From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DC765218AAB for ; Wed, 20 Aug 2025 16:52:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755708765; cv=none; b=nHXUOHeKiX53MxPEoVKXw00DgPGzYG8A5OJSTlxGHfv0Ttwblk4EO3rmmBnIWEuAnc8peMtSTc1PIwh12pOpjFJ8wAD6HZZbc+urxluCEFJUcLcxaQwRRfa9ZU0UfYF7tydCgweHgA6e7WeNtn2Qsy8VwvQP17930rW8GxJ+s6M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755708765; c=relaxed/simple; bh=a1rgKl6oWJaDB7bfW8fBe6cuwkiaEm4eGDouUvg8XA0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ISDQygYTIR2iM1Sfgw+j+J4gi70IVC5xr4r4gBWrHuefH/GC3JYBExJ3dYCTGuL9fztmjGHziISK/ygJLZ0fzMNMqYNbiDboYfHBcSrvHkpXwheM9YVdWrjzeN+1quC9UBd63lyqkgrVS2gOuoDJt4179oiogClh2MqcROCLK/I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=tvUwppo8; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="tvUwppo8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5141DC4CEE7; Wed, 20 Aug 2025 16:52:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1755708765; bh=a1rgKl6oWJaDB7bfW8fBe6cuwkiaEm4eGDouUvg8XA0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=tvUwppo8/zu71lfF5eSoj9Onptm2Qv/FcBLneR56M4Cr/PsGQK7RdJXvmgkIh3Hx5 TJM/FlDjid3rc/cOLkeLMP9dPhsFZHwwJBCXH4AzCLoIqXHKwYMvvO5plMQmXEekHf +YlC6YvV9daF+MoD5LqF+KgkBqU+DI2HgMDZegsLEzD9wI4l2/3MgINelor4SxKETE vGDMHEeCi89wBHiim0bcXI0W3dsDTfTP66RFj7nazXVH6f7bPHtX/sBf2trgT9ONHn SL9yqUzP+eEaWkJZJ8H4qSVNzt86xPRS9CtBPc4I4Qg19cjMz27SAmw1kjivZHBseA 72xvHiAdJSfgg== Date: Wed, 20 Aug 2025 06:52:44 -1000 From: 'Tejun Heo' To: Peter Zijlstra Cc: liuwenfang , 'David Vernet' , 'Andrea Righi' , 'Changwoo Min' , 'Ingo Molnar' , 'Juri Lelli' , 'Vincent Guittot' , 'Dietmar Eggemann' , 'Steven Rostedt' , 'Ben Segall' , 'Mel Gorman' , 'Valentin Schneider' , "'linux-kernel@vger.kernel.org'" , Joel Fernandes Subject: Re: [PATCH v4 2/3] sched_ext: Fix cpu_released while RT task and SCX task are scheduled concurrently Message-ID: References: <0144ab66963248cf8587c47bf900aabb@honor.com> <814bebd2ad844b08993836fd8e7274b8@honor.com> <228ebd9e6ed3437996dffe15735a9caa@honor.com> <8d64c74118c6440f81bcf5a4ac6b9f00@honor.com> <20250819074736.GD3245006@noisy.programming.kicks-ass.net> <20250820091810.GK3245006@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250820091810.GK3245006@noisy.programming.kicks-ass.net> Hello, On Wed, Aug 20, 2025 at 11:18:10AM +0200, Peter Zijlstra wrote: > > This guarantee changes if we add @rf to pick_task() and let it unlock and > > relock. A higher priority task may get queued while the rq lock is > > released and then the lower priority pick_task() may still return a task > > of its own. > > No, this would be broken. This guarantee must not change. > > What you can do however is something like: > > again: > p = pick_local_task(); > if (!p) { > unlock(rq, rf); > // get yourself a local task > lock(rq, rf); > if (higher-class-task-available(rq)) { > // roll back whatever state > return RETRY_TASK; > } > goto again; > } > > return p; Isn't that kinda nasty as we'd have to do that in every pick_task(). What'd be equivalent but in a central location would be making sure that wakeup_preempt() asserts a resched event if it hits a race window like that. I haven't thought too much about how that should be done exactly but the races should be pretty rare, so it'd be surprising if the behavior difference is noticeable. Thanks. -- tejun