linux-kernel - Re: [PATCH -v6 2/3] lib, Make gen

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Date:	Tue, 30 Nov 2010 09:08:09 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Len Brown <lenb@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH -v6 2/3] lib, Make gen_pool memory allocator lockless

Hi, Peter,

On Mon, 2010-11-29 at 20:11 +0800, Peter Zijlstra wrote:
> On Mon, 2010-11-29 at 15:03 +0800, Huang Ying wrote:
> > This version of the gen_pool memory allocator supports lockless
> > operation.
> > 
> > This makes it safe to use in NMI handlers and other special
> > unblockable contexts that could otherwise deadlock on locks.  This is
> > implemented by using atomic operations and retries on any conflicts.
> > The disadvantage is that there may be livelocks in extreme cases.  For
> > better scalability, one gen_pool allocator can be used for each CPU.
> > 
> > The lockless operation only works if there is enough memory available.
> > If new memory is added to the pool a lock has to be still taken.  So
> > any user relying on locklessness has to ensure that sufficient memory
> > is preallocated.
> > 
> > The basic atomic operation of this allocator is cmpxchg on long.  On
> > architectures that don't have NMI-safe cmpxchg implementation, a
> > spin_trylock_irqsave based fallback is used for gen_pool_alloc, so it
> > can be used in NMI handler safely.  But gen_pool_free can not be used
> > in NMI handler in these architectures, because memory free can not
> > fail. 
> 
> I still don't see a reason to merge this.
> 
> It makes the genalloc thing slower for every other user (more LOCK'ed
> ops) and there is no new user presented in this series.

As far as I know, all genalloc users are not performance sensitive.
After all, it is mainly used to manage some device memory before. As for
users, I think this is a "chicken and egg" problem. And I have plan to
use it in APEI code.

Best Regards,
Huang Ying



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