From owner-freebsd-arch@FreeBSD.ORG Sat Feb 28 15:46:12 2015 Return-Path: Delivered-To: freebsd-arch@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 993AE220 for ; Sat, 28 Feb 2015 15:46:12 +0000 (UTC) Received: from mailout.easymail.ca (mailout.easymail.ca [64.68.201.169]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 4F29032A for ; Sat, 28 Feb 2015 15:46:11 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by mailout.easymail.ca (Postfix) with ESMTP id 3C6DEE4E4 for ; Sat, 28 Feb 2015 10:46:10 -0500 (EST) X-Virus-Scanned: Debian amavisd-new at mailout.easymail.ca X-Spam-Flag: NO X-Spam-Score: -3.854 X-Spam-Level: X-Spam-Status: No, score=-3.854 required=5 tests=[ALL_TRUSTED=-1.8, AWL=-0.147, BAYES_00=-2.599, DNS_FROM_AHBL_RHSBL=0.692] Received: from mailout.easymail.ca ([127.0.0.1]) by localhost (easymail-mailout.easydns.vpn [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id NyNyrMowlR-9 for ; Sat, 28 Feb 2015 10:46:09 -0500 (EST) Received: from bsddt1241.lv01.astrodoggroup.com (unknown [40.141.24.126]) (using TLSv1 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) by mailout.easymail.ca (Postfix) with ESMTPSA id 9BACFE4E2 for ; Sat, 28 Feb 2015 10:46:09 -0500 (EST) Message-ID: <54F1E25F.5040905@astrodoggroup.com> Date: Sat, 28 Feb 2015 07:44:31 -0800 From: Harrison Grundy User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 MIME-Version: 1.0 To: freebsd-arch@freebsd.org Subject: Re: Minor ULE changes and optimizations References: <54EF2C54.7030207@astrodoggroup.com> <2311645.BNIPBaFv2E@ralph.baldwin.cx> <54F0925F.30002@astrodoggroup.com> <1547642.s3cC06khRt@ralph.baldwin.cx> In-Reply-To: <1547642.s3cC06khRt@ralph.baldwin.cx> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 28 Feb 2015 15:46:12 -0000 On 02/28/15 04:24, John Baldwin wrote: > On Friday, February 27, 2015 07:50:55 AM Harrison Grundy wrote: >> On 02/27/15 06:14, John Baldwin wrote: >>> On Thursday, February 26, 2015 06:23:16 AM Harrison Grundy >>> wrote: >>>> https://reviews.freebsd.org/D1969 This allows a >>>> non-migratable thread to pin itself to a CPU if it is already >>>> running on that CPU. >>>> >>>> I've been running these patches for the past week or so >>>> without issue. Any additional testing or comments would be >>>> greatly appreciated. >>> >>> Can you explain the reason / use case for this? This seems to >>> be allowing an API violation. sched_pin() was designed to be >>> a lower-level API than sched_bind(), so you wouldn't call >>> sched_bind() if you were already pinned. In addition, >>> sched_pin() is sometimes used by code that assumes it won't >>> migrate until sched_unpin() (e.g. temporary mappings inside an >>> sfbuf). If you allow sched_bind() to move a thread that is >>> pinned you will allow someone to unintentionally break those >>> sort of things instead of getting an assertion failure panic. >> >> For a pinned thread, the underlying idea is that if you're >> already on the CPU you pinned to, calling sched_bind with that >> CPU specified allows you to set TSF_BOUND without calling >> sched_unpin first. >> >> If a pinned thread were to call sched_bind for a CPU it isn't >> pinned to, it would still hit the assert and fail. >> >> For any unpinned thread, if you're already running on the correct >> CPU, you can skip the THREAD_CAN_MIGRATE check and the call to >> mi_switch. > > Ah, ok, so you aren't allowing migration in theory. However, I'm > still curious as to why you want/need this. This makes the API > usage a bit more complex to reason about (sched_bind() can > sometimes be called while pinned but not always after this change), > so I think that extra complexity needs a reason to exist. Primarily, it allows those threads already on a CPU to skip the call to mi_switch and get out of sched_bind a bit faster. Additionally, it allows a driver to call sched_pin, then bind to that same cpu later without having to write something like "critical_enter(); sched_unpin(); sched_bind(foo, bar); critical_exit();", since otherwise it could be migrated/preempted between unpin and bind.