Messages in this thread Patch in this message | | | Date | Tue, 29 Oct 2019 17:02:10 +0100 | From | Vincent Guittot <> | Subject | Re: [PATCH] Revert "sched/fair: Fix O(nr_cgroups) in the load balancing path" |
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Le Tuesday 29 Oct 2019 à 07:55:26 (-0700), Doug Smythies a écrit : > On 2019.10.28 01:22 Vincent Guittot wrote: > > On Sat, 26 Oct 2019 at 08:59, Doug Smythies <dsmythies@telus.net> wrote: > >> 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). > > > > Ok. > > > > I have studied a bit more your results below and IIUC your problem, > > some periodic wakes up (every 4sec) are missing with kernel 5.4-rc2 > > Actually, I don't know that the periodic wake ups are missing, I only > know that the intel_pstate CPU scaling driver is not being called. > This has been since kernel 5.1-rc1. > I bisected the kernel and found the patch that is this subject. > Then I used kernel 5.4-rc2 as my baseline for the data submitted. > > Looking at the number of clocks cycles that have being used since the last > call to the driver suggests that the CPU has been doing something between > the long time between calls: > For the example given, there were 20,716,861 active clocks in 327.175 > seconds. At 4 seconds per doing something that's 253,286 clocks each, > which is consistent (o.k. the variability is high) with actual data > (and, for example, see the aperf number of 231,813 clocks for the > 4 second example given below). > > > that helps cpuidle to enters deeper idle state after each new wake up > > until reaching the deepest state, isn't it ? > > Well, it is the delay in the intel_pstate driver calls that is the > root issue. Dragging idle into it was just the use case example that > started this investigation.
Ok, I misuderstood your explanation. Your point is that cfs_rq_util_change is not called anymore and as a result intel pstate driver.
Could you try the patch below ? It ensures that at least the root cfs rq stays in the list so each time update_blocked_averages is called, we will call update_cfs_rq_load_avg() for the root cfs_rq at least and even if everything already reach zero. This will ensure that cfs_rq_util_change is called even if nothing has changed.
--- kernel/sched/fair.c | 5 +++++ 1 file changed, 5 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 151c0b7..ac0a549 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7552,6 +7552,8 @@ static inline void update_blocked_load_status(struct rq *rq, bool has_blocked) { static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) { + struct rq *rq = rq_of(cfs_rq); + if (cfs_rq->load.weight) return false; @@ -7564,6 +7566,9 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) if (cfs_rq->avg.runnable_load_sum) return false; + if (cfs_rq == &rq->cfs) + return false; + return true; } -- 2.7.4
> > > My 1st point is that this code doesn't use timer or hrtimer to wake up > > the system but only take advantage of the wake up of something else to > > update the blocked load. So I don't see how this patch could remove > > the 4sec periodic wakeup of the watchdog timer that you are > > mentioning. > > I don't know that it is, as mentioned above. > > > Then, when a system is idle and not used, the load should obviously be > > null most of the time and the update of decayed load should not happen > > anyway. It looks like you take advantage of some spurious and > > un-necessary wake up to help cpuidle to reach deeper idle state. Is > > this understanding correct ? > > I don't know. > > I only know that the call to the intel_pstate driver doesn't > happen, and that it is because cfs_rq_is_decayed returns TRUE. > So, I am asserting that the request is not actually decayed, and > should not have been deleted. Furthermore, I am wondering if other > tasks that should be run are suffering the same fate. > > Now, if we also look back at the comments for the original commit: > > "In an edge case where temporary cgroups were leaking, this > caused the kernel to consume good several tens of percents of > CPU cycles running update_blocked_averages(), each run taking > multiple millisecs." > > To my way of thinking: Fix the leak, don't program around it; The > commit breaks something else, so revert it. > > > > > Then, without or without removing the cfs_rq from the list, we should > > end up with rq->has_blocked_load == 0 and nohz.has_blocked == 0 too. > > The only main impact will be the duration of the loop that can be > > significantly shorter when you have a lot of rqs and cgroups. > > I'm not following. > > > > >>>> > >>>> 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. > > The difference in probabilities of the issue occurring does appear to be > somewhat consistent. Not sure what it means, if anything. > > >>>> 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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