linux-kernel - Re: scheduling anomaly on uml (was: -rt doesn't compile for UML)

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Message-Id: <E1IyBmq-0005t7-Re@dorka.pomaz.szeredi.hu>
Date:	Fri, 30 Nov 2007 20:44:32 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	jdike@...toit.com
CC:	mingo@...e.hu, user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: scheduling anomaly on uml (was: -rt doesn't compile for UML)

Found some interesting things, the delays are caused by

 1) throttle_vm_writeout()

     I removed it, Andrew worries about that, but hopefully there's a
     better solution to his worries

 2) atime updates

     my uml image did not have them turned off

 3) UML timer tick does not seem very reliable, here's what the below
    patch outputs during writing.  It shows the delay between ticks in
    milliseconds:

[42949415.110000] zzz - 267
[42949415.120000] zzz - 862
[42949415.130000] zzz - 71
[42949415.140000] zzz - 599
[42949415.170000] zzz - 3746
[42949415.190000] zzz - 2961
[42949415.200000] zzz - 3272
[42949415.210000] zzz - 253
[42949415.220000] zzz - 1692
[42949415.230000] zzz - 823
[42949415.280000] zzz - 344
[42949415.310000] zzz - 224

I guess, the ubd interrupts confuse the timekeeping somehow, but the
details are unclear.

Miklos


Index: linux/arch/um/kernel/time.c
===================================================================
--- linux.orig/arch/um/kernel/time.c	2007-11-30 20:19:17.000000000 +0100
+++ linux/arch/um/kernel/time.c	2007-11-30 20:21:24.000000000 +0100
@@ -23,6 +23,12 @@ unsigned long long sched_clock(void)
 void timer_handler(int sig, struct uml_pt_regs *regs)
 {
 	unsigned long flags;
+	static long long last;
+	long long now = os_nsecs() / 1000000;
+
+	if (now - last > 50)
+		printk("zzz - %lli\n", now-last);
+	last = now;
 
 	local_irq_save(flags);
 	do_IRQ(TIMER_IRQ, regs);
-
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