Re: [PATCH] sched_ext/scx_flatcg: expire cached hweights on weight changes

> sched_ext/scx_flatcg: expire cached hweights on weight changes
>
> fcg_cgroup_set_weight() updates cgc->weight and the parent's
> child_weight_sum but doesn't bump hweight_gen, so the hweights cached by
> cgrp_refresh_hweight() stay stale until some task activation bumps the
> generation. For cgroups whose tasks never go through a 0->n runnable
> transition (e.g. persistently busy ones), a cpu.weight change never
> propagates to scheduling at all.
>
> Bump hweight_gen on weight changes so the next refresh recomputes with
> the new weight.
>
> Verified on a flatcg VM: a live cpu.weight 100->800 change on a busy
> cgroup leaves HWT update at 0 and the distribution unchanged; with it,
> hweight_gen increments and the refresh recomputes.

This looks like a bug fix. Should this include a Fixes: tag pointing to
the commit that introduced the scx_flatcg scheduler?

  Fixes: a4103eacc2ab4 ("sched_ext: Add a cgroup scheduler which uses flattened hierarchy")

This isn't a bug, but should the verification sentence say "without this
change" to make the "with it" contrast more explicit? The first clause
lacks a matching counterpart, so on first read the "with it" appears to
refer back to something unstated.

>
> Signed-off-by: Tao Cui <[email protected]>

> diff --git a/tools/sched_ext/scx_flatcg.bpf.c b/tools/sched_ext/scx_flatcg.bpf.c
> index fec3595818269..059b68ca00cbc 100644
> --- a/tools/sched_ext/scx_flatcg.bpf.c
> +++ b/tools/sched_ext/scx_flatcg.bpf.c
> @@ -604,6 +604,9 @@ void BPF_STRUCT_OPS(fcg_cgroup_set_weight, struct cgroup *cgrp, u32 weight)
>  		pcgc->child_weight_sum += (s64)weight - cgc->weight;
>  	cgc->weight = weight;
>  	bpf_spin_unlock(&cgv_tree_lock);
> +
> +	/* expire cached hweights so the new weight propagates */
> +	__sync_fetch_and_add(&hweight_gen, 1);
>  }
>
>  static bool try_pick_next_cgroup(u64 *cgidp)


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