linux-kernel - Re: [PATCH] mm: percpu: Add PCPU_FC_FIXED to pcpu_fc for setting fixed pcpu_atom

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Message-ID: <20120427163430.GQ27486@google.com>
Date:	Fri, 27 Apr 2012 09:34:30 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Yanmin Zhang <yanmin_zhang@...ux.intel.com>
Cc:	ShuoX Liu <shuox.liu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: percpu: Add PCPU_FC_FIXED to pcpu_fc for setting
 fixed pcpu_atom_size.

Hello,

On Fri, Apr 27, 2012 at 09:09:24AM +0800, Yanmin Zhang wrote:
> > > We can't fix FC_PAGE power regression. If we do so, we need contact many
> > > hardware architects. Current kernel supports FC_PAGE and PMD_SIZE, why
> > > not to allow admin to choose other values?
> > 
> > If this is something which is met in the field commonly, we need to
> > fix the default behavior rather than introducing some arcane boot
> > param.
>
> We just add a new value input method instead of introducing new parameter.

Ummm... I don't know what you meant by the above sentence but adding a
new magic kernel param whether it's part of an existing one or not, is
not a good solution.  They're difficult to discover and not many
actually understand what they do.  If you *have* to add some, then you
better make it clear where it's being applied for what.  ie. in this
case, add something like x86_percpu_embed_unit_size.

> >   IIRC, the reasons PMD_SIZE is used for atom_size are so that
> > percpu areas are aligned to PSE mapping, maybe later we can make use
> > of PSE mapping in vmalloc area too, and it didn't seem to hurt
> > anything.
>
> Well, vmalloc area might use different prot to map physical pages.
> So sharing one PMD huge page by many vmalloc areas might be not good.

Percpu allocator uses the whole vmalloc chunk, so there's no prot
problem.  They're all percpu memory.

Thanks.

-- 
tejun
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