Attachment 159610 Details for Bug 248491 – Add a command-line option to disable lost tick compensation

[patch] Add a command-line option to disable lost tick compensation

ignore_lost_ticks.patch (text/plain), 3.94 KB, created by Tim Mann on 2007-07-19 21:19:57 UTC

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Creator: Tim Mann

Created: 2007-07-19 21:19:57 UTC

Size: 3.94 KB

patch

obsolete

>Subject: [PATCH 2.6.9-55.0.2.EL] add ignore_lost_ticks option
>
>As discussed in http://bugzilla.kernel.org/show_bug.cgi?id=5127, Linux
>kernels prior to the clocksource patches (i.e., 2.6.17 and earlier)
>have buggy code for detecting and compensating for lost timer
>interrupts (ticks).  The bug results in spurious detection of lost
>ticks and thus makes the clock run faster than real time.
>
>This problem is observable on real hardware, but is much more severe
>in a virtual machine.
>
>In 32-bit kernels, the command line option clock=pit effectively
>disables lost-tick compensation.  (That's not the primary intent of
>the option; it's just that basing timekeeping only on the PIT, as
>clock=pit does, gives the kernel no way to detect lost ticks, so it
>doesn't try.)  On 64-bit kernels, however, there has been no way to
>disable lost-tick compensation from the command line.
>
>This patch adds a kernel command-line option "ignore_lost_ticks" that
>turns off lost-tick compensation in 64-bit kernels.  The patch is kept
>very simple to make it obvious that it has no effect if the command
>line option is not given.
>
>Signed-off-by: Tim Mann <mann@vmware.com>
>
>diff -u -r linux-2.6.9/arch/x86_64/kernel/pmtimer.c linux-2.6.9-mann/arch/x86_64/kernel/pmtimer.c
>--- linux-2.6.9/arch/x86_64/kernel/pmtimer.c	2007-07-17 18:20:01.000000000 -0700
>+++ linux-2.6.9-mann/arch/x86_64/kernel/pmtimer.c	2007-07-17 18:59:24.000000000 -0700
>@@ -53,6 +53,7 @@
> 	static int first_run = 1;
> 	unsigned long tsc;
> 	u32 lost;
>+        extern int ignore_lost_ticks; /* from time.c */
> 
> 	u32 tick = inl(pmtmr_ioport);
> 	u32 delta;
>@@ -62,10 +63,15 @@
> 	last_pmtmr_tick = tick;
> 	monotonic_base += delta * NSEC_PER_USEC;
> 
>-	delta += offset_delay;
>-
>-	lost = delta / (USEC_PER_SEC / HZ);
>-	offset_delay = delta % (USEC_PER_SEC / HZ);
>+        if (ignore_lost_ticks) {
>+                lost = 1;
>+                offset_delay = 0;
>+        } else {
>+                delta += offset_delay;
>+
>+                lost = delta / (USEC_PER_SEC / HZ);
>+                offset_delay = delta % (USEC_PER_SEC / HZ);
>+        }
> 
> 	rdtscll(tsc);
> 	vxtime.last_tsc = tsc - offset_delay * cpu_khz;
>diff -u -r linux-2.6.9/arch/x86_64/kernel/time.c linux-2.6.9-mann/arch/x86_64/kernel/time.c
>--- linux-2.6.9/arch/x86_64/kernel/time.c	2007-07-17 18:20:01.000000000 -0700
>+++ linux-2.6.9-mann/arch/x86_64/kernel/time.c	2007-07-17 19:12:14.000000000 -0700
>@@ -67,6 +67,7 @@
> static int hpet_use_timer;
> unsigned long vxtime_hz = PIT_TICK_RATE;
> int report_lost_ticks;				/* command line option */
>+int ignore_lost_ticks;
> unsigned long long monotonic_base;
> 
> struct vxtime_data __vxtime __section_vxtime;	/* for vsyscalls */
>@@ -139,8 +140,16 @@
> 		   in arch/x86_64/kernel/vsyscall.c and export all needed
> 		   variables in vmlinux.lds. -AK */ 
> 
>-		t = (jiffies - wall_jiffies) * (1000000L / HZ) +
>-			do_gettimeoffset();
>+                /*
>+                 * If lost tick compensation is turned off, we must
>+                 * cap the offset at one tick.  Otherwise when we do
>+                 * lose a tick, gettimeofday may go backward when the
>+                 * next tick arrives.
>+                 */
>+                t = (jiffies - wall_jiffies) * (1000000L / HZ) +
>+                        ignore_lost_ticks ?
>+                        min(USEC_PER_SEC / HZ, do_gettimeoffset()) :
>+                        do_gettimeoffset();
> 		usec += t;
> 
> 	} while (read_seqretry(&xtime_lock, seq));
>@@ -436,7 +445,7 @@
> 				(((long) offset << 32) / vxtime.tsc_quot) - 1;
> 	}
> 
>-	if (lost > 0) {
>+	if (lost > 0 && !ignore_lost_ticks) {
> 		handle_lost_ticks(lost, regs);
> 		jiffies += lost;
> 	}
>@@ -823,6 +832,12 @@
> 	return 1;
> }
> 
>+int __init ignore_lost_ticks_setup(char *str)
>+{
>+	ignore_lost_ticks = 1;
>+	return 1;
>+}
>+
> static struct irqaction irq0 = {
> 	timer_interrupt, SA_INTERRUPT, CPU_MASK_NONE, "timer", NULL, NULL
> };
>@@ -971,6 +986,7 @@
> }
> 
> __setup("report_lost_ticks", time_setup);
>+__setup("ignore_lost_ticks", ignore_lost_ticks_setup);
> 
> static long clock_cmos_diff;
>

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Attachments on bug 248491: 159610