Date: Sun, 17 Feb 2002 10:46:12 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Bruce Evans <bde@zeta.org.au> Cc: <freebsd-current@FreeBSD.ORG> Subject: Re: 'microuptime() went backwards ...' using ACPI timer. Shouldn't that be impossible? Message-ID: <200202171846.g1HIkCQ71367@apollo.backplane.com> References: <20020217184436.M934-100000@gamplex.bde.org>
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:I just wrote the following fix for some of the overflow problems.
I don't understand how this code is supposed to handle overflows.
You seem only to be checking to see if the master timecounter has
changed to a different type.
-Matt
Matthew Dillon
<dillon@backplane.com>
:%%%
:Index: kern_tc.c
:===================================================================
:RCS file: /home/ncvs/src/sys/kern/kern_tc.c,v
:retrieving revision 1.113
:diff -c -2 -r1.113 kern_tc.c
:*** kern_tc.c 7 Feb 2002 21:21:55 -0000 1.113
:--- kern_tc.c 17 Feb 2002 06:25:14 -0000
:***************
:*** 108,114 ****
: struct timecounter *tc;
:
:! tc = timecounter;
:! *bt = tc->tc_offset;
:! bintime_addx(bt, tc->tc_scale * tco_delta(tc));
: }
:
:--- 95,129 ----
: struct timecounter *tc;
:
:! /*
:! * The loop is to handle changes of timecounter underneath us.
:! * Such changes may even become normal for preemptive kernels.
:! * It is quite reasonable for idle priority processes to not
:! * run for many seconds, and if they are not running after
:! * being preempted here, the timecounter may cycle many times
:! * underneath them. An NTIMECOUNTER of > 2 is neither necessary
:! * or sufficient for fixing this problem, unless NTIMECOUNTER is
:! * preposterously large. NTIMECOUNTER == 2 suffices for most
:! * cases, and something more is required to fix the general case.
:! *
:! * I hope this also fixes problems with overflow of the
:! * multiplication. We depend on tc not becoming stale by more
:! * than 1 second. We will now normally see such staleness
:! * because it will cause the timecounter to change many times
:! * underneath us. There will only be problems if hardclock()
:! * doesn't run for many seconds, but hardclock() is a very
:! * high priority interrupt, so such problems "can't happen".
:! *
:! * XXX should use a generation count.
:! *
:! * XXX problems with hardclock() can happen, e.g., at boot time
:! * if you have fixed hardclock() to not be a broken fast interrupt
:! * handler, or if you sit at the ddb prompt for several seconds.
:! * Should do something to make them harmless.
:! */
:! do {
:! tc = timecounter;
:! *bt = tc->tc_offset;
:! bintime_addx(bt, tc->tc_scale * tco_delta(tc));
:! } while (tc != timecounter);
: }
:
:%%%
:
:Bruce
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