Attachment 317400 Details for Bug 461184 – RHEL5 fix for this issue

[patch] RHEL5 fix for this issue

461184.patch (text/plain), 2.88 KB, created by Prarit Bhargava on 2008-09-22 18:01:53 UTC

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Creator: Prarit Bhargava

Created: 2008-09-22 18:01:53 UTC

Size: 2.88 KB

patch

obsolete

>Revert time() syscall changes made in 250708.
>
>In the past two or three years, RH has not run any gettimeofday() or time()
>tests on the RHEL5 kernel because of what appeared to be HW issues.
>
>I finally upon the real issue behind the problems with gettimeofday() in 250708,
>rounding errors were compounding and returning an occasional "off-by-one"
>result on gettimeofday() queries.
>
>After patching the kernel, I turned on the gettimeofday testsuite in RHTS and
>noted that all tests but one passed.  The test that didn't pass was
>
>tests/kernel/syscalls/gettimeofday/timedifftest-qa.c
>
>This test does
>
>time()
>gettimeofday()
>time()
>
>and compares the values to determine if "time" is increasing.  If time
>decreases, the test fails.
>
>[Aside: The difference between time() and gettimeofday() is that time() returns
>the # of seconds since Jan 1, 1970 and gettimeofday() returns the # of seconds
>and microseconds since Jan 1, 1970.]
>
>The test failed after my initial patch for 250708 because the value returned in
>time() does not take into account the amount of time since the last timer
>interrupt and "now", whereas gettimeofday() does take this small amount into
>account in the calculation.  This resulted in an occasional error in the value
>that time() reports.
>
>For example,
>
>A timer interrupt is handled.  t(int) = 10001 seconds
>
>time() is called and returns t(time) = t(int) = 10001 seconds
>
>gettimeofday is called and returns t(time) = t(int) + delta(now-t(int)) = 10001
>\+ 1 = 10002 seconds
>
>time() is called again, and returns t(time) = t(int) = 10001 seconds
>
>To fix this problem I changed time() to now call gettimeofday() and return the
>macroscopic portion of the gettimeofday() calculation.  Coincidentally, this
>solution matched upstream.
>
>With the change a call to time() results in a read of the clock which takes
>some time and IMO is responsible for the performance hit.
>
>Without this fix, the timedifftest-qa.c test _will fail_ 100% of the time.
>IIRC, a similar test is performed in the ltp testsuite and it will fail as
>well.
>
>However, jturner noted that performance testing on the new time() syscall
>resulted in a 2200% increase in the amount of time to call time().  This is
>unacceptable so I am reverting this small change which will leave time()
>and gettimeofday() unsynchronized in RHEL5.
>
>Note that this revert does not effect the gettimeofday() syscall modifications.
>
>jburke has already disabled the timedifftest-qa.c test in RHTS.
>
>Resolves BZ 461184.
>
>diff --git a/arch/x86_64/kernel/vsyscall.c b/arch/x86_64/kernel/vsyscall.c
>index e8357cd..7bf3a59 100644
>--- a/arch/x86_64/kernel/vsyscall.c
>+++ b/arch/x86_64/kernel/vsyscall.c
>@@ -141,13 +141,9 @@ time_t __vsyscall(1) vtime(time_t *t)
> 
> 	if (!__sysctl_vsyscall)
> 		return time_syscall(t);
>-
>-	do_vgettimeofday(&tv);
>-
>-	if (t)
>-		*t = tv.tv_sec;
>-
>-	return tv.tv_sec;
>+	else if (t)
>+		*t = __xtime.tv_sec;		
>+	return __xtime.tv_sec;
> }
> 
> /* Fast way to get current CPU and node.

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