Date: Wed, 7 Aug 2013 14:08:29 -0600 From: Scott Long <scott4long@yahoo.com> To: Andre Oppermann <andre@freebsd.org> Cc: Adrian Chadd <adrian@freebsd.org>, mike@karels.net, Bryan Venteicher <bryanv@daemoninthecloset.org>, Navdeep Parhar <np@freebsd.org>, net@freebsd.org, Giuseppe Lettieri <g.lettieri@iet.unipi.it>, Luigi Rizzo <rizzo@iet.unipi.it> Subject: Re: [net] protecting interfaces from races between control and data ? Message-ID: <DBE2F2CD-02F7-4738-B4FE-446C0B45C941@yahoo.com> In-Reply-To: <5202A74E.4060602@freebsd.org> References: <20130805215319.GA43271@onelab2.iet.unipi.it> <201308070326.r773QCLD035541@mail.karels.net> <CA%2BhQ2%2Bi2MEKw-A=T-x1gw87P5G0g-xko-XPmE__=Cy=Airb1vg@mail.gmail.com> <5202A74E.4060602@freebsd.org>
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On Aug 7, 2013, at 2:00 PM, Andre Oppermann <andre@freebsd.org> wrote: > On 07.08.2013 09:18, Luigi Rizzo wrote: >> On Wed, Aug 7, 2013 at 5:26 AM, Mike Karels <mike@karels.net <mailto:mike@karels.net>> wrote: >> Jumping to (near) the end of the thread, I like most of Andre's proposal. >> Running with minimal locks at this layer is an admirable goal, and I agree >> with most of what was said. I have a few observations on the general changes, >> or related issues: >> >> There was mention of taskqueues. I think that with MSI-X, taskqueues >> should not be needed or used. More specifically, having separate ithreads >> and taskqueues, with ithreads deferring to taskqueues after some limit, makes >> sense only for MSI and legacy interrupts. With MSI-X, an interrupt thread >> should be able to process packets indefinitely with sufficient CPU resources, >> and there is no reason to context switch to a different thread periodically. >> A periodic "yield" might be reasonable, but if it is necessary, small packet >> performance will suffer. However, most of this is internal to the driver. >> >> >> i am not completely clear on what is the difference between ithreads and taskqueues. > > The difference between ithreads and taskqueues is actually very small and mostly in > name and how they're set up and kicked off when work is to be done. Both are real > kernel threads. Most of the confusion, also in Mikes response, seems to come from > the name ithreads for interrupt-threads. However an ithread is not running in > interrupt context, only the interrupt handler (called filter) does. Scheduling a > taskqueue from an ithread is a bit round-about but commonly done. The bus_setup_intr(9) > man page isn't helping to clear up the confusion. > I think it might still be the case that threads are not allowed to sleep. That means no sx locks and no malloc(M_WAITOK), along with the obvious tsleep/msleep. Taskqueues have no such restriction. An even rore relevant difference is that taskqueues have a much stronger management API. Ithreads can only be scheduled by generating a hardware interrupt, can only be drained by calling bus_teardown_intr(), and cannot be paused. Taskqueues give you direct access to this control. Scott
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