Messages in this thread | | | From | "Doug Smythies" <> | Subject | RE: [PATCH] Revert "sched/fair: Fix O(nr_cgroups) in the load balancing path" | Date | Fri, 25 Oct 2019 23:59:41 -0700 |
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Hi Vincent,
Thank you for your quick reply.
On 2010.10.25 09:51 Vincent Guittot wrote: > On Fri, 25 Oct 2019 at 17:55, Doug Smythies <doug.smythies@gmail.com> wrote: >> >> This reverts commit 039ae8bcf7a5f4476f4487e6bf816885fb3fb617, >> which, in turn, was a re-apply of >> commit a9e7f6544b9c ("sched/fair: Fix O(nr_cgroups) in load balance path") >> after it was reverted via >> commit c40f7d74c741 ("sched/fair: Fix infinite loop in update_blocked_averages() by reverting a9e7f6544b9c") >> >> For an idle system, the cfs_rq_is_decayed function components can underflow to 0 and >> incorrectly return TRUE, when the item should not be deleted from the list. > > The patch from Rik solves the problem of cfs_rq_is_decayed wrongly returns true > https://lore.kernel.org/lkml/20190906191237.27006-6-riel@surriel.com/
Not for my use case.
I applied Rik's patch to kernel 5.4-rc2 (since all my other reference test data had been acquired against a base of 5.4-rc2). Tests results are similar to the non-reverted kernel (results added in-line below).
>> >> Signed-off-by: Doug Smythies <dsmythies@telus.net> >> Cc: Vincent Guittot <vincent.guittot@linaro.org> >> Cc: Peter Zijlstra (Intel) <peterz@infradead.org> >> Cc: Ingo Molnar <mingo@kernel.org> >> Cc: Linus Torvalds <torvalds@linux-foundation.org> >> Cc: Peter Zijlstra <peterz@infradead.org> >> Cc: Thomas Gleixner <tglx@linutronix.de> >> Cc: sargun@sargun.me >> Cc: tj@kernel.org >> Cc: xiexiuqi@huawei.com >> Cc: xiezhipeng1@huawei.com >> Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com> >> >> --- >> Note 1: Both this reversion and just deleting the cfs_rq_is_decayed function >> and it's call and leaving the other changes have been tested. I do not know >> which solution is better. (ie for the "list_for_each_entry_rcu" part of it.) >> >> Note 2: Previous controversy over this patch was based on heavy workloads, >> but this is based on minimal or no workload, or "idle". >> Where "idle" on my test server, with no gui and many services disabled, >> tends to mean more "idle" than most systems. >> >> Note 3: While this supporting data only involves the intel_pstate CPU >> frequency scaling driver as a casualty, it is beyond my capabilities >> to determine what other tasks that should be running might be omitted. >> >> Use case example 1: >> System Idle: The intel pstate CPU frequency scaling driver: >> Mode: Active, non-hwp, powersave governor. >> Expected behaviour: There is never ever a duration (time between calls to >> the driver / per CPU) longer than 4 seconds (the watchdog time, I think). >> Actual behaviour: There are long long gaps between calls to the driver: >> >> Kernel: 5.4-rc2 CPU:7 >> duration: 327.17 Seconds. (this is one of many hundreds of examples.) >> mpref: 44023326 >> apref: 20716861 >> tsc: 1.11604E+12 >> load: 0 >> CPU frequency: 1.6053 GHz (average over this 327 second sample period). >> old pstate: 16 (the lowest for my processor) >> new pstate: 16 >> >> Kernel: 5.4-rc2 + reversion (either method) >> After several hours of testing, maximum durations were never more >> than 4 seconds (well plus some jitter). >> reversion method: max=4.07908 seconds >> CPU:7 >> mperf: 492578 >> apref: 231813 (56,829 per second average is consistent with other tests) >> tsc: 13914264074 >> load: 0 >> CPU frequency: 1.6052 GHz >> old pstate: 16 (the lowest for my precessor) >> new pstate: 16 >> >> On average, the non-reverted kernel executes the driver 25% less >> than the reverted kernel during idle.
On (shorter)average, the Rik patched kernel executes the driver 14% less than the reverted kernel during idle.
Longer and repeated testing would be required to determine if this is a trend or simply non-repeatable noise.
>> O.K. so who cares, the requested pstate doesn't change? >> First, one wonders if the math could overflow. >> (although 7180ddd suggests maybe it won't) >> Second, the sample is largely dominated by obsolete information. >> Third, this can be problematic, and potentially wastes energy, >> for the busy to idle transition. >> >> Use case example 2: >> The busy to idle transition: >> >> Typically, the pstate request response to a busy to idle transition >> is very slow because the duration suddenly goes from, typically, >> 10 milliseconds to much much longer, up to 4 seconds. Transition >> times to the system being fully idle, with all requested pstates >> being at minimum, takes around 8 seconds with this reversion, >> and, potentially, a very very long time (over 100 seconds has been >> measured) without. >> >> Again, so who cares, if the processor is in a deep idle state anyway, >> not consuming much energy? O.K. but what if it is in an idle state >> where energy consumption is a function of the requested pstate? >> For example, for my processor (i7-2600K), idle state 1, then processor >> package energy can be over double what it should be for many 10s of >> seconds. >> >> Experiment method: >> >> enable only idle state 1 >> Dountil stopped >> apply a 100% load (all CPUs) >> after awhile (about 50 seconds) remove the load. >> allow a short transient delay (1 second). >> measure the processor package joules used over the next 149 seconds. >> Enduntil >> >> Kernel k5.4-rc2 + reversion (this method) >> Average processor package power: 9.148 watts (128 samples, > 7 hours) >> Minimum: 9.02 watts >> Maximum: 9.29 watts >> Note: outlyer data point group removed, as it was assumed the computer >> had something to do and wasn't actually "idle". >> >> Kernel 5.4-rc2: >> Average processor package power: 9.969 watts (150 samples, > 8 hours) >> Or 9% more energy for the idle phases of the work load. >> Minimum: 9.15 watts >> Maximum: 13.79 watts (51% more power)
Kernel 5.4-rc2 + Rik-patch: Average processor package power: 9.85 watts (53 samples, < 3 hours) Or 7.7% more energy for the idle phases of the work load. Minimum: 9.23 watts Maximum: 12.79 watts (40% more power)
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