Date: Sat, 12 Sep 2009 17:48:36 +0200 From: Luigi Rizzo <rizzo@iet.unipi.it> To: John Baldwin <jhb@freebsd.org> Cc: Juergen Lock <nox@jelal.kn-bremen.de>, Avi Kivity <avi@qumranet.com>, qemu-devel@nongnu.org, freebsd-current@freebsd.org, Jan Kiszka <jan.kiszka@web.de>, Mohammed Gamal <m.gamal005@gmail.com> Subject: Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing) Message-ID: <20090912154836.GA47410@onelab2.iet.unipi.it> In-Reply-To: <200909111301.55692.jhb@freebsd.org> References: <4A93BF0C.8040601@web.de> <200909111123.00257.jhb@freebsd.org> <20090911170317.GA33232@onelab2.iet.unipi.it> <200909111301.55692.jhb@freebsd.org>
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On Fri, Sep 11, 2009 at 01:01:54PM -0400, John Baldwin wrote: > On Friday 11 September 2009 1:03:17 pm Luigi Rizzo wrote: ... > > Note that the per-cpu ticks i was proposing were only visible to the > > timing wheels, which don't use absolute timeouts anyways. > > So i think the mechanism would be quite safe: right now, when you > > request a callout after x ticks, the code first picks a CPU > > (with some criteria), then puts the request in the timer wheel for > > that CPU using (now) the global 'ticks'. Replacing ticks with cc->cc_ticks, > > would completely remove the races in insertion and removal. > > > > I actually find the per-cpu ticks even less intrusive than this change. > > Well, it depends. If TCP ever started using per-CPU callouts (i.e. > callout_reset_on()) It seems that this is already the case in practice. callout_reset() is just #defined to callout_reset_on(c, ... c->cc_cpu) so all calls end up there. c->cc_cpu is initialized in callout_init as c->c_cpu = timeout_cpu; (which is a static int variable; i still don't understand what is the final value it gets, because the comment says that kern_timeout_callwheel_alloc() can be called multiple times and here is where timeout_cpu is initialized.) cheers luigi
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