Re: [RFC PATCH v7] sched/fair: select idle cpu from idle cpumask for task wakeup

[Vincent Guittot]

On Fri, 11 Dec 2020 at 23:50, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Dec 11, 2020 at 11:19:05PM +0100, Peter Zijlstra wrote:
> > On Fri, Dec 11, 2020 at 08:43:37PM +0000, Mel Gorman wrote:
> > > One bug is in __select_idle_core() though. It's scanning the SMT mask,
> > > not select_idle_mask so it can return an idle candidate that is not in
> > > p->cpus_ptr.
> >
> > D'0h.. luckily the benchmarks don't hit that :-)
> >
>
> Yep, neither do mine for the most part which is why I ran it as-is but
> eventually someone would complain that sched_setscheduler was being
> ignored.
>
> Trick is whether a failed check should continue searching for an idle core
> or terminate early and just pick an allowed idle CPU. I tend to favour
> the latter but it's hard to predict what sort of reasonable workloads
> would be affected.
>
> > > There are some other potential caveats.
> > >
> > > This is a single pass so when test_idle_cores() is true, __select_idle_core
> > > is used to to check all the siblings even if the core is not idle. That
> > > could have been cut short if __select_idle_core checked *idle_cpu ==
> > > 1 and terminated the SMT scan if an idle candidate had already been found.
> >
> > So I did that on purpose, so as to track the last/most-recent idle cpu,
> > with the thinking that that cpu has the higher chance of still being
> > idle vs one we checked earlier/longer-ago.
> >
> > I suppose we benchmark both and see which is liked best.
> >
>
> I originally did something like that on purpose too but Vincent called
> it out so it is worth mentioning now to avoid surprises. That said, I'm
> at the point where anything SIS-related simply needs excessive exposure
> because no matter what it does, someone is unhappy with it.

Yeah, I don't like extending the idle core search loop for something
that is not related to the idle core search. This adds more work out
of  control of the sis_prop. So I'm still against hiding some idle cpu
search in idle core search.

>
> > > Second downside is related. If test_idle_cpus() was true but no idle
> > > CPU is found then __select_idle_core has been called enough to scan
> > > the entire domain. In this corner case, the new code does *more* work
> > > because the old code would have failed select_idle_core() quickly and
> > > then select_idle_cpu() would be throttled by SIS_PROP. I think this will
> > > only be noticable in the heavily overloaded case but if the corner case
> > > hits enough then the new code will be slower than the old code for the
> > > over-saturated case (i.e. hackbench with lots of groups).
> >
> > Right, due to scanning siblings, even if the first inspected thread is
> > not idle, we scan more.
> >
>
> Yep.
>
> > > The third potential downside is that the SMT sibling is not guaranteed to
> > > be checked due to SIS_PROP throttling but in the old code, that would have
> > > been checked by select_idle_smt(). That might result in premature stacking
> > > of runnable tasks on the same CPU. Similarly, as __select_idle_core may
> > > find multiple idle candidates, it will not pick the targets SMT sibling
> > > if it is idle like select_idle_smt would have.
> > >
> > > That said, I am skeptical that select_idle_smt() matters all that often.
> >
> > This, I didn't really believe in it either.
> >
>
> Good because I think any benefit from select_idle_smt is so marginal
> that it should be ignored if the full scan is simpler overall.
>
> > The benchmarks I started are mostly noise, with a few wins for
> > TCP_STREAM and UDP_RR around the 50% mark. Although I should run
> > longer variants to make sure.
>
> So far I have one benchmark from one machine. It's tbench again because
> it's a reasonable communicating workload that is trivial to vary the
> level of utilisation.
>
> 80-cpu CascadeLake, 2 sockets, HT enabled
> tbench4
>                           5.10.0-rc6             5.10.0-rc6             5.10.0-rc6
>                        baseline-v1r1        singlescan-v1r1        singlescan-v1r3
> Hmean     1        503.90 (   0.00%)      505.19 *   0.26%*      499.20 *  -0.93%*
> Hmean     2        980.80 (   0.00%)      975.15 *  -0.58%*      983.79 *   0.31%*
> Hmean     4       1912.37 (   0.00%)     1883.25 *  -1.52%*     1923.76 *   0.60%*
> Hmean     8       3741.47 (   0.00%)     3568.66 *  -4.62%*     3690.60 *  -1.36%*
> Hmean     16      6552.90 (   0.00%)     6549.97 *  -0.04%*     6478.37 *  -1.14%*
> Hmean     32     10217.34 (   0.00%)    10266.66 *   0.48%*    10291.60 *   0.73%*
> Hmean     64     13604.93 (   0.00%)    11240.88 * -17.38%*    12045.87 * -11.46%*
> Hmean     128    21194.11 (   0.00%)    21316.08 *   0.58%*    21868.39 *   3.18%*
> Hmean     256    21163.19 (   0.00%)    20989.14 *  -0.82%*    20831.20 *  -1.57%*
> Hmean     320    20906.29 (   0.00%)    21024.11 *   0.56%*    20756.88 *  -0.71%*
> Stddev    1          3.14 (   0.00%)        1.17 (  62.93%)        1.05 (  66.61%)
> Stddev    2          4.44 (   0.00%)        3.72 (  16.35%)        2.20 (  50.56%)
> Stddev    4          9.09 (   0.00%)       18.67 (-105.32%)        3.66 (  59.71%)
> Stddev    8         12.87 (   0.00%)       18.96 ( -47.31%)       11.90 (   7.58%)
> Stddev    16         8.34 (   0.00%)        8.77 (  -5.22%)       36.25 (-334.84%)
> Stddev    32        27.05 (   0.00%)       20.90 (  22.74%)       28.57 (  -5.61%)
> Stddev    64       720.66 (   0.00%)       20.12 (  97.21%)       32.10 (  95.55%)
> Stddev    128       17.49 (   0.00%)       52.33 (-199.22%)      137.68 (-687.25%)
> Stddev    256       57.17 (   0.00%)       18.87 (  67.00%)       38.98 (  31.81%)
> Stddev    320       29.87 (   0.00%)       46.49 ( -55.64%)       31.48 (  -5.39%)
>
>                   5.10.0-rc6  5.10.0-rc6  5.10.0-rc6
>                 baseline-v1r1singlescan-v1r1singlescan-v1r3
> Duration User        9771.18     9435.64     9353.29
> Duration System     34224.28    33829.01    33802.34
> Duration Elapsed     2218.87     2218.87     2218.69
>
> baseline is roughly what's in tip for 5.11-rc1 with patches 1-2 from my
> series like you had.
>
> singlescan-v1r1 is your patch
>
> singlescan-v1r3 is your patch with my "untested" patch on top that
> enforces p->cpus_ptr and short-cuts corner cases.
>
> Few points of interest
>
> 1. 64 clients regresses. With 64 clients, this is roughly the point
>    where test_idle_cores() returns false positives and we hit the worst
>    corner cases. Your patch regresses 17%, mine is only a marginal
>    improvement and still a regression slower.
>
> 2. Variations are all over the place. Your patch is great at low
>    utilisation and stabilises overall. After that, your milage varies a lot
>
> 3. The system CPu usage summarised over the entire workload drops quite
>    a bit. Whether it's your patch or minor changes on top, there is less
>    work being done within the kernel overall
>
> A wider battery of tests is still running and a second set is queued
> that adds the debugging schedstats but they take ages to run.
>
> I'm currently on "holidays" so response time will be variable :P
>
> --
> Mel Gorman
> SUSE Labs