Re: [PATCH 1/3] sched/fair: Add tg_load_contrib cfs_rq decay checking

[Vincent Guittot]

On Thu, 27 May 2021 at 09:51, Odin Ugedal <odin@xxxxxxx> wrote:
>
> Hi,
>
> Did some quick testing with bpftrace, and it looks like it is
> "update_tg_cfs_load" that does this;
>
>  kretfunc:attach_entity_load_avg: cfs_rq: 0xffff8a3e6773e400,
> se.load_sum: 0, se.load_avg: 0, se->load.weight: 1048576 diff.sum: 0,
> diff.load: 0, new_sum: 0, new_load: 0, period_contrib: 821
>
>  // quite some time passes
>
>  kretfunc:update_tg_cfs_load: cfs_rq: 0xffff8a3e6773e400, old load_sum
> 0, old load_avg: 0, new load_sum: 47876096, new load_avg: 1022
> tg_load_avg_contrib: 0, period_contrib: 82, on_list: 0, throttled: 0/0
> stack:
>          bpf_prog_a37cf01922e02958_update_tg_cfs_l+504
>          bpf_get_stackid_raw_tp+115
>          bpf_prog_a37cf01922e02958_update_tg_cfs_l+504
>          bpf_trampoline_6442466486_1+144
>          update_tg_cfs_load+5
>          update_load_avg+450
>          enqueue_entity+91
>          enqueue_task_fair+134
>          move_queued_task+172
>          affine_move_task+1282
>          __set_cpus_allowed_ptr+317
>          update_tasks_cpumask+70
>          update_cpumasks_hier+448
>          update_cpumask+345
>          cpuset_write_resmask+796
>          cgroup_file_write+162
>          kernfs_fop_write_iter+284
>          new_sync_write+345
>          vfs_write+511
>          ksys_write+103
>          do_syscall_64+51
>          entry_SYSCALL_64_after_hwframe+68
>
>  // There might be more stuff happening here
>  // __update_load_avg_cfs_rq runs way too often to be able to
>  // trace it without filtering, and didn't look into that.
>
>  kretfunc:update_tg_cfs_load: cfs_rq: 0xffff8a3e6773e400, old load_sum
> 48117265, old load_avg: 1025, new load_sum: 0, new load_avg: 2
> tg_load_avg_contrib: 1022, period_contrib: 223, on_list: 1, throttled:
> 1/1 stack:
>          bpf_prog_a37cf01922e02958_update_tg_cfs_l+504
>          bpf_get_stackid_raw_tp+115
>          bpf_prog_a37cf01922e02958_update_tg_cfs_l+504
>          bpf_trampoline_6442466486_1+144
>          update_tg_cfs_load+5
>          update_load_avg+450
>          update_blocked_averages+723
>          newidle_balance+533
>          pick_next_task_fair+57
>          __schedule+376
>          schedule+91
>          schedule_hrtimeout_range_clock+164
>          do_sys_poll+1043
>          __x64_sys_poll+179
>          do_syscall_64+51
>          entry_SYSCALL_64_after_hwframe+68
>
> This particular example resulted in a 75/25 share of cpu time for the
> two groups.
>
> From this, it also looks like it doesn't matter if new load_avg is 0 or not
> (when new load_sum is 0), since tg_load_avg_contrib will not be properly
> removed either way (without this patch). There might be some other precision
>
>
> > I have added a trace in cfs_rq_is_decayed() but I'm not able to
> > reproduce a situation where load_sum == 0 but not load_avg  even with
> > the script in the cover letter
>
> Hmm, strange. I am able to reproduce on both v5.12 and v5.13. I am running on
> a non SMT 4 core CPU (4670k), with CONFIG_HZ=250. Since this is timing
> sensitive,
> you might need to tweak the timings for either the CFS bw period/quota, or the
> time waiting between changing what cpu it runs on. If you have more than 4 cpus,
> you can also try to start on CPU 0 and move it through all cores and
> then onto CPU n.
> Maybe that makes it possible to reproduce.

I finally got it this morning with your script and I confirm that the
problem of load_sum == 0 but load_avg != 0 comes from
update_tg_cfs_load(). Then, it seems that we don't call
update_tg_load_avg for this cfs_rq in __update_blocked_fair() because
of a recent update while propagating child's load changes. At the end
we remove the cfs_rq from the list without updating its contribution.

I'm going to prepare a patch to fix this


>
> Since bpftrace doesn't work properly in v5.13, this particular test was
> on v1.12 with cherry-pick of 0258bdfaff5bd13c4d2383150b7097aecd6b6d82 and
> the other patch in this patchset.
>
> testscript.bt:
>
>   kfunc:attach_entity_load_avg {@a_sum[tid] =
> args->cfs_rq->avg.load_sum; @a_load[tid] =
> args->cfs_rq->avg.load_avg;}
>   kretfunc:attach_entity_load_avg{printf("%s: cfs_rq: %p, se.load_sum:
> %llu, se.load_avg: %llu, se->load.weight: %llu diff.sum: %llu,
> diff.load: %llu, new_sum: %llu, new_load: %llu, period_contrib:
> %llu\n", probe, args->cfs_rq,args->se->avg.load_sum,
> args->se->avg.load_avg,
> args->se->load.weight,args->cfs_rq->avg.load_sum - @a_sum[tid],
> args->cfs_rq->avg.load_avg - @a_load[tid], args->cfs_rq->avg.load_sum,
> args->cfs_rq->avg.load_avg, args->cfs_rq->avg.period_contrib)}
>
>   kfunc:update_tg_cfs_load {@beg_load_avg[tid] =
> args->cfs_rq->avg.load_avg; @beg_load_sum[tid] =
> args->cfs_rq->avg.load_sum}
>   kretfunc:update_tg_cfs_load {printf("%s: cfs_rq: %p, old load_sum
> %llu, old load_avg: %llu, new load_sum: %llu, new load_avg: %llu
> tg_load_avg_contrib: %llu, period_contrib: %llu, on_list: %d,
> throttled: %d/%d stack: %s\n",probe, args->cfs_rq, @beg_load_sum[tid],
> @beg_load_avg[tid], args->cfs_rq->avg.load_sum,
> args->cfs_rq->avg.load_avg, args->cfs_rq->tg_load_avg_contrib,
> args->cfs_rq->avg.period_contrib,args->cfs_rq->on_list,
> args->cfs_rq->throttled, args->cfs_rq->throttle_count,kstack)}
>
> Thanks
> Odin