Date: Mon, 18 Feb 2002 11:26:54 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: <freebsd-current@FreeBSD.ORG> Subject: Re: 'microuptime() went backwards ...' using ACPI timer. Shouldn't that be impossible? Message-ID: <20020218112157.F3970-100000@gamplex.bde.org> In-Reply-To: <200202172025.g1HKPc589840@apollo.backplane.com>
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On Sun, 17 Feb 2002, Matthew Dillon wrote: > Ok, I've looked at the code more carefully and I understand how this > works now. However, it is not enough in an SMP environment. You > need a generation count in the timecounter structure and you also need > a synchronization point when you switch time counters or a process > running on a different cpu may wind up using a time counter that is being > actively updated. Er, the comment in patch says it needs a generation count. Unfortunately, we only have half a synchronization point now -- there is a sched_lock() in the update code but nothing in the code that reads the pointer. Maybe a large value of NTIMECOUNTER would help after all. We can afford to o miss a few timecounter changes if we have a lot of timecounter states. Bruce To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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