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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 autolearn=no 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 DCCA8C7618F for ; Fri, 26 Jul 2019 20:44:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ACBE022ADA for ; Fri, 26 Jul 2019 20:44:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727239AbfGZUoc (ORCPT ); Fri, 26 Jul 2019 16:44:32 -0400 Received: from mail.kernel.org ([198.145.29.99]:38632 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726305AbfGZUob (ORCPT ); Fri, 26 Jul 2019 16:44:31 -0400 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E778E218B8; Fri, 26 Jul 2019 20:44:29 +0000 (UTC) Date: Fri, 26 Jul 2019 16:44:28 -0400 From: Steven Rostedt To: Thomas Gleixner Cc: LKML , x86@kernel.org, Sebastian Siewior , Anna-Maria Gleixner , Arnaldo Carvalho de Melo , Jiri Olsa , Paolo Bonzini , Juergen Gross Subject: Re: [patch 10/12] hrtimer: Determine hard/soft expiry mode for hrtimer sleepers on RT Message-ID: <20190726164428.40a4e4b4@gandalf.local.home> In-Reply-To: <20190726185753.645792403@linutronix.de> References: <20190726183048.982726647@linutronix.de> <20190726185753.645792403@linutronix.de> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 26 Jul 2019 20:30:58 +0200 Thomas Gleixner wrote: > +++ b/kernel/time/hrtimer.c > @@ -1662,6 +1662,30 @@ static enum hrtimer_restart hrtimer_wake > static void __hrtimer_init_sleeper(struct hrtimer_sleeper *sl, > clockid_t clock_id, enum hrtimer_mode mode) > { > + /* > + * On PREEMPT_RT enabled kernels hrtimers which are not explicitely > + * marked for hard interrupt expiry mode are moved into soft > + * interrupt context either for latency reasons or because the > + * hrtimer callback takes regular spinlocks or invokes other > + * functions which are not suitable for hard interrupt context on > + * PREEMPT_RT. Have we marked all timer handlers that have normal spin_locks as HRTIMER_MODE_SOFT? Otherwise, can't we switch one to hard below and having their handler grab a spin_lock/mutex in hard interrupt context in RT? -- Steve > + * > + * The hrtimer_sleeper callback is RT compatible in hard interrupt > + * context, but there is a latency concern: Untrusted userspace can > + * spawn many threads which arm timers for the same expiry time on > + * the same CPU. That causes a latency spike due to the wakeup of > + * a gazillion threads. > + * > + * OTOH, priviledged real-time user space applications rely on the > + * low latency of hard interrupt wakeups. If the current task is in > + * a real-time scheduling class, mark the mode for hard interrupt > + * expiry. > + */ > + if (IS_ENABLED(CONFIG_PREEMPT_RT)) { > + if (task_is_realtime(current) && !(mode & HRTIMER_MODE_SOFT)) > + mode |= HRTIMER_MODE_HARD; > + } > + > __hrtimer_init(&sl->timer, clock_id, mode); > sl->timer.function = hrtimer_wakeup; > sl->task = current;