Re: [PATCH] Fix dirty page accounting in redirty_page_for_writepage()

Ingo Molnar
2009-04-30T16:08:17Z

* Christoph Lameter <cl@linux.com> wrote:

> On Thu, 30 Apr 2009, Ingo Molnar wrote:
>
> > nice. Do these all get called with irqs off, all the time?
>
> In most cases yes. The VMstat patch includes an example were we
> may not care too much about a event counter missing a beat once in
> a while for platforms not supporting atomic per cpu ops. I know
> this affects IA64. The cost of an atomic operations for an event
> counter update (which would have avoided the potential of a
> concurrent update) was not justifiable.

when you say "atomics", do you mean the classic meaning of atomics?
Because there are no classic atomics involved. This is the
before/after disassembly from Eric's commit 4e69489a0:

before:

c0436274: b8 01 00 00 00 mov $0x1,%eax
c0436279: e8 42 40 df ff call c022a2c0 <add_preempt_count>
c043627e: bb 20 4f 6a c0 mov $0xc06a4f20,%ebx
c0436283: e8 18 ca f0 ff call c0342ca0 <debug_smp_processor_id>
c0436288: 03 1c 85 60 4a 65 c0 add -0x3f9ab5a0(,%eax,4),%ebx
c043628f: ff 03 incl (%ebx)
c0436291: b8 01 00 00 00 mov $0x1,%eax
c0436296: e8 75 3f df ff call c022a210 <sub_preempt_count>
c043629b: 89 e0 mov %esp,%eax
c043629d: 25 00 e0 ff ff and $0xffffe000,%eax
c04362a2: f6 40 08 08 testb $0x8,0x8(%eax)
c04362a6: 75 07 jne c04362af <sock_alloc+0x7f>
c04362a8: 8d 46 d8 lea -0x28(%esi),%eax
c04362ab: 5b pop %ebx
c04362ac: 5e pop %esi
c04362ad: c9 leave
c04362ae: c3 ret
c04362af: e8 cc 5d 09 00 call c04cc080 <preempt_schedule>
c04362b4: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi
c04362b8: eb ee jmp c04362a8 <sock_alloc+0x78>

after:

c0436275: 64 83 05 20 5f 6a c0 addl $0x1,%fs:0xc06a5f20

There's no atomic instructions at all - the counters here are only
accessed locally. They are local-irq-atomic, but not
cacheline-atomic.

Ingo