From: Christoph Lameter <cl@linux.com>
To: Tejun Heo <tj@kernel.org>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>,
David Rientjes <rientjes@google.com>,
Eric Dumazet <eric.dumazet@gmail.com>,
"H. Peter Anvin" <hpa@zytor.com>,
linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [slubllv7 04/17] x86: Add support for cmpxchg_double
Date: Wed, 15 Jun 2011 12:19:35 -0500 (CDT) [thread overview]
Message-ID: <alpine.DEB.2.00.1106151213180.4224@router.home> (raw)
In-Reply-To: <20110615163952.GE8141@htj.dyndns.org>
On Wed, 15 Jun 2011, Tejun Heo wrote:
> > > > +#define system_has_cmpxchg_double() cpu_has_cx16
> > >
> > > Where's the fallback %false definition for the above feature macro for
> > > archs which don't support cmpxchg_double? Also, is system_has_*()
> > > conventional? Isn't arch_has_*() more conventional for this purpose?
> >
> > There is a convention for querying processor flags from core code?
>
> At least generic ptrace code uses arch_has_block/single_step().
> Probably better than introducing something completely new.
Its not a property of the arch that we are after. We need to know of the
hardware that is running the kernel (the system) has that capability.
> > The system_has_cmpxchg_double() is only used if the arch defines
> > CONFIG_CMPXCHG_DOUBLE
>
> Why? What's the benefit of that?
You dont have to define system_has_cmpxchg_double() for systems without
CONFIG_CMPXCHG_DOUBLE.
> > This way it is done in the same way on 32 bit than on 64 bit. The use of
> > cmpxchg64 also means that some of the parameters would have to be combined
> > to form 64 bit ints from the 32 bit ones before __cmpxchg64 could be used.
> >
> > __cmpxchg64 has different parameter conventions.
>
> But they all just deal with the starting addresses and the _local
> version already has proper fallback implementation.
The local version is not that problematic since it is just provided for
completeness. If you want to go through the gyrations of providing a
conversion layer with casting and conversion between 32 and 64 bit
entities then please do so. But its not going to be nice.
> > The fallback through the subsystem means that the subsystem can do locking
> > that scales better. In the case of SLUB we fall back to a bit lock in the
> > page struct which is a hot cache line in the hotpaths. This is the same
> > approach as used before the lockless patches and we expect the performance
> > on platforms not supporting cmpxchg_double to stay the same.
>
> Yes, that's nice but you're introducing new operations and they should
> meet the usual conventions and cmpxchg fallback on the arch which I
> don't recall now already uses hashed lock so it's not like this is
> completely new. As added, the interface basically requires extreme
> ifdeffery which isn't good.
I'd be glad if you could improve on it. But the fallback to a hashed lock
would also mean additional cache footprint which would not be acceptable
for the hotpaths that this is used for now.
next prev parent reply other threads:[~2011-06-15 17:19 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-01 17:25 [slubllv7 00/17] SLUB: Lockless freelists for objects V7 Christoph Lameter
2011-06-01 17:25 ` [slubllv7 01/17] slub: Push irq disable into allocate_slab() Christoph Lameter
2011-06-01 17:25 ` [slubllv7 02/17] slub: Do not use frozen page flag but a bit in the page counters Christoph Lameter
2011-06-01 17:25 ` [slubllv7 03/17] slub: Move page->frozen handling near where the page->freelist handling occurs Christoph Lameter
2011-06-01 17:25 ` [slubllv7 04/17] x86: Add support for cmpxchg_double Christoph Lameter
2011-06-09 9:53 ` Pekka Enberg
2011-06-10 15:17 ` Christoph Lameter
2011-06-11 9:50 ` Pekka Enberg
2011-06-11 17:02 ` Christoph Lameter
2011-06-14 5:49 ` Pekka Enberg
2011-06-14 8:04 ` Ingo Molnar
2011-06-14 14:04 ` Christoph Lameter
2011-06-14 15:05 ` H. Peter Anvin
2011-06-15 8:55 ` Tejun Heo
2011-06-15 14:26 ` Christoph Lameter
2011-06-15 16:39 ` Tejun Heo
2011-06-15 17:19 ` Christoph Lameter [this message]
2011-06-25 23:49 ` [tip:x86/atomic] " tip-bot for Christoph Lameter
2011-06-01 17:25 ` [slubllv7 05/17] mm: Rearrange struct page Christoph Lameter
2011-06-09 9:57 ` Pekka Enberg
2011-06-09 16:45 ` Andrew Morton
2011-06-09 17:03 ` [PATCH] checkpatch: Add a "prefer __aligned" check Joe Perches
2011-06-01 17:25 ` [slubllv7 06/17] slub: Add cmpxchg_double_slab() Christoph Lameter
2011-07-11 19:55 ` Eric Dumazet
2011-07-12 15:59 ` Christoph Lameter
2011-07-12 16:06 ` Eric Dumazet
2011-07-12 16:47 ` Christoph Lameter
2011-07-12 18:40 ` H. Peter Anvin
2011-07-12 18:53 ` Christoph Lameter
2011-07-12 20:40 ` H. Peter Anvin
2011-06-01 17:25 ` [slubllv7 07/17] slub: explicit list_lock taking Christoph Lameter
2011-06-01 17:25 ` [slubllv7 08/17] slub: Pass kmem_cache struct to lock and freeze slab Christoph Lameter
2011-06-01 17:25 ` [slubllv7 09/17] slub: Rework allocator fastpaths Christoph Lameter
2011-06-01 17:25 ` [slubllv7 10/17] slub: Invert locking and avoid slab lock Christoph Lameter
2011-06-01 17:25 ` [slubllv7 11/17] slub: Disable interrupts in free_debug processing Christoph Lameter
2011-06-01 17:25 ` [slubllv7 12/17] slub: Avoid disabling interrupts in free slowpath Christoph Lameter
2011-06-01 17:25 ` [slubllv7 13/17] slub: Get rid of the another_slab label Christoph Lameter
2011-06-01 17:25 ` [slubllv7 14/17] slub: Add statistics for the case that the current slab does not match the node Christoph Lameter
2011-06-01 17:25 ` [slubllv7 15/17] slub: fast release on full slab Christoph Lameter
2011-06-01 17:25 ` [slubllv7 16/17] slub: Not necessary to check for empty slab on load_freelist Christoph Lameter
2011-06-01 17:26 ` [slubllv7 17/17] slub: slabinfo update for cmpxchg handling Christoph Lameter
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