lkml.org 
[lkml]   [2020]   [Mar]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 1/1] ppc/crash: Skip spinlocks during crash
From
Date
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,
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2020-03-30 16:34    [W:0.076 / U:0.324 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site