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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9341DC4332F for ; Wed, 8 Nov 2023 16:29:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230081AbjKHQ3x convert rfc822-to-8bit (ORCPT ); Wed, 8 Nov 2023 11:29:53 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49702 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229460AbjKHQ3v (ORCPT ); Wed, 8 Nov 2023 11:29:51 -0500 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 204A0131 for ; Wed, 8 Nov 2023 08:29:48 -0800 (PST) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-202-TeiFwQDsO6-x3H0sA3xebA-1; Wed, 08 Nov 2023 16:29:45 +0000 X-MC-Unique: TeiFwQDsO6-x3H0sA3xebA-1 Received: from AcuMS.Aculab.com (10.202.163.6) by AcuMS.aculab.com (10.202.163.6) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Wed, 8 Nov 2023 16:29:40 +0000 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Wed, 8 Nov 2023 16:29:40 +0000 From: David Laight To: 'Steven Rostedt' CC: 'Ankur Arora' , "linux-kernel@vger.kernel.org" , "tglx@linutronix.de" , "peterz@infradead.org" , "torvalds@linux-foundation.org" , "paulmck@kernel.org" , "linux-mm@kvack.org" , "x86@kernel.org" , "akpm@linux-foundation.org" , "luto@kernel.org" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "hpa@zytor.com" , "mingo@redhat.com" , "juri.lelli@redhat.com" , "vincent.guittot@linaro.org" , "willy@infradead.org" , "mgorman@suse.de" , "jon.grimm@amd.com" , "bharata@amd.com" , "raghavendra.kt@amd.com" , "boris.ostrovsky@oracle.com" , "konrad.wilk@oracle.com" , "jgross@suse.com" , "andrew.cooper3@citrix.com" , "mingo@kernel.org" , "bristot@kernel.org" , "mathieu.desnoyers@efficios.com" , "geert@linux-m68k.org" , "glaubitz@physik.fu-berlin.de" , "anton.ivanov@cambridgegreys.com" , "mattst88@gmail.com" , "krypton@ulrich-teichert.org" , "richard@nod.at" , "mjguzik@gmail.com" Subject: RE: [RFC PATCH 00/86] Make the kernel preemptible Thread-Topic: [RFC PATCH 00/86] Make the kernel preemptible Thread-Index: AQHaEcV0+BoURT5JbUqhFq41r7bm+rBwJNlwgABkiACAAAiUQA== Date: Wed, 8 Nov 2023 16:29:40 +0000 Message-ID: <19a53508f44049a39272e5bd89ade4ab@AcuMS.aculab.com> References: <20231107215742.363031-1-ankur.a.arora@oracle.com> <20231108101552.7ebbb52d@gandalf.local.home> In-Reply-To: <20231108101552.7ebbb52d@gandalf.local.home> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Steven Rostedt > Sent: 08 November 2023 15:16 > > On Wed, 8 Nov 2023 09:43:10 +0000 > David Laight wrote: > > > > Policies: > > > > > > A - preemption=none: run to completion > > > B - preemption=voluntary: run to completion, unless a task of higher > > > sched-class awaits > > > C - preemption=full: optimized for low-latency. Preempt whenever a higher > > > priority task awaits. > > > > If you remove cond_resched() then won't both B and C require an extra IPI. > > That is probably OK for RT tasks but could get expensive for > > normal tasks that aren't bound to a specific cpu. > > What IPI is extra? I was thinking that you wouldn't currently need an IPI if the target cpu was running in-kernel because nothing would happen until cond_resched() was called. > > I suspect C could also lead to tasks being pre-empted just before > > they sleep (eg after waking another task). > > There might already be mitigation for that, I'm not sure if > > a voluntary sleep can be done in a non-pre-emptible section. > > No, voluntary sleep can not be done in a preemptible section. I'm guessing you missed out a negation in that (or s/not/only/). I was thinking about sequences like: wake_up(); ... set_current_state(TASK_UNINTERRUPTIBLE) add_wait_queue(); spin_unlock(); schedule(); Where you really don't want to be pre-empted by the woken up task. For non CONFIG_RT the lock might do it - if held long enough. Otherwise you'll need to have pre-emption disabled and enable it just after the set_current_state(). And then quite likely disable again after the schedule() to balance things out. So having the scheduler save the pre-empt disable count might be useful. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)