From: Tejun Heo <tj@kernel.org>
To: Tao Cui <cui.tao@linux.dev>
Cc: void@manifault.com, arighi@nvidia.com, changwoo@igalia.com,
liwanwu@kylinos.cn, usama.arif@linux.dev,
sched-ext@lists.linux.dev, linux-kernel@vger.kernel.org,
bpf@vger.kernel.org, Tao Cui <cuitao@kylinos.cn>
Subject: Re: [PATCH 6/6] sched_ext: Clear the built-in idle masks on CPU hotplug transitions
Date: Thu, 12 Nov 2026 07:34:38 -1000 [thread overview]
Message-ID: <7a17d1d32232abe85b559d61b5d2c536@kernel.org> (raw)
In-Reply-To: <20261112130934.563321-7-cui.tao@linux.dev>
Hello, Tao.
On Thu, Nov 12, 2026 at 09:09:34PM +0800, Tao Cui wrote:
> offline CPU as an idle candidate, and the built-in select_cpu path may
> claim it once before the guard layers bounce the wakeup, costing a
> failed pick and a bumped SCX_EV_SELECT_CPU_FALLBACK each time. This is
> not the documented racy and self-correcting regime of the idle SMT
> handling: the stale bit is a deterministic leftover, and read-only
> kfunc consumers never trigger the claim-time self-correction.
The failure mode is benign, and the patch doesn't address even what it
describes. With a scheduler which implements the hotplug ops and only reads
the idle mask, the offline CPU stays set in the mask for the whole offline
window with the patch applied, the same as without it.
Thanks.
--
tejun
next prev parent reply other threads:[~2026-11-12 17:34 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-12 13:09 [PATCH 0/6] sched_ext: fix kfunc, sub-sched, flatcg and idle-mask issues Tao Cui
2026-11-12 13:09 ` [PATCH 1/6] sched_ext: Reject DSQ iterator kfuncs in tracing programs Tao Cui
2026-11-12 17:34 ` Tejun Heo
2026-11-12 13:09 ` [PATCH 2/6] sched_ext: Make scx_bpf_cid_node() a benign lookup for invalid cids Tao Cui
2026-11-12 17:34 ` Tejun Heo
2026-11-12 13:09 ` [PATCH 3/6] sched_ext: Return -EBUSY from a pre-init scx_bpf_sub_caps() read Tao Cui
2026-11-12 17:34 ` Tejun Heo
2026-11-12 13:09 ` [PATCH 4/6] sched_ext: Reject cid_online/cid_offline on sub-schedulers Tao Cui
2026-11-12 17:34 ` Tejun Heo
2026-11-12 13:09 ` [PATCH 5/6] tools/sched_ext: scx_flatcg: Don't strand a cgroup on the ENQ_RACE path Tao Cui
2026-11-12 13:26 ` sashiko-bot
2026-11-12 17:34 ` Tejun Heo
2026-11-12 13:09 ` [PATCH 6/6] sched_ext: Clear the built-in idle masks on CPU hotplug transitions Tao Cui
2026-11-12 13:32 ` sashiko-bot
2026-11-12 17:34 ` Tejun Heo [this message]
2026-11-12 17:34 ` [PATCH 0/6] sched_ext: fix kfunc, sub-sched, flatcg and idle-mask issues Tejun Heo
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