From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jack Steiner <steiner@sgi.com>
Date: Thu, 22 Feb 2001 20:48:03 +0000
Subject: Re: Re: Re: [Linux-ia64] Re: Lockups on 2.4.1
Message-Id: <marc-linux-ia64-105590693005190@msgid-missing>
List-Id: <linux-ia64.vger.kernel.org>
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To: linux-ia64@vger.kernel.org

> > Anyway, I have ITPs connected to the IBM hardware and have noticed that
> > when the lockup occurs, and we lose video, at least one of the CPUs is
> > executing in flush_tlb_no_ptcg() or handle_IPI(), in the 'do' loop where
> > TLB
> > entries are being purged. What I have observed is that the end address and
> > the start address are in completely different regions. Usually, the start
> > address
> > is in region register 1 (address of 0x2000XXXXXXXXXXXX) and the end address
> > is in region register 3 (address of 0x6000XXXXXXXXXXXX). I don't know if
> > this
> > is the same problem I am seeing on the Lion, but I plan to connect and ITP
> > and
> > a serial console (although we haven't been able to get one to work yet on
> > the
> > Lion with BIOS 71) to see if the symptoms are the same.
> 
> FWIW, we have seen EXACTLY the same hang running here on our system.
> The start/end addresses for the purge cross region boundaries.
> 
> 
> We are running a 2.4.0 kernel.

I found a problem that was causing the lockup described above & I suspect this
may responsible for some of the other hangs various folks have seen.

There is code in flush_tlb_no_ptcg() that resends the IPI if other
cpus have not responded within a short time. If this code get invoked, then
it is possible for flush_cpu_count to get corrupted. When that happens, a cpu
can be executing in handle_IPI() while flush_start/flush_end are changing.
A cpu can pick up a non-matching flush_start/flush_end. This leads to  hangs or
lost TLB flushes.

To verify that this could cause the hang, I changed the timeout in
flush_tlb_no_ptcg() from 40000UL to 400UL. I hung before getting to multiuser mode
with flush_start/flush_end in different regions.

Here is the patch I used. Note: this is against 2.4.0,


--- linux-trillian/arch/ia64/kernel/smp.c	Thu Feb 22 14:35:28 2001
+++ linux/arch/ia64/kernel/smp.c	Thu Feb 22 14:19:46 2001
@@ -321,6 +321,16 @@
 {
 	send_IPI_allbutself(IPI_FLUSH_TLB);
 }
+
+void
+smp_resend_flush_tlb(void)
+{
+	/*
+	 * Really need a null IPI but since this rarely should happen &
+	 * since this code will go away, lets not add one.
+	 */
+	send_IPI_allbutself(IPI_RESCHEDULE);
+}
 #endif	/* !CONFIG_ITANIUM_PTCG */
 
 /*
--- linux-trillian/arch/ia64/mm/tlb.c	Thu Feb 22 14:35:28 2001
+++ linux/arch/ia64/mm/tlb.c	Thu Feb 22 14:19:50 2001
@@ -59,6 +59,7 @@
 flush_tlb_no_ptcg (unsigned long start, unsigned long end, unsigned long nbits)
 {
 	extern void smp_send_flush_tlb (void);
+	extern void smp_resend_flush_tlb (void);
 	unsigned long saved_tpr = 0;
 	unsigned long flags;
 
@@ -101,9 +102,8 @@
 	{
 		unsigned long start = ia64_get_itc();
 		while (atomic_read(&flush_cpu_count) > 0) {
-			if ((ia64_get_itc() - start) > 40000UL) {
-				atomic_set(&flush_cpu_count, smp_num_cpus - 1);
-				smp_send_flush_tlb();
+			if ((ia64_get_itc() - start) > 400UL) {
+				smp_resend_flush_tlb();
 				start = ia64_get_itc();
 			}
 		}

-- 
Thanks

Jack Steiner    (651-683-5302)   (vnet 233-5302)      steiner@sgi.com