Re: [PATCH v4 00/10] sched/fair: rework the CFS load balance

[Vincent Guittot]

Hi Phil,

On Fri, 25 Oct 2019 at 15:33, Phil Auld <pauld@xxxxxxxxxx> wrote:
>
>
> Hi Vincent,
>
>
> On Thu, Oct 24, 2019 at 04:59:05PM +0200 Vincent Guittot wrote:
> > On Thu, 24 Oct 2019 at 15:47, Phil Auld <pauld@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Oct 24, 2019 at 08:38:44AM -0400 Phil Auld wrote:
> > > > Hi Vincent,
> > > >
> > > > On Mon, Oct 21, 2019 at 10:44:20AM +0200 Vincent Guittot wrote:
> > > > > On Mon, 21 Oct 2019 at 09:50, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> > > > > >
> >
> > [...]
> >
> > > > > > A full run on Mel Gorman's magic scalability test-suite would be super
> > > > > > useful ...
> > > > > >
> > > > > > Anyway, please be on the lookout for such performance regression reports.
> > > > >
> > > > > Yes I monitor the regressions on the mailing list
> > > >
> > > >
> > > > Our kernel perf tests show good results across the board for v4.
> > > >
> > > > The issue we hit on the 8-node system is fixed. Thanks!
> > > >
> > > > As we didn't see the fairness issue I don't expect the results to be
> > > > that different on v4a (with the followup patch) but those tests are
> > > > queued up now and we'll see what they look like.
> > > >
> > >
> > > Initial results with fix patch (v4a) show that the outlier issues on
> > > the 8-node system have returned.  Median time for 152 and 156 threads
> > > (160 cpu system) goes up significantly and worst case goes from 340
> > > and 250 to 550 sec. for both. And doubles from 150 to 300 for 144
> >
> > For v3, you had a x4 slow down IIRC.
> >
>
> Sorry, that was a confusing change of data point :)
>
>
> That 4x was the normal versus group result for v3.  I.e. the usual
> view of this test case's data.
>
> These numbers above are the group vs group difference between
> v4 and v4a.

ok. Thanks for the clarification

>
> The similar data points are that for v4 there was no difference
> in performance between group and normal at 152 threads and a 35%
> drop off from normal to group at 156.
>
> With v4a there was 100% drop (2x slowdown) normal to group at 152
> and close to that at 156 (~75-80% drop off).
>
> So, yes, not as severe as v3. But significantly off from v4.

Thanks for the details

>
> >
> > > threads. These look more like the results from v3.
> >
> > OK. For v3, we were not sure that your UC triggers the slow path but
> > it seems that we have the confirmation now.
> > The problem happens only for this  8 node 160 cores system, isn't it ?
>
> Yes. It only shows up now on this 8-node system.

The input could mean that this system reaches a particular level of
utilization and load that is close to the threshold between 2
different behavior like spare capacity and fully_busy/overloaded case.
But at the opposite, there is less threads that CPUs in your UCs so
one group at least at NUMA level should be tagged as
has_spare_capacity and should pull tasks.

>
> >
> > The fix favors the local group so your UC seems to prefer spreading
> > tasks at wake up
> > If you have any traces that you can share, this could help to
> > understand what's going on. I will try to reproduce the problem on my
> > system
>
> I'm not actually sure the fix here is causing this. Looking at the data
> more closely I see similar imbalances on v4, v4a and v3.
>
> When you say slow versus fast wakeup paths what do you mean? I'm still
> learning my way around all this code.

When task wakes up, we can decide to
- speedup the wakeup and shorten the list of cpus and compare only
prev_cpu vs this_cpu (in fact the group of cpu that share their
respective LLC). That's the fast wakeup path that is used most of the
time during a wakeup
- or start to find the idlest CPU of the system and scan all domains.
That's the slow path that is used for new tasks or when a task wakes
up a lot of other tasks at the same time


>
> This particular test is specifically designed to highlight the imbalance
> cause by the use of group scheduler defined load and averages. The threads
> are mostly CPU bound but will join up every time step. So if each thread

ok the fact that they join up might be the root cause of your problem.
They will wake up at the same time by the same task and CPU.

> more or less gets its own CPU (we run with fewer threads than CPUs) they
> all finish the timestep at about the same time.  If threads are stuck
> sharing cpus then those finish later and the whole computation is slowed
> down.  In addition to the NAS benchmark threads there are 2 stress CPU
> burners. These are either run in their own cgroups (thus having full "load")
> or all in the same cgroup with the benchmarck, thus all having tiny "loads".
>
> In this system, there are 20 cpus per node. We track average number of
> benchmark threads running in each node. Generally for a balanced case
> we should not have any much over 20 and indeed in the normal case (every
> one in one cgroup) we see pretty nice balance. In the cgroup case we are
> still seeing numbers much higher than 20.
>
> Here are some eye charts:
>
> This is the GROUP numbers from that machine on the v1 series (I don't have the
> NORMAL lines handy for this one):
> lu.C.x_152_GROUP_1 Average   18.08  18.17  19.58  19.29  19.25  17.50  21.46  18.67
> lu.C.x_152_GROUP_2 Average   17.12  17.48  17.88  17.62  19.57  17.31  23.00  22.02
> lu.C.x_152_GROUP_3 Average   17.82  17.97  18.12  18.18  24.55  22.18  16.97  16.21
> lu.C.x_152_GROUP_4 Average   18.47  19.08  18.50  18.66  21.45  25.00  15.47  15.37
> lu.C.x_152_GROUP_5 Average   20.46  20.71  27.38  24.75  17.06  16.65  12.81  12.19
>
> lu.C.x_156_GROUP_1 Average   18.70  18.80  20.25  19.50  20.45  20.30  19.55  18.45
> lu.C.x_156_GROUP_2 Average   19.29  19.90  17.71  18.10  20.76  21.57  19.81  18.86
> lu.C.x_156_GROUP_3 Average   25.09  29.19  21.83  21.33  18.67  18.57  11.03  10.29
> lu.C.x_156_GROUP_4 Average   18.60  19.10  19.20  18.70  20.30  20.00  19.70  20.40
> lu.C.x_156_GROUP_5 Average   18.58  18.95  18.63  18.1   17.32  19.37  23.92  21.08
>
> There are a couple that did not balance well but the overall results were good.
>
> This is v4:
> lu.C.x_152_GROUP_1   Average    18.80  19.25  21.95  21.25  17.55  17.25  17.85  18.10
> lu.C.x_152_GROUP_2   Average    20.57  20.62  19.76  17.76  18.95  18.33  18.52  17.48
> lu.C.x_152_GROUP_3   Average    15.39  12.22  13.96  12.19  25.51  28.91  21.88  21.94
> lu.C.x_152_GROUP_4   Average    20.30  19.75  20.75  19.45  18.15  17.80  18.15  17.65
> lu.C.x_152_GROUP_5   Average    15.13  12.21  13.63  11.39  25.42  30.21  21.55  22.46
> lu.C.x_152_NORMAL_1  Average    17.00  16.88  19.52  18.28  19.24  19.08  21.08  20.92
> lu.C.x_152_NORMAL_2  Average    18.61  16.56  18.56  17.00  20.56  20.28  20.00  20.44
> lu.C.x_152_NORMAL_3  Average    19.27  19.77  21.23  20.86  18.00  17.68  17.73  17.45
> lu.C.x_152_NORMAL_4  Average    20.24  19.33  21.33  21.10  17.33  18.43  17.57  16.67
> lu.C.x_152_NORMAL_5  Average    21.27  20.36  20.86  19.36  17.50  17.77  17.32  17.55
>
> lu.C.x_156_GROUP_1   Average    18.60  18.68  21.16  23.40  18.96  19.72  17.76  17.72
> lu.C.x_156_GROUP_2   Average    22.76  21.71  20.55  21.32  18.18  16.42  17.58  17.47
> lu.C.x_156_GROUP_3   Average    13.62  11.52  15.54  15.58  25.42  28.54  23.22  22.56
> lu.C.x_156_GROUP_4   Average    17.73  18.14  21.95  21.82  19.73  19.68  18.55  18.41
> lu.C.x_156_GROUP_5   Average    15.32  15.14  17.30  17.11  23.59  25.75  20.77  21.02
> lu.C.x_156_NORMAL_1  Average    19.06  18.72  19.56  18.72  19.72  21.28  19.44  19.50
> lu.C.x_156_NORMAL_2  Average    20.25  19.86  22.61  23.18  18.32  17.93  16.39  17.46
> lu.C.x_156_NORMAL_3  Average    18.84  17.88  19.24  17.76  21.04  20.64  20.16  20.44
> lu.C.x_156_NORMAL_4  Average    20.67  19.44  20.74  22.15  18.89  18.85  18.00  17.26
> lu.C.x_156_NORMAL_5  Average    20.12  19.65  24.12  24.15  17.40  16.62  17.10  16.83
>
> This one is better overall, but there are some mid 20s abd 152_GROUP_5 is pretty bad.
>
>
> This is v4a
> lu.C.x_152_GROUP_1   Average    28.64  34.49  23.60  24.48  10.35  11.99  8.36  10.09
> lu.C.x_152_GROUP_2   Average    17.36  17.33  15.48  13.12  24.90  24.43  18.55  20.83
> lu.C.x_152_GROUP_3   Average    20.00  19.92  20.21  21.33  18.50  18.50  16.50  17.04
> lu.C.x_152_GROUP_4   Average    18.07  17.87  18.40  17.87  23.07  22.73  17.60  16.40
> lu.C.x_152_GROUP_5   Average    25.50  24.69  21.48  21.46  16.85  16.00  14.06  11.96
> lu.C.x_152_NORMAL_1  Average    22.27  20.77  20.60  19.83  16.73  17.53  15.83  18.43
> lu.C.x_152_NORMAL_2  Average    19.83  20.81  23.06  21.97  17.28  16.92  15.83  16.31
> lu.C.x_152_NORMAL_3  Average    17.85  19.31  18.85  19.08  19.00  19.31  19.08  19.54
> lu.C.x_152_NORMAL_4  Average    18.87  18.13  19.00  20.27  18.20  18.67  19.73  19.13
> lu.C.x_152_NORMAL_5  Average    18.16  18.63  18.11  17.00  19.79  20.63  19.47  20.21
>
> lu.C.x_156_GROUP_1   Average    24.96  26.15  21.78  21.48  18.52  19.11  12.98  11.02
> lu.C.x_156_GROUP_2   Average    18.69  19.00  18.65  18.42  20.50  20.46  19.85  20.42
> lu.C.x_156_GROUP_3   Average    24.32  23.79  20.82  20.95  16.63  16.61  18.47  14.42
> lu.C.x_156_GROUP_4   Average    18.27  18.34  14.88  16.07  27.00  21.93  20.56  18.95
> lu.C.x_156_GROUP_5   Average    19.18  20.99  33.43  29.57  15.63  15.54  12.13  9.53
> lu.C.x_156_NORMAL_1  Average    21.60  23.37  20.11  19.60  17.11  17.83  18.17  18.20
> lu.C.x_156_NORMAL_2  Average    21.00  20.54  19.88  18.79  17.62  18.67  19.29  20.21
> lu.C.x_156_NORMAL_3  Average    19.50  19.94  20.12  18.62  19.88  19.50  19.00  19.44
> lu.C.x_156_NORMAL_4  Average    20.62  19.72  20.03  22.17  18.21  18.55  18.45  18.24
> lu.C.x_156_NORMAL_5  Average    19.64  19.86  21.46  22.43  17.21  17.89  18.96  18.54
>
>
> This shows much more imblance in the GROUP case. There are some single digits
> and some 30s.
>
> For comparison here are some from my 4-node (80 cpu) system:
>
> v4
> lu.C.x_76_GROUP_1.ps.numa.hist   Average    19.58  17.67  18.25  20.50
> lu.C.x_76_GROUP_2.ps.numa.hist   Average    19.08  19.17  17.67  20.08
> lu.C.x_76_GROUP_3.ps.numa.hist   Average    19.42  18.58  18.42  19.58
> lu.C.x_76_NORMAL_1.ps.numa.hist  Average    20.50  17.33  19.08  19.08
> lu.C.x_76_NORMAL_2.ps.numa.hist  Average    19.45  18.73  19.27  18.55
>
>
> v4a
> lu.C.x_76_GROUP_1.ps.numa.hist   Average    19.46  19.15  18.62  18.77
> lu.C.x_76_GROUP_2.ps.numa.hist   Average    19.00  18.58  17.75  20.67
> lu.C.x_76_GROUP_3.ps.numa.hist   Average    19.08  17.08  20.08  19.77
> lu.C.x_76_NORMAL_1.ps.numa.hist  Average    18.67  18.93  18.60  19.80
> lu.C.x_76_NORMAL_2.ps.numa.hist  Average    19.08  18.67  18.58  19.67
>
> Nicely balanced in both kernels and normal and group are basically the
> same.

That fact that the 4 nodes works well but not the 8 nodes is a bit
surprising except if this means more NUMA level in the sched_domain
topology
Could you give us more details about the sched domain topology ?

>
> There's still something between v1 and v4 on that 8-node system that is
> still illustrating the original problem.  On our other test systems this
> series really works nicely to solve this problem. And even if we can't get
> to the bottom if this it's a significant improvement.
>
>
> Here is v3 for the 8-node system
> lu.C.x_152_GROUP_1  Average    17.52  16.86  17.90  18.52  20.00  19.00  22.00  20.19
> lu.C.x_152_GROUP_2  Average    15.70  15.04  15.65  15.72  23.30  28.98  20.09  17.52
> lu.C.x_152_GROUP_3  Average    27.72  32.79  22.89  22.62  11.01  12.90  12.14  9.93
> lu.C.x_152_GROUP_4  Average    18.13  18.87  18.40  17.87  18.80  19.93  20.40  19.60
> lu.C.x_152_GROUP_5  Average    24.14  26.46  20.92  21.43  14.70  16.05  15.14  13.16
> lu.C.x_152_NORMAL_1 Average    21.03  22.43  20.27  19.97  18.37  18.80  16.27  14.87
> lu.C.x_152_NORMAL_2 Average    19.24  18.29  18.41  17.41  19.71  19.00  20.29  19.65
> lu.C.x_152_NORMAL_3 Average    19.43  20.00  19.05  20.24  18.76  17.38  18.52  18.62
> lu.C.x_152_NORMAL_4 Average    17.19  18.25  17.81  18.69  20.44  19.75  20.12  19.75
> lu.C.x_152_NORMAL_5 Average    19.25  19.56  19.12  19.56  19.38  19.38  18.12  17.62
>
> lu.C.x_156_GROUP_1  Average    18.62  19.31  18.38  18.77  19.88  21.35  19.35  20.35
> lu.C.x_156_GROUP_2  Average    15.58  12.72  14.96  14.83  20.59  19.35  29.75  28.22
> lu.C.x_156_GROUP_3  Average    20.05  18.74  19.63  18.32  20.26  20.89  19.53  18.58
> lu.C.x_156_GROUP_4  Average    14.77  11.42  13.01  10.09  27.05  33.52  23.16  22.98
> lu.C.x_156_GROUP_5  Average    14.94  11.45  12.77  10.52  28.01  33.88  22.37  22.05
> lu.C.x_156_NORMAL_1 Average    20.00  20.58  18.47  18.68  19.47  19.74  19.42  19.63
> lu.C.x_156_NORMAL_2 Average    18.52  18.48  18.83  18.43  20.57  20.48  20.61  20.09
> lu.C.x_156_NORMAL_3 Average    20.27  20.00  20.05  21.18  19.55  19.00  18.59  17.36
> lu.C.x_156_NORMAL_4 Average    19.65  19.60  20.25  20.75  19.35  20.10  19.00  17.30
> lu.C.x_156_NORMAL_5 Average    19.79  19.67  20.62  22.42  18.42  18.00  17.67  19.42
>
>
> I'll try to find pre-patched results for this 8 node system.  Just to keep things
> together for reference here is the 4-node system before this re-work series.
>
> lu.C.x_76_GROUP_1  Average    15.84  24.06  23.37  12.73
> lu.C.x_76_GROUP_2  Average    15.29  22.78  22.49  15.45
> lu.C.x_76_GROUP_3  Average    13.45  23.90  22.97  15.68
> lu.C.x_76_NORMAL_1 Average    18.31  19.54  19.54  18.62
> lu.C.x_76_NORMAL_2 Average    19.73  19.18  19.45  17.64
>
> This produced a 4.5x slowdown for the group runs versus the nicely balance
> normal runs.
>
>
>
> I can try to get traces but this is not my system so it may take a little
> while. I've found that the existing trace points don't give enough information
> to see what is happening in this problem. But the visualization in kernelshark
> does show the problem pretty well. Do you want just the existing sched tracepoints
> or should I update some of the traceprintks I used in the earlier traces?

The standard tracepoint is a good starting point but tracing the
statistings for find_busiest_group and find_idlest_group should help a
lot.

Cheers,
Vincent

>
>
>
> Cheers,
> Phil
>
>
> >
> > >
> > > We're re-running the test to get more samples.
> >
> > Thanks
> > Vincent
> >
> > >
> > >
> > > Other tests and systems were still fine.
> > >
> > >
> > > Cheers,
> > > Phil
> > >
> > >
> > > > Numbers for my specific testcase (the cgroup imbalance) are basically
> > > > the same as I posted for v3 (plus the better 8-node numbers). I.e. this
> > > > series solves that issue.
> > > >
> > > >
> > > > Cheers,
> > > > Phil
> > > >
> > > >
> > > > >
> > > > > >
> > > > > > Also, we seem to have grown a fair amount of these TODO entries:
> > > > > >
> > > > > >   kernel/sched/fair.c: * XXX borrowed from update_sg_lb_stats
> > > > > >   kernel/sched/fair.c: * XXX: only do this for the part of runnable > running ?
> > > > > >   kernel/sched/fair.c:     * XXX illustrate
> > > > > >   kernel/sched/fair.c:    } else if (sd_flag & SD_BALANCE_WAKE) { /* XXX always ? */
> > > > > >   kernel/sched/fair.c: * can also include other factors [XXX].
> > > > > >   kernel/sched/fair.c: * [XXX expand on:
> > > > > >   kernel/sched/fair.c: * [XXX more?]
> > > > > >   kernel/sched/fair.c: * [XXX write more on how we solve this.. _after_ merging pjt's patches that
> > > > > >   kernel/sched/fair.c:             * XXX for now avg_load is not computed and always 0 so we
> > > > > >   kernel/sched/fair.c:            /* XXX broken for overlapping NUMA groups */
> > > > > >
> > > > >
> > > > > I will have a look :-)
> > > > >
> > > > > > :-)
> > > > > >
> > > > > > Thanks,
> > > > > >
> > > > > >         Ingo
> > > >
> > > > --
> > > >
> > >
> > > --
> > >
>
> --
>