Re: [ltt-dev] [PATCH] Fix dirty page accounting inredirty_page_for_writepage()

[Ingo Molnar]

* Mathieu Desnoyers <compudj@xxxxxxxxxxxxxxxxxx> wrote:

> * Ingo Molnar (mingo@xxxxxxx) wrote:
> > 
> > * Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote:
> > 
> > > And thanks for the review! This excercise only convinced me that 
> > > the kernel memory accounting works as expected. All this gave me 
> > > the chance to have a good look at the memory accounting code. We 
> > > could probably benefit of Christoph Lameter's cpu ops (using 
> > > segment registers to address per-cpu variables with atomic 
> > > inc/dec) in there. Or at least removing interrupt disabling by 
> > > using preempt disable and local_t variables for the per-cpu 
> > > counters could bring some benefit.
> > 
> > Note, optimized per cpu ops are already implemented upstream, by 
> > Tejun Heo's percpu patches in .30:
> > 
> >  #define percpu_read(var)	percpu_from_op("mov", per_cpu__##var)
> >  #define percpu_write(var, val)	percpu_to_op("mov", per_cpu__##var, val)
> >  #define percpu_add(var, val)	percpu_to_op("add", per_cpu__##var, val)
> >  #define percpu_sub(var, val)	percpu_to_op("sub", per_cpu__##var, val)
> >  #define percpu_and(var, val)	percpu_to_op("and", per_cpu__##var, val)
> >  #define percpu_or(var, val)	percpu_to_op("or", per_cpu__##var, val)
> >  #define percpu_xor(var, val)	percpu_to_op("xor", per_cpu__##var, val)
> > 
> > See:
> > 
> >   6dbde35: percpu: add optimized generic percpu accessors
> > 
> > From the changelog:
> > 
> >     [...]
> >     The advantage is that for example to read a local percpu variable,
> >     instead of this sequence:
> >     
> >      return __get_cpu_var(var);
> >     
> >      ffffffff8102ca2b:  48 8b 14 fd 80 09 74    mov    -0x7e8bf680(,%rdi,8),%rdx
> >      ffffffff8102ca32:  81
> >      ffffffff8102ca33:  48 c7 c0 d8 59 00 00    mov    $0x59d8,%rax
> >      ffffffff8102ca3a:  48 8b 04 10             mov    (%rax,%rdx,1),%rax
> >     
> >     We can get a single instruction by using the optimized variants:
> >     
> >      return percpu_read(var);
> >     
> >      ffffffff8102ca3f:  65 48 8b 05 91 8f fd    mov    %gs:0x7efd8f91(%rip),%rax
> >     [...]
> > 
> > So if you want to make use of it, percpu_add()/percpu_sub() would be 
> > the place to start.
> > 
> 
> Great !
> 
> I see however that it's only guaranteed to be atomic wrt preemption.

That's really only true for the non-x86 fallback defines. If we so 
decide, we could make the fallbacks in asm-generic/percpu.h irq-safe 
...

> What would be even better would be to have the atomic ops wrt local irqs
> (as local.h does) available in this percpu flavor. By doing this, we
> could have interrupt and nmi-safe per-cpu counters, without even the
> need to disable preemption.

nmi-safe isnt a big issue (we have no NMI code that interacts with 
MM counters) - and we could make them irq-safe by fixing the 
wrapper. (and on x86 they are NMI-safe too.)

	Ingo
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