linux-kernel - Re: Per-CPU data as a structure

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Message-ID: <p73ps5hx1t2.fsf@bingen.suse.de>
Date:	03 May 2007 19:19:21 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Julio M. Merino Vidal" <jmerino@...upc.edu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Per-CPU data as a structure

"Julio M. Merino Vidal" <jmerino@...upc.edu> writes:

> Similarly, and if I understood it correctly, the PDA (Per-processor
> Data Area) also aims to do the above, but at the moment it only
> contains some fields and is not defined in all platforms.  There are
> still a lot of usages of the percpu functionality (such as, e.g., in
> kernel/sched.c).

PDA is an earlier version of percpu; it still can be more efficiently
accessed so it is kept for some low level code.

> As far as I can tell, the advantage of percpu is that you can define
> new "fields" anywhere in the code and independently from the rest of
> the system. 

- Independent maintenance as you noted
- Fast access and relatively compact code
- Avoids false sharing by keeping cache lines of different CPUs separate
- Doesn't waste a lot of memory in padding like NR_CPUs arrays usually
need to to avoid the previous point.

Any replacement that doesn't have these properties too will probably
be not useful.

-Andi

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