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 F1CF06DCE1; Sun, 19 Apr 2026 16:18:53 +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=1776615534; cv=none; b=N1a1BZMGn6zYuacQycahN9sc9ivh7MmDv4RqzgFXFPnTR36T6Rc6NC0K6l+FrhZxJ+nK1Cd33RCVJggPJhRTYJPiCsOMlxon9v5K2H/A0AEEz6FdJv425W+BBSbW8LqFmo7/6Kr6fWhTtS6gDZITvMclkHp8+HIe5wqXJwE60zo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776615534; c=relaxed/simple; bh=beOyohRE8VULu8jwTBTLt685jL408kRf3prLZt0G/hs=; h=Date:Message-ID:From:To:Cc:Subject; b=twisq/yidEr8SObzsVQdJU0LBuXmLT5pI0DlhU7DkOMDtEDIbSca14mNmckFNOjRg2TscE4xuaONHCyk60nzNpF4+jGnbNIuxbSJvbK29No9cLJS/HC8WiI8QOfcDp4wuOGbny7mx/vQQJ7NGdrVRyCCll47Ci8VSbf+danTPRU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=drHm3Kq9; 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="drHm3Kq9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 66D8DC2BCAF; Sun, 19 Apr 2026 16:18:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776615533; bh=beOyohRE8VULu8jwTBTLt685jL408kRf3prLZt0G/hs=; h=Date:From:To:Cc:Subject:From; b=drHm3Kq9D+4/Y/8N7F8aZwsju37rpnnpeuNTsvKN4ZOsYXt/V5ifI/Y1muNAGEjca QLLj36WkAzwYtxf/E9fusFIClO1XTzgMh8Y490Ewp7+7epKj+EN2soH7sdX89ofVGo eRJgFpLItd07RdcNm1/12uaFZ9n2ApSjqXXEz8Kip2A7HcRwgcY/EbLDw53Japryji G6dwKyUeL0OI4l367ZsqkPlfK1E6he85X8p17DIYBO/utOSS+LB8MQtHF/7NuLueyf csg1WFZae8bivUV7XuD7w3Sq1wXJ3ekV4EGJ/It0zB6xb1hoFEfSEZeRsZGQBb2UMe 52Kwd9fUyyU4A== Date: Sun, 19 Apr 2026 06:18:52 -1000 Message-ID: <75aaaf7a4dd540191bd8550dd181528b@kernel.org> From: Tejun Heo To: David Vernet , Andrea Righi , Changwoo Min , sched-ext@lists.linux.dev Cc: Emil Tsalapatis , linux-kernel@vger.kernel.org Subject: [PATCH sched_ext/for-7.2] sched_ext: Document the ops compat strategy in compat.h/compat.bpf.h Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The comments around SCX_OPS_DEFINE() and SCX_OPS_OPEN() were vague about how backward compatibility actually works. Expand them to describe the two mechanisms: load-time BTF fix-up for additive changes, and multi-variant struct_ops for incompatible ones. Signed-off-by: Tejun Heo --- tools/sched_ext/include/scx/compat.bpf.h | 6 ++++-- tools/sched_ext/include/scx/compat.h | 23 +++++++++++++++++++---- 2 files changed, 23 insertions(+), 6 deletions(-) --- a/tools/sched_ext/include/scx/compat.bpf.h +++ b/tools/sched_ext/include/scx/compat.bpf.h @@ -423,8 +423,10 @@ static inline void scx_bpf_dsq_reenq(u64 } /* - * Define sched_ext_ops. This may be expanded to define multiple variants for - * backward compatibility. See compat.h::SCX_OPS_LOAD/ATTACH(). + * Define sched_ext_ops. See compat.h::SCX_OPS_OPEN() for how backward + * compatibility is handled (this macro can be expanded to emit multiple + * variants for incompatible op changes; SCX_OPS_OPEN() handles purely + * additive changes at load time). */ #define SCX_OPS_DEFINE(__name, ...) \ SEC(".struct_ops.link") \ --- a/tools/sched_ext/include/scx/compat.h +++ b/tools/sched_ext/include/scx/compat.h @@ -149,10 +149,24 @@ static inline long scx_hotplug_seq(void) } /* - * struct sched_ext_ops can change over time. If compat.bpf.h::SCX_OPS_DEFINE() - * is used to define ops and compat.h::SCX_OPS_LOAD/ATTACH() are used to load - * and attach it, backward compatibility is automatically maintained where - * reasonable. + * Open the sched_ext_ops skeleton. + * + * struct sched_ext_ops can change over time. Two complementary mechanisms + * keep BPF schedulers built against newer headers running on older kernels: + * + * 1. Load-time fix-up (this macro). For each optional ops callback or field + * added to struct sched_ext_ops, an explicit stanza below probes the + * running kernel's BTF via __COMPAT_struct_has_field() and, if the field + * is missing, clears it in the in-memory struct_ops (with a warning to + * stderr) before load. Handles additive changes - a new stanza must be + * added here for each new optional field. + * + * 2. Multi-variant struct_ops via compat.bpf.h::SCX_OPS_DEFINE(). That + * macro can be expanded to emit several variants of struct sched_ext_ops, + * and SCX_OPS_LOAD()/ATTACH() can pick the right one based on what the + * kernel supports. Needed when an existing operation has to change + * incompatibly (e.g. a callback signature changes); the load-time + * fix-up above only handles purely additive changes. * * ec7e3b0463e1 ("implement-ops") in https://github.com/sched-ext/sched_ext is * the current minimum required kernel version. @@ -225,6 +239,7 @@ static inline void __scx_ops_assoc_prog( } #endif +/* See SCX_OPS_OPEN() above for backward-compatibility handling. */ #define SCX_OPS_LOAD(__skel, __ops_name, __scx_name, __uei_name) ({ \ struct bpf_program *__prog; \ UEI_SET_SIZE(__skel, __ops_name, __uei_name); \