From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.6 required=5.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI, SPF_HELO_NONE,SPF_NONE autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id D502D7D90F for ; Thu, 1 Aug 2019 09:51:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726125AbfHAJvZ (ORCPT ); Thu, 1 Aug 2019 05:51:25 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:59302 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725379AbfHAJvZ (ORCPT ); Thu, 1 Aug 2019 05:51:25 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; 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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=/RCQXkoLdx1Dap2zHV/paZLxYVJHeWju5RP8ynoxTPI=; b=UluEH/75fC5+qgudhOiGsRDkg Dmv3F33wa/UY2uDdoFibQqQZ/AqqbiXyF/wqMePOsemH4nmb1rKJpHHfYVfEv3f3ai4nBEp7Ug+sL SWzd5y8YOKyQ/tSuRq0AiUJXLq4NeSuDbMiZURN9ycAr1WLCUOUT5OizKLOhC9xB/AVC02rjw3CtK Wax8iH9jNoLz0U6pypJXmZ8HPc5NNfEIKf4EOPSG1OTeIXbzBGbcVP41xY+kvR+e+I/f8rggEFydN eqt4MyjaUuiqqF6LW21UPHkR1sy64DUUDe4i1To53kp8ma1FLTaZT+7cLoi8LygcrYfmuoqz0PCAh nXgvvDuHQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1ht7ju-0003e9-F5; Thu, 01 Aug 2019 09:51:14 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 149A52029F4CD; Thu, 1 Aug 2019 11:51:12 +0200 (CEST) Date: Thu, 1 Aug 2019 11:51:12 +0200 From: Peter Zijlstra To: Suren Baghdasaryan Cc: Ingo Molnar , lizefan@huawei.com, Johannes Weiner , axboe@kernel.dk, Dennis Zhou , Dennis Zhou , Andrew Morton , linux-mm , linux-doc@vger.kernel.org, LKML , kernel-team , Nick Kralevich , Thomas Gleixner Subject: Re: [PATCH 1/1] psi: do not require setsched permission from the trigger creator Message-ID: <20190801095112.GA31381@hirez.programming.kicks-ass.net> References: <20190730013310.162367-1-surenb@google.com> <20190730081122.GH31381@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Tue, Jul 30, 2019 at 10:44:51AM -0700, Suren Baghdasaryan wrote: > On Tue, Jul 30, 2019 at 1:11 AM Peter Zijlstra wrote: > > > > On Mon, Jul 29, 2019 at 06:33:10PM -0700, Suren Baghdasaryan wrote: > > > When a process creates a new trigger by writing into /proc/pressure/* > > > files, permissions to write such a file should be used to determine whether > > > the process is allowed to do so or not. Current implementation would also > > > require such a process to have setsched capability. Setting of psi trigger > > > thread's scheduling policy is an implementation detail and should not be > > > exposed to the user level. Remove the permission check by using _nocheck > > > version of the function. > > > > > > Suggested-by: Nick Kralevich > > > Signed-off-by: Suren Baghdasaryan > > > --- > > > kernel/sched/psi.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c > > > index 7acc632c3b82..ed9a1d573cb1 100644 > > > --- a/kernel/sched/psi.c > > > +++ b/kernel/sched/psi.c > > > @@ -1061,7 +1061,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group, > > > mutex_unlock(&group->trigger_lock); > > > return ERR_CAST(kworker); > > > } > > > - sched_setscheduler(kworker->task, SCHED_FIFO, ¶m); > > > + sched_setscheduler_nocheck(kworker->task, SCHED_FIFO, ¶m); > > > > ARGGH, wtf is there a FIFO-99!! thread here at all !? > > We need psi poll_kworker to be an rt-priority thread so that psi There is a giant difference between 'needs to be higher than OTHER' and FIFO-99. > notifications are delivered to the userspace without delay even when > the CPUs are very congested. Otherwise it's easy to delay psi > notifications by running a simple CPU hogger executing "chrt -f 50 dd > if=/dev/zero of=/dev/null". Because these notifications are So what; at that point that's exactly what you're asking for. Using RT is for those who know what they're doing. > time-critical for reacting to memory shortages we can't allow for such > delays. Furthermore, actual RT programs will have pre-allocated and locked any memory they rely on. They don't give a crap about your pressure nonsense. > Notice that this kworker is created only if userspace creates a psi > trigger. So unless you are using psi triggers you will never see this > kthread created. By marking it FIFO-99 you're in effect saying that your stupid statistics gathering is more important than your life. It will preempt the task that's in control of the band-saw emergency break, it will preempt the task that's adjusting the electromagnetic field containing this plasma flow. That's insane. I'm going to queue a patch to reduce this to FIFO-1, that will preempt regular OTHER tasks but will not perturb (much) actual RT bits.