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Date: Thu, 27 Jun 2013 17:44:44 -0500
From: Shawn Bohrer <sbohrer@...advisors.com>
To: Rick Jones <rick.jones2@...com>
Cc: netdev@...r.kernel.org
Subject: Re: Understanding lock contention in __udp4_lib_mcast_deliver
On Thu, Jun 27, 2013 at 03:03:15PM -0700, Rick Jones wrote:
> On 06/27/2013 02:54 PM, Shawn Bohrer wrote:
> >On Thu, Jun 27, 2013 at 01:46:58PM -0700, Rick Jones wrote:
> >>How do you know that time is actually contention and not simply
> >>acquire and release overhead?
> >
> >Excellent point, and that could be the problem with my thinking. I
> >just now tried (unsuccessfully) to use lockstat to see if there was
> >any contention reported. I read Documentation/lockstat.txt and
> >followed the instructions but the lock in question did not appear to
> >be in the output. I think I'm going to have to go with the assumption
> >that this is just acquire and release overhead.
>
> I think there is a way to get perf to "annotate" (iirc that is the
> term it uses) the report to show hits at the instruction level.
> Ostensibly one could then look and see how many of the hits were for
> the acquire/release part of the routine, and how much was for the
> actual contention.
Yep, so ~1% of my total time is in _raw_spin_lock and using perf
annotate it appears that maybe only 5-6% percent of that is actually
contention and the rest is acquire/release. Looks like I need to look
elsewhere for my performance improvements. Thanks Rick for your help!
Below is the output of perf annotate if your curious.
Percent | Source code & Disassembly of vmlinux
------------------------------------------------
:
:
:
: Disassembly of section .text:
:
: ffffffff814c72d0 <_raw_spin_lock>:
: EXPORT_SYMBOL(_raw_spin_trylock_bh);
: #endif
:
: #ifndef CONFIG_INLINE_SPIN_LOCK
: void __lockfunc _raw_spin_lock(raw_spinlock_t *lock)
: {
2.43 : ffffffff814c72d0: callq ffffffff814cf440 <__fentry__>
1.23 : ffffffff814c72d5: push %rbp
1.66 : ffffffff814c72d6: mov %rsp,%rbp
: */
: static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
: {
: register struct __raw_tickets inc = { .tail = 1 };
:
: inc = xadd(&lock->tickets, inc);
0.71 : ffffffff814c72d9: mov $0x10000,%eax
0.00 : ffffffff814c72de: lock xadd %eax,(%rdi)
86.07 : ffffffff814c72e2: mov %eax,%edx
0.05 : ffffffff814c72e4: shr $0x10,%edx
:
: for (;;) {
: if (inc.head == inc.tail)
0.00 : ffffffff814c72e7: cmp %ax,%dx
0.00 : ffffffff814c72ea: je ffffffff814c72fa <_raw_spin_lock+0x2a>
0.04 : ffffffff814c72ec: nopl 0x0(%rax)
: }
:
: /* REP NOP (PAUSE) is a good thing to insert into busy-wait loops. */
: static inline void rep_nop(void)
: {
: asm volatile("rep; nop" ::: "memory");
0.47 : ffffffff814c72f0: pause
: break;
: cpu_relax();
: inc.head = ACCESS_ONCE(lock->tickets.head);
2.85 : ffffffff814c72f2: movzwl (%rdi),%eax
: register struct __raw_tickets inc = { .tail = 1 };
:
: inc = xadd(&lock->tickets, inc);
:
: for (;;) {
: if (inc.head == inc.tail)
3.53 : ffffffff814c72f5: cmp %ax,%dx
0.00 : ffffffff814c72f8: jne ffffffff814c72f0 <_raw_spin_lock+0x20>
: __raw_spin_lock(lock);
: }
0.91 : ffffffff814c72fa: pop %rbp
0.00 : ffffffff814c72fb: retq
--
Shawn
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