From: Rusty Russell <rusty@rustcorp•com.au>
To: Tejun Heo <tj@kernel•org>
Cc: Ingo Molnar <mingo@elte•hu>,
Herbert Xu <herbert@gondor•apana.org.au>,
akpm@linux-foundation•org, hpa@zytor•com, brgerst@gmail•com,
ebiederm@xmission•com, cl@linux-foundation•org, travis@sgi•com,
linux-kernel@vger•kernel.org, steiner@sgi•com, hugh@veritas•com,
"David S. Miller" <davem@davemloft•net>,
netdev@vger•kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@polymtl•ca>
Subject: Re: [PATCH] percpu: add optimized generic percpu accessors
Date: Wed, 28 Jan 2009 21:08:49 +1030 [thread overview]
Message-ID: <200901282108.51864.rusty@rustcorp.com.au> (raw)
In-Reply-To: <497E705B.5000302@kernel.org>
On Tuesday 27 January 2009 12:54:27 Tejun Heo wrote:
> Hello, Rusty.
Hi Tejun!
> There actually were quite some places where atomic add ops would be
> useful, especially the places where statistics are collected. For
> logical bitops, I don't think we'll have too many of them.
If the stats are only manipulated in one context, than an atomic requirement is overkill (and expensive on non-x86).
> > If they are worth doing generically, should the ops be atomic? To
> > extrapolate from x86 usages again, it seems to be happy with
> > non-atomic (tho of course it is atomic on x86).
>
> If atomic rw/add/sub are implementible on most archs (and judging from
> local_t, I suppose it is), I think it should. So that it can replace
> local_t and we won't need something else again in the future.
This is more like Christoph's CPU_OPS: they were special operators on normal per-cpu vars/ptrs. Generic version was irqsave+op+irqrestore.
I actually like this idea, but Mathieu insists that the ops be NMI-safe, for ftrace. Hence local_t needing to be atomic_t for generic code.
AFAICT we'll need a hybrid: HAVE_NMISAFE_CPUOPS, and if not, use atomic_t
in ftrace (which isn't NMI safe on parisc or sparc/32 anyway, but I don't think we care).
Other than the shouting, I liked Christoph's system:
- CPU_INC = always safe (eg. local_irq_save/per_cpu(i)++/local_irq_restore)
- _CPU_INC = not safe against interrupts (eg. get_cpu/per_cpu(i)++/put_cpu)
- __CPU_INC = not safe against anything (eg. per_cpu(i)++)
I prefer the name 'local' to the name 'cpu', but I'm not hugely fussed.
> >> Another question to ask is whether to keep using separate
> >> interfaces for static and dynamic percpu variables or migrate to
> >> something which can take both.
> >
> > Well, IA64 can do stuff with static percpus that it can't do with
> > dynamic (assuming we get expanding dynamic percpu areas
> > later). That's because they use TLB tricks for a static 64k per-cpu
> > area, but this doesn't scale. That might not be vital: abandoning
> > that trick will mean they can't optimise read_percpu/read_percpu_var
> > etc as much.
>
> Isn't something like the following possible?
>
> #define pcpu_read(ptr) \
> ({ \
> if (__builtin_constant_p(ptr) && \
> ptr >= PCPU_STATIC_START && ptr < PCPU_STATIC_END) \
> do 64k TLB trick for static pcpu; \
> else \
> do generic stuff; \
> })
No, that will be "do generic stuff", since it's a link-time constant. I don't know that this is a huge worry, to be honest. We can leave the __ia64_per_cpu_var for their arch-specific code (I feel the same way about x86 to be honest).
> > Tejun, any chance of you updating the tj-percpu tree? My current
> > patches are against Linus's tree, and rebasing them on yours
> > involves some icky merging.
>
> If Ingo is okay with it, I'm fine with it too. Unless Ingo objects,
> I'll do it tomorrow-ish (still big holiday here).
Ah, I did not realize that you celebrated Australia day :)
Cheers!
Rusty.
next prev parent reply other threads:[~2009-01-28 10:39 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20090115183942.GA6325@elte.hu>
[not found] ` <20090116001200.GA9137@gondor.apana.org.au>
[not found] ` <20090116001544.GA11073@elte.hu>
2009-01-16 0:18 ` [PATCH] percpu: add optimized generic percpu accessors Herbert Xu
[not found] ` <200901170827.33729.rusty@rustcorp.com.au>
2009-01-16 22:08 ` Ingo Molnar
[not found] ` <200901201328.24605.rusty@rustcorp.com.au>
2009-01-20 6:25 ` Tejun Heo
2009-01-20 10:36 ` Ingo Molnar
[not found] ` <200901271213.18605.rusty@rustcorp.com.au>
2009-01-27 2:24 ` Tejun Heo
2009-01-27 13:13 ` Ingo Molnar
2009-01-27 23:07 ` Tejun Heo
2009-01-28 3:36 ` Tejun Heo
2009-01-28 8:12 ` Tejun Heo
2009-01-27 20:08 ` Christoph Lameter
2009-01-27 21:47 ` David Miller
2009-01-27 22:47 ` Rick Jones
2009-01-28 0:17 ` Luck, Tony
2009-01-28 16:48 ` Christoph Lameter
2009-01-28 17:15 ` Luck, Tony
2009-01-28 16:45 ` Christoph Lameter
2009-01-28 20:47 ` David Miller
2009-01-28 10:38 ` Rusty Russell [this message]
2009-01-28 10:56 ` Tejun Heo
2009-01-29 2:06 ` Rusty Russell
2009-01-31 6:11 ` Tejun Heo
2009-01-28 16:50 ` Christoph Lameter
2009-01-28 18:07 ` Mathieu Desnoyers
2009-01-29 18:33 ` Christoph Lameter
2009-01-29 18:48 ` H. Peter Anvin
2009-01-20 10:40 ` Ingo Molnar
2009-01-21 5:52 ` Tejun Heo
2009-01-21 10:05 ` Ingo Molnar
2009-01-21 11:21 ` Eric W. Biederman
2009-01-21 12:45 ` Stephen Hemminger
2009-01-21 14:13 ` Eric W. Biederman
2009-01-21 20:34 ` David Miller
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