Re: [BUG] long freezes on thinkpad t60

[Ingo Molnar <mingo@elte.hu>]

* Ingo Molnar <mingo@elte.hu> wrote:

> > > > > This change causes the memory access of the "easy" spin-loop 
> > > > > portion to be more agressive: after the REP; NOP we'd not do 
> > > > > the 'easy-loop' with a simple CMPB, but we'd re-attempt the 
> > > > > atomic op.
> > > > 
> > > > It looks as if this is going to overflow of the lock counter, 
> > > > no?
> > > 
> > > hm, what do you mean? There's no lock counter.
> > 
> > I mean, the repeated calls to decb will pretty soon make lock->slock 
> > wrap around.
> 
> ugh, indeed, bad thinko on my part. I'll rework this.

how about the patch below? Boot-tested on 32-bit. As a side-effect this 
change also removes the 255 CPUs limit from the 32-bit kernel.

	Ingo

------------------------->
Subject: [patch] x86: fix spin-loop starvation bug
From: Ingo Molnar <mingo@elte.hu>

Miklos Szeredi reported very long pauses (several seconds, sometimes
more) on his T60 (with a Core2Duo) which he managed to track down to
wait_task_inactive()'s open-coded busy-loop. He observed that an
interrupt on one core tries to acquire the runqueue-lock but does not
succeed in doing so for a very long time - while wait_task_inactive() on
the other core loops waiting for the first core to deschedule a task
(which it wont do while spinning in an interrupt handler).

The problem is: both the spin_lock() code and the wait_task_inactive()
loop uses cpu_relax()/rep_nop(), so in theory the CPU should have
guaranteed MESI-fairness to the two cores - but that didnt happen: one
of the cores was able to monopolize the cacheline that holds the
runqueue lock, for extended periods of time.

This patch changes the spin-loop to assert an atomic op after every REP
NOP instance - this will cause the CPU to express its "MESI interest" in
that cacheline after every REP NOP.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 include/asm-i386/spinlock.h   |   27 ++++++++++-----------------
 include/asm-x86_64/spinlock.h |   33 ++++++++++++++++-----------------
 2 files changed, 26 insertions(+), 34 deletions(-)

Index: linux-cfs-2.6.22-rc5.q/include/asm-i386/spinlock.h
===================================================================
--- linux-cfs-2.6.22-rc5.q.orig/include/asm-i386/spinlock.h
+++ linux-cfs-2.6.22-rc5.q/include/asm-i386/spinlock.h
@@ -35,15 +35,12 @@ static inline int __raw_spin_is_locked(r
 static inline void __raw_spin_lock(raw_spinlock_t *lock)
 {
 	asm volatile("\n1:\t"
-		     LOCK_PREFIX " ; decb %0\n\t"
-		     "jns 3f\n"
-		     "2:\t"
-		     "rep;nop\n\t"
-		     "cmpb $0,%0\n\t"
-		     "jle 2b\n\t"
+		     LOCK_PREFIX " ; btrl %[zero], %[slock]\n\t"
+		     "jc 3f\n"
+		     "rep; nop\n\t"
 		     "jmp 1b\n"
 		     "3:\n\t"
-		     : "+m" (lock->slock) : : "memory");
+		     : [slock] "+m" (lock->slock) : [zero] "Ir" (0) : "memory");
 }
 
 /*
@@ -59,27 +56,23 @@ static inline void __raw_spin_lock_flags
 {
 	asm volatile(
 		"\n1:\t"
-		LOCK_PREFIX " ; decb %[slock]\n\t"
+		LOCK_PREFIX " ; btrl %[zero], %[slock]\n\t"
 		"jns 5f\n"
 		"2:\t"
 		"testl $0x200, %[flags]\n\t"
 		"jz 4f\n\t"
 		STI_STRING "\n"
-		"3:\t"
-		"rep;nop\n\t"
-		"cmpb $0, %[slock]\n\t"
-		"jle 3b\n\t"
+		"rep; nop\n\t"
 		CLI_STRING "\n\t"
 		"jmp 1b\n"
 		"4:\t"
-		"rep;nop\n\t"
-		"cmpb $0, %[slock]\n\t"
-		"jg 1b\n\t"
+		"rep; nop\n\t"
 		"jmp 4b\n"
 		"5:\n\t"
 		: [slock] "+m" (lock->slock)
-		: [flags] "r" (flags)
-	 	  CLI_STI_INPUT_ARGS
+		: [zero] "Ir" (0),
+		  [flags] "r" (flags)
+		  CLI_STI_INPUT_ARGS
 		: "memory" CLI_STI_CLOBBERS);
 }
 #endif
Index: linux-cfs-2.6.22-rc5.q/include/asm-x86_64/spinlock.h
===================================================================
--- linux-cfs-2.6.22-rc5.q.orig/include/asm-x86_64/spinlock.h
+++ linux-cfs-2.6.22-rc5.q/include/asm-x86_64/spinlock.h
@@ -26,14 +26,15 @@ static inline void __raw_spin_lock(raw_s
 {
 	asm volatile(
 		"\n1:\t"
-		LOCK_PREFIX " ; decl %0\n\t"
+		LOCK_PREFIX " ; btrl %[zero], %[slock]\n\t"
 		"jns 2f\n"
-		"3:\n"
-		"rep;nop\n\t"
-		"cmpl $0,%0\n\t"
-		"jle 3b\n\t"
+		"rep; nop\n\t"
 		"jmp 1b\n"
-		"2:\t" : "=m" (lock->slock) : : "memory");
+		"2:\t"
+		: [slock] "+m" (lock->slock)
+		: [zero] "Ir" (0)
+		: "memory"
+	);
 }
 
 /*
@@ -44,24 +45,22 @@ static inline void __raw_spin_lock_flags
 {
 	asm volatile(
 		"\n1:\t"
-		LOCK_PREFIX " ; decl %0\n\t"
+		LOCK_PREFIX " ; btrl %[zero], %[slock]\n\t"
 		"jns 5f\n"
-		"testl $0x200, %1\n\t"	/* interrupts were disabled? */
+		"testl $0x200, %[flags]\n\t"	/* were interrupts disabled? */
 		"jz 4f\n\t"
 	        "sti\n"
-		"3:\t"
-		"rep;nop\n\t"
-		"cmpl $0, %0\n\t"
-		"jle 3b\n\t"
+		"rep; nop\n\t"
 		"cli\n\t"
 		"jmp 1b\n"
-		"4:\t"
-		"rep;nop\n\t"
-		"cmpl $0, %0\n\t"
-		"jg 1b\n\t"
+		"rep; nop\n\t"
 		"jmp 4b\n"
 		"5:\n\t"
-		: "+m" (lock->slock) : "r" ((unsigned)flags) : "memory");
+		: [slock] "+m" (lock->slock)
+		: [zero] "Ir" (0),
+		  [flags] "r" ((unsigned)flags)
+		: "memory"
+	);
 }
 #endif
 
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