Re: [PATCH 1/1] ppc/crash: Skip spinlocks during crash

[Leonardo Bras <leonardo@linux.ibm.com>]

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Hello Christophe,

On Sat, 2020-03-28 at 10:19 +0000, Christophe Leroy wrote:
> Hi Leonardo,
> 
> 
> > On 03/27/2020 03:51 PM, Leonardo Bras wrote:
> > > 
> > [SNIP]
> > - If the lock is already free, it would change nothing,
> > - Otherwise, the lock will wait.
> > - Waiting cycle just got bigger.
> > - Worst case scenario: running one more cycle, given lock->slock can
> > turn to 0 just after checking.Could you please point where I failed to see the performance penalty?
> > (I need to get better at this :) )
> 
> You are right that when the lock is free, it changes nothing. However 
> when it is not, it is not just one cycle.

Sorry, what I meant here is one "waiting cycle", meaning that in WCS
there would be 1 extra iteration on that while. Or it would 'spin' one
more time.

> 
> Here is arch_spin_lock() without your patch:
> 
> 00000440 <my_lock>:
>   440:	39 40 00 01 	li      r10,1
>   444:	7d 20 18 28 	lwarx   r9,0,r3
>   448:	2c 09 00 00 	cmpwi   r9,0
>   44c:	40 82 00 10 	bne     45c <my_lock+0x1c>
>   450:	7d 40 19 2d 	stwcx.  r10,0,r3
>   454:	40 a2 ff f0 	bne     444 <my_lock+0x4>
>   458:	4c 00 01 2c 	isync
>   45c:	2f 89 00 00 	cmpwi   cr7,r9,0
>   460:	4d be 00 20 	bclr+   12,4*cr7+eq
>   464:	7c 21 0b 78 	mr      r1,r1
>   468:	81 23 00 00 	lwz     r9,0(r3)
>   46c:	2f 89 00 00 	cmpwi   cr7,r9,0
>   470:	40 be ff f4 	bne     cr7,464 <my_lock+0x24>
>   474:	7c 42 13 78 	mr      r2,r2
>   478:	7d 20 18 28 	lwarx   r9,0,r3
>   47c:	2c 09 00 00 	cmpwi   r9,0
>   480:	40 82 00 10 	bne     490 <my_lock+0x50>
>   484:	7d 40 19 2d 	stwcx.  r10,0,r3
>   488:	40 a2 ff f0 	bne     478 <my_lock+0x38>
>   48c:	4c 00 01 2c 	isync
>   490:	2f 89 00 00 	cmpwi   cr7,r9,0
>   494:	40 be ff d0 	bne     cr7,464 <my_lock+0x24>
>   498:	4e 80 00 20 	blr
> 
> Here is arch_spin_lock() with your patch. I enclose with === what comes 
> in addition:
> 
> 00000440 <my_lock>:
>   440:	39 40 00 01 	li      r10,1
>   444:	7d 20 18 28 	lwarx   r9,0,r3
>   448:	2c 09 00 00 	cmpwi   r9,0
>   44c:	40 82 00 10 	bne     45c <my_lock+0x1c>
>   450:	7d 40 19 2d 	stwcx.  r10,0,r3
>   454:	40 a2 ff f0 	bne     444 <my_lock+0x4>
>   458:	4c 00 01 2c 	isync
>   45c:	2f 89 00 00 	cmpwi   cr7,r9,0
>   460:	4d be 00 20 	bclr+   12,4*cr7+eq
> =====================================================
>   464:	3d 40 00 00 	lis     r10,0
> 			466: R_PPC_ADDR16_HA	crash_skip_spinlock
>   468:	39 4a 00 00 	addi    r10,r10,0
> 			46a: R_PPC_ADDR16_LO	crash_skip_spinlock
>   46c:	39 00 00 01 	li      r8,1
>   470:	89 2a 00 00 	lbz     r9,0(r10)
>   474:	2f 89 00 00 	cmpwi   cr7,r9,0
>   478:	4c 9e 00 20 	bnelr   cr7
> =====================================================
>   47c:	7c 21 0b 78 	mr      r1,r1
>   480:	81 23 00 00 	lwz     r9,0(r3)
>   484:	2f 89 00 00 	cmpwi   cr7,r9,0
>   488:	40 be ff f4 	bne     cr7,47c <my_lock+0x3c>
>   48c:	7c 42 13 78 	mr      r2,r2
>   490:	7d 20 18 28 	lwarx   r9,0,r3
>   494:	2c 09 00 00 	cmpwi   r9,0
>   498:	40 82 00 10 	bne     4a8 <my_lock+0x68>
>   49c:	7d 00 19 2d 	stwcx.  r8,0,r3
>   4a0:	40 a2 ff f0 	bne     490 <my_lock+0x50>
>   4a4:	4c 00 01 2c 	isync
>   4a8:	2f 89 00 00 	cmpwi   cr7,r9,0
>   4ac:	40 be ff c4 	bne     cr7,470 <my_lock+0x30>
>   4b0:	4e 80 00 20 	blr
> 
> 
> Christophe

I agree. When there is waiting, it will usually add some time to it.
Accounting that spinlocks are widely used, it will cause a slowdown in
the whole system.

Thanks for the feedback,
Best regards,

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