From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F2B70C48BD1 for ; Fri, 11 Jun 2021 09:20:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CB1FB613D0 for ; Fri, 11 Jun 2021 09:20:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231509AbhFKJWv (ORCPT ); Fri, 11 Jun 2021 05:22:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42130 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230478AbhFKJWt (ORCPT ); Fri, 11 Jun 2021 05:22:49 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A60C8C061574 for ; Fri, 11 Jun 2021 02:20:49 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=EswCt/Z76atEagNm6Jm4YHwcVOamUfUCApeR1/tGBR8=; b=hhu/UUOXb5j/CbCXszQLcFUk6/ qqF+dD6bXT9oZ7NvMyZyRn1OIM+vX4eZCVQeDDcIAUFEg6IN2I//fQY/x9YFKiZVyDSNBWYuNHxoc FbnXMmots3TuqMYJ7TNzDN9cnOIrOB/xX88p3hsZGAA4pGoo2DQD1JnSHQKh4edUTFyW6Jl2+6fZg GkH34vRNN4HGHyA+QwtP43eFPWgiqRLwTkjl2pkrDANu4znV3sD/qo5p143daIRx1kPfjucNkBvZH NyYIAeHap6/qmvzL0V3X9RYDKBAXeU0aSIBJlHTIGFvdLUoku2V65mMywmtrKG8v4A9BwWz+y5gll h+qdQdqQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1lrdLA-005ptM-BT; Fri, 11 Jun 2021 09:20:43 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id AFF2030008D; Fri, 11 Jun 2021 11:20:42 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 90C5821AB0AC0; Fri, 11 Jun 2021 11:20:42 +0200 (CEST) Date: Fri, 11 Jun 2021 11:20:42 +0200 From: Peter Zijlstra To: Quentin Perret Cc: mingo@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, qais.yousef@arm.com, rickyiu@google.com, wvw@google.com, patrick.bellasi@matbug.net, xuewen.yan94@gmail.com, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v2 2/3] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS Message-ID: References: <20210610151306.1789549-1-qperret@google.com> <20210610151306.1789549-3-qperret@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 11, 2021 at 08:59:25AM +0000, Quentin Perret wrote: > On Thursday 10 Jun 2021 at 21:15:45 (+0200), Peter Zijlstra wrote: > > On Thu, Jun 10, 2021 at 03:13:05PM +0000, Quentin Perret wrote: > > > SCHED_FLAG_KEEP_PARAMS can be passed to sched_setattr to specify that > > > the call must not touch scheduling parameters (nice or priority). This > > > is particularly handy for uclamp when used in conjunction with > > > SCHED_FLAG_KEEP_POLICY as that allows to issue a syscall that only > > > impacts uclamp values. > > > > > > However, sched_setattr always checks whether the priorities and nice > > > values passed in sched_attr are valid first, even if those never get > > > used down the line. This is useless at best since userspace can > > > trivially bypass this check to set the uclamp values by specifying low > > > priorities. However, it is cumbersome to do so as there is no single > > > expression of this that skips both RT and CFS checks at once. As such, > > > userspace needs to query the task policy first with e.g. sched_getattr > > > and then set sched_attr.sched_priority accordingly. This is racy and > > > slower than a single call. > > > > > > As the priority and nice checks are useless when SCHED_FLAG_KEEP_PARAMS > > > is specified, simply inherit them in this case to match the policy > > > inheritance of SCHED_FLAG_KEEP_POLICY. > > > > > > Reported-by: Wei Wang > > > Signed-off-by: Quentin Perret > > > --- > > > kernel/sched/core.c | 4 ++++ > > > 1 file changed, 4 insertions(+) > > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > > index 3b213402798e..1d4aedbbcf96 100644 > > > --- a/kernel/sched/core.c > > > +++ b/kernel/sched/core.c > > > @@ -6585,6 +6585,10 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr, > > > rcu_read_unlock(); > > > > > > if (likely(p)) { > > > + if (attr.sched_flags & SCHED_FLAG_KEEP_PARAMS) { > > > + attr.sched_priority = p->rt_priority; > > > + attr.sched_nice = task_nice(p); > > > + } > > > retval = sched_setattr(p, &attr); > > > put_task_struct(p); > > > } > > > > I don't like this much... afaict the KEEP_PARAMS clause in > > __setscheduler() also covers the DL params, and you 'forgot' to copy > > those. > > > > Can't we short circuit the validation logic? > > I think we can but I didn't like the look of it, because we end up > sprinkling checks all over the place. KEEP_PARAMS doesn't imply > KEEP_POLICY IIUC, and the policy and params checks are all mixed up. > > But maybe that wants fixing too? If you can make that code nicer, I'm all for it, it's a bit of a mess. But failing that, I suppose the alternative is extracting something like get_params from sched_getattr() and sharing that bit of code to do what you do above. > I guess it could make sense to switch > policies without touching the params in some cases (e.g switching > between FIFO and RR, or BATCH and NORMAL), but I'm not sure what that > would mean for cross-sched_class transitions. You're right, cross-class needs both.