frequency > >> >> step (freq_lo_jiffies) but a larger one (sampling period * scaling > >> >> down factor). The ratio between low and high frequency was not the > >> >> right one. > >> >> > >> >> Do you use the powersave bias mode ? > >> > > >> > No. > >> > > >> >> Could you give us more statistics : the number of state transition > >> >> could be an interesting value. Is there a difference with and without > >> >> CONFIG_NO_HZ ? What is your sampling rate ? > >> > > >> > These are my settings: > >> > > >> > ignore_nice_load 0 > >> > io_is_busy 0 > >> > powersave_bias 0 > >> > sampling_down_factor 200 > >> > sampling_rate 10000 > >> > sampling_rate_min 10000 > >> > up_threshold 95 > >> > > >> > cat sys/devices/system/cpu/cpu0/cpufreq/stats/* on an otherwise idle > >> > machine with CONFIG_NO_HZ and 5cb2c3bd0c5e0f reverted: > >> > 3200000 532 > >> > 2500000 172 > >> > 2100000 2703 > >> > 800000 20995 > >> > 153 > >> > > >> > >> With this configuration (without the patch), there is a period of 2 > >> seconds with a low frequency when the governor comes back from the > >> highest frequency. During these 2 seconds, you will not be able to go > >> back to max frequency. So, if your cpu is overloaded during this 2 > >> seconds period, you will not increase your frequency. For this use > >> case, your cpufreq responsiveness is more then 2 seconds. > > > > I don't see these 2 second delays (being stuck on a low frequency) on my > > system. On the contrary as soon as there is sufficient load it switches > > to the highest frequency immediately. > > > > Let assume that your system is at the highest frequency > > without the patch, you have the following sequence : > > ->do_dbs_timer > -> delay = usecs_to_jiffies(dbs_tuners_ins.sampling_rate * > dbs_info->rate_mult); // delay will be equal to 10000*200=2000000us > -> dbs_check_cpu > Let assume that your cpu load is quite small > -> freq_next = max_load_freq / (dbs_tuners_ins.up_threshold > - dbs_tuners_ins.down_differential); //freq_next is set to your lowest > frequency > -> __cpufreq_driver_target(policy, freq_next, CPUFREQ_RELATION_L); > -> queue_delayed_work_on(cpu, kondemand_wq, &dbs_info->work, delay); > > the delay value is set to sampling_rate * rate_mult but the frequency > is the lowest one which is not the correct behavior of the > sampling_down_factor feature. > the patch only solves this issue. > > >> > and with your patch and also CONFIG_NO_HZ: > >> > 3200000 11795 > >> > 2500000 0 > >> > 2100000 0 > >> > 800000 20620 > >> > 213 > >> > > >> > Which shows the problem very nicely. > >> > > >> > >> My understand is that your idle activity is made of cpu activities > >> which are 10ms long and which trigs the increase of the frequency. > > > > Could it be that the call to dbs_check_cpu(dbs_info) itself is the > > reason for these activities? > > > >> >> One difference with CONFIG_NO_HZ is the real sampling period which can > >> >> be greater than the timer configuration because of the deferrable > >> >> mode. The deferrable mode has nearly no effect when CONFIG_NO_HZ is > >> >> not set because the tick timer will ensure enough cpu activity to > >> >> trigger the governor. When CONFIG_NO_HZ is set, the ondemand governor > >> >> work is triggered at the beginning of a cpu activity so we have more > >> >> chance to have a short cpu load in one period instead of splitting it > >> >> into 2 differents periods. This behavior is quite useful for > >> >> responsiveness but can generates spurious frequency increase if the > >> >> sampling rate is too short. > >> > > >> > Hm, my sampling rate (10000) is already the most minimal rate available. > >> > > >> > >> It's seems that your sampling period is too small and the ondemand > >> governor detects your idle activity as an increase of the cpu activity > >> and as a result, it increases the frequency. Have you tried to > >> increase the sampling rate and decrease your sampling_down_factor > >> which seems to be also quite high ? > > > > Please note that these are all default values (with the exception of > > sampling_down_factor). So why should I fiddle with the parameters when > > everything was working fine before your patch went in? And even if I > > increase the sampling rate and decrease the sampling_down_factor, I > > cannot replicate the old behavior. So IMHO it's a regression. > > > > IMHO, the previous results were "good" because of the bug in the > sampling_down_factor which was "filtering" some cpu activities after > decreasing the frequency. > > The best cpufreq statistic should be achieved in idle when the > sampling_down_factor is set to 1 because the sampling_down_factor > feature has been done to "improve performance by reducing the overhead > of load evaluation and helping the CPU stay at its top speed" > (Documentation/cpu-freq/governors.txt). > > Could you make some measurements with sampling_down_factor set to 1 > and sampling_down_factor set to 200 ? The cpufreq statistic starts at > system boot but we are interested in idle use case result so we should > use the delta between 2 statistics outputs in order to remove boot > measurements. Using the following command in idle should be enough # > cat /sys/devices/system/cpu/cpu0/cpufreq/stats/* && sleep 60 && cat > /sys/devices/system/cpu/cpu0/cpufreq/stats/* OK. On a totally idle system: 1) With your patch: * sampling_down_factor=200 cat /sys/devices/system/cpu/cpu0/cpufreq/stats/* && sleep 60 && cat /sys/devices/system/cpu/cpu0/cpufreq/stats/* 3200000 507 2500000 0 2100000 0 800000 903 13 3200000 533 2500000 0 2100000 0 800000 6876 14 diff: 3200000 26 2500000 0 2100000 0 800000 5973 * sampling_down_factor=1 3200000 1078 2500000 3 2100000 49 800000 15632 79 3200000 1078 2500000 3 2100000 49 800000 21632 79 diff: 3200000 0 2500000 0 2100000 0 800000 6000 2) Without your patch (reverted): * sampling_down_factor=200 3200000 106 2500000 0 2100000 339 800000 1260 15 3200000 106 2500000 0 2100000 339 800000 7259 15 diff: 3200000 0 2500000 0 2100000 0 800000 5999 * sampling_down_factor=1 3200000 134 2500000 142 2100000 694 800000 13006 30 3200000 134 2500000 142 2100000 694 800000 19005 30 diff: 3200000 0 2500000 0 2100000 0 800000 5999 And now the same measurements while running: watch -n.1 'cat /proc/cpuinfo|grep MHz' in another terminal. 1) With your patch: * sampling_down_factor=200 3200000 1243 2500000 4 2100000 68 800000 36493 187 3200000 1373 2500000 4 2100000 68 800000 42363 192 diff: 3200000 130 2500000 0 2100000 0 800000 5870 * sampling_down_factor=1 3200000 1205 2500000 4 2100000 67 800000 27873 171 3200000 1209 2500000 4 2100000 67 800000 33869 179 diff: 3200000 4 2500000 0 2100000 0 800000 5996 2) Without your patch (reverted): * sampling_down_factor=200 3200000 240 2500000 0 2100000 505 800000 12842 41 3200000 245 2500000 0 2100000 505 800000 18836 51 diff: 3200000 5 2500000 0 2100000 0 800000 5994 * sampling_down_factor=1 3200000 230 2500000 0 2100000 505 800000 5497 31 3200000 234 2500000 0 2100000 505 800000 11493 39 diff: 3200000 4 2500000 0 2100000 0 800000 5996 So, with sampling_down_factor=200 and "watch -n.1" running, the CPU spends 1300 msec on top speed vs. 50 msec without your patch. BTW what irritates me is that "watch -n.1 'cat /proc/cpuinfo|grep MHz'" shows way more frequency changes than what is reported in cpufreq/stats/. -- Markus -- To unsubscribe from this list: send the line "unsubscribe cpufreq" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
