From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-193.mta0.migadu.com [91.218.175.193]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 29FD742B33D for ; Tue, 1 Sep 2026 02:40:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.193 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788230473; cv=none; b=Xs8r24G6zZEjRG5vmpqqoA2LuaTwsAR0y3oRFr8y6VzjpXZe+1uJ2E/LMy0X9lukhjReSQmz2jhk1G7OKMKNhrw+BIE5UxV7GyM6PPboLiOzMCdPgT0NlA+Hqx4hnrmRWFW3ttNPaUdIcroClFXJjXYVGX9wfSQIO/lHJ2N+qNM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788230473; c=relaxed/simple; bh=zGl9k2EKaQ/cypP2a3HZEV5eAmxgn5HG0stOMh5zxuc=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=DrTHkQDQgRZLRZm8NM/VRFaksWwTo71tzP616ZNEIb4GkjqU4sJ0Bn9+nCyo0SFj4ZgEJ3s+dKx92Lpnz7klKaMHjszUAWM0bNIeH42tHvTb1zjzAQx6aPvenfaBTZlMrhi1mRsq/Etht4/XK4pEP5nfUMXxpWEZlsoHGJiu8jM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=HoiwX54D; arc=none smtp.client-ip=91.218.175.193 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="HoiwX54D" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=zGl9k2EKaQ/cypP2a3HZEV5eAmxgn5HG0stOMh5zxuc=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1788230454; v=1; x=1788835254; b=HoiwX54DQrfYZJWKPMKZesULS90u1xJUpAMjWosCmIkwXCMLXkscriPhKHbeTBn2d3fCAXhC xmFdJfSo0UhBSd3njfcbaE/MECAdjAgAwp9cn0yH/UNwImWsnrgvq7EzlQGGjqpwDK+Qxyqiukz yQTTtetUoJomPe/KOMzrm0j0= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 8df81cf4b717920e; Tue, 01 Sep 2026 02:40:54 +0000 X-Mizu-Trace-ID: 8df81cf4b717920e X-Migadu-Flow: FLOW_OUT From: Tao Cui To: tj@kernel.org, void@manifault.com Cc: arighi@nvidia.com, changwoo@igalia.com, michalblk@google.com, liwanwu@kylinos.cn, sched-ext@lists.linux.dev, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, cui.tao@linux.dev, Tao Cui Subject: [PATCH 1/2] sched_ext: fix vtime priority queue inversion on wide vtime spread Date: Tue, 1 Sep 2026 10:40:37 +0800 Message-ID: <20260901024038.730424-2-cui.tao@linux.dev> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260901024038.730424-1-cui.tao@linux.dev> References: <20260901024038.730424-1-cui.tao@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Tao Cui scx_dsq_priq_less() compares dsq_vtime with time_before64(), a cyclic comparison that is only valid when the values in the queue are less than 2^63 apart. Unlike CFS, which enforces that invariant with min_vruntime clamping, sched_ext takes dsq_vtime directly from the BPF scheduler and cannot bound the spread. A scheduler that inserts tasks with vtimes wider than 2^63 apart into one DSQ gets the order inverted: the tasks it placed last run first and the rest starve. Reproduced with a probe scheduler assigning half its tasks vtimes near 0 and the other half vtimes above 2^63 -- four of eight busy tasks monopolized the CPU while the other four starved. Compare with plain u64 < instead, which is a total order and always honors the order the scheduler asked for. The transient misordering around the natural 2^64 wrap is the same class of anomaly the cyclic comparison trades it for, but bounded. Fixes: 06e51be3d5e7 ("sched_ext: Add vtime-ordered priority queue to dispatch_q's") Signed-off-by: Tao Cui --- kernel/sched/ext/ext.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c index 8041c87a3562..dd0ce01370d1 100644 --- a/kernel/sched/ext/ext.c +++ b/kernel/sched/ext/ext.c @@ -1417,7 +1417,8 @@ static bool scx_dsq_priq_less(struct rb_node *node_a, const struct task_struct *b = container_of(node_b, struct task_struct, scx.dsq_priq); - return time_before64(a->scx.dsq_vtime, b->scx.dsq_vtime); + /* dsq_vtime is arbitrary BPF input: keep a total order */ + return a->scx.dsq_vtime < b->scx.dsq_vtime; } static void dsq_inc_nr(struct scx_dispatch_q *dsq, struct task_struct *p, u64 enq_flags) -- 2.43.0