From owner-freebsd-arch@FreeBSD.ORG Sat Feb 28 12:41:31 2015 Return-Path: Delivered-To: freebsd-arch@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 171A2D65 for ; Sat, 28 Feb 2015 12:41:31 +0000 (UTC) Received: from bigwig.baldwin.cx (bigwig.baldwin.cx [IPv6:2001:470:1f11:75::1]) (using TLSv1 with cipher DHE-RSA-CAMELLIA256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id E3E1BB6 for ; Sat, 28 Feb 2015 12:41:30 +0000 (UTC) Received: from ralph.baldwin.cx (pool-173-54-116-245.nwrknj.fios.verizon.net [173.54.116.245]) by bigwig.baldwin.cx (Postfix) with ESMTPSA id 077E5B9B1; Sat, 28 Feb 2015 07:41:30 -0500 (EST) From: John Baldwin To: freebsd-arch@freebsd.org Subject: Re: Minor ULE changes and optimizations Date: Sat, 28 Feb 2015 07:24:30 -0500 Message-ID: <1547642.s3cC06khRt@ralph.baldwin.cx> User-Agent: KMail/4.14.2 (FreeBSD/10.1-STABLE; KDE/4.14.2; amd64; ; ) In-Reply-To: <54F0925F.30002@astrodoggroup.com> References: <54EF2C54.7030207@astrodoggroup.com> <2311645.BNIPBaFv2E@ralph.baldwin.cx> <54F0925F.30002@astrodoggroup.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.2.7 (bigwig.baldwin.cx); Sat, 28 Feb 2015 07:41:30 -0500 (EST) Cc: Harrison Grundy 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 12:41:31 -0000 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. -- John Baldwin