From: Dario Faggioli <firstname.lastname@example.org> To: email@example.com Cc: George Dunlap <firstname.lastname@example.org>, Ian Jackson <email@example.com> Subject: [Bugfix PATCH for-4.15] xen: credit2: fix per-entity load tracking when continuing running Date: Fri, 19 Mar 2021 12:14:17 +0000 [thread overview] Message-ID: <161615605709.5036.4052641880659992679.stgit@Wayrath> (raw) If we schedule, and the current vCPU continues to run, its statistical load is not properly updated, resulting in something like this, even if all the 8 vCPUs are 100% busy: (XEN) Runqueue 0: (XEN) [...] (XEN) aveload = 2097152 (~800%) (XEN) [...] (XEN) Domain: 0 w 256 c 0 v 8 (XEN) 1: [0.0] flags=2 cpu=4 credit=9996885 [w=256] load=35 (~0%) (XEN) 2: [0.1] flags=2 cpu=2 credit=9993725 [w=256] load=796 (~0%) (XEN) 3: [0.2] flags=2 cpu=1 credit=9995885 [w=256] load=883 (~0%) (XEN) 4: [0.3] flags=2 cpu=5 credit=9998833 [w=256] load=487 (~0%) (XEN) 5: [0.4] flags=2 cpu=6 credit=9998942 [w=256] load=1595 (~0%) (XEN) 6: [0.5] flags=2 cpu=0 credit=9994669 [w=256] load=22 (~0%) (XEN) 7: [0.6] flags=2 cpu=7 credit=9997706 [w=256] load=0 (~0%) (XEN) 8: [0.7] flags=2 cpu=3 credit=9992440 [w=256] load=0 (~0%) As we can see, the average load of the runqueue as a whole is, instead, computed properly. This issue would, in theory, potentially affect Credit2 load balancing logic. In practice, however, the problem only manifests (at least with these characteristics) when there is only 1 runqueue active in the cpupool, which also means there is no need to do any load-balancing. Hence its real impact is pretty much limited to wrong per-vCPU load percentages, when looking at the output of the 'r' debug-key. With this patch, the load is updated and displayed correctly: (XEN) Runqueue 0: (XEN) [...] (XEN) aveload = 2097152 (~800%) (XEN) [...] (XEN) Domain info: (XEN) Domain: 0 w 256 c 0 v 8 (XEN) 1: [0.0] flags=2 cpu=4 credit=9995584 [w=256] load=262144 (~100%) (XEN) 2: [0.1] flags=2 cpu=6 credit=9992992 [w=256] load=262144 (~100%) (XEN) 3: [0.2] flags=2 cpu=3 credit=9998918 [w=256] load=262118 (~99%) (XEN) 4: [0.3] flags=2 cpu=5 credit=9996867 [w=256] load=262144 (~100%) (XEN) 5: [0.4] flags=2 cpu=1 credit=9998912 [w=256] load=262144 (~100%) (XEN) 6: [0.5] flags=2 cpu=2 credit=9997842 [w=256] load=262144 (~100%) (XEN) 7: [0.6] flags=2 cpu=7 credit=9994623 [w=256] load=262144 (~100%) (XEN) 8: [0.7] flags=2 cpu=0 credit=9991815 [w=256] load=262144 (~100%) Signed-off-by: Dario Faggioli <firstname.lastname@example.org> --- Cc: George Dunlap <email@example.com> Cc: Ian Jackson <firstname.lastname@example.org> --- Despite the limited effect, it's a bug. So: - it should be backported; - I think it should be included in 4.15. The risk is pretty low, for the same reasons already explained when describing its limited impact. --- xen/common/sched/credit2.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/xen/common/sched/credit2.c b/xen/common/sched/credit2.c index eb5e5a78c5..b3b5de94cf 100644 --- a/xen/common/sched/credit2.c +++ b/xen/common/sched/credit2.c @@ -3646,6 +3646,8 @@ static void csched2_schedule( runq_remove(snext); __set_bit(__CSFLAG_scheduled, &snext->flags); } + else + update_load(ops, rqd, snext, 0, now); /* Clear the idle mask if necessary */ if ( cpumask_test_cpu(sched_cpu, &rqd->idle) )
next reply other threads:[~2021-03-19 12:14 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-03-19 12:14 Dario Faggioli [this message] 2021-04-27 8:35 ` Ping: " Jan Beulich 2021-05-28 15:18 ` Dario Faggioli 2021-06-07 11:44 ` George Dunlap
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