Re: [PATCH v2 1/1] doc: Add remote CPU access details and others to this_cpu_ops.txt

From: Christoph Lameter
Date: Thu Jul 17 2014 - 11:26:32 EST


On Thu, 17 Jul 2014, Pranith Kumar wrote:

> I can mention that IPI is preferable. What is that you don't want mentioned? atomic_t?

Definitely not as an example. atomic_t in per cpu areas is self
contradicting. The per cpu area is exclusively for that processor whereas
an atomic_t is supposed to be accessed from multiple processors.

> > Remote percpu updates are extremely rare events. If the cpu is idle/asleep
> > then usually no updates are needed because no activity is occurring on
> > that cpu.
> >
>
> Yes, -usually- that is the case. But we are talking about the extreme rare event
> where we need to update some remote CPU`s per-cpu data without waking it up from
> sleep/idle. How do you suggest we handle this? I don't think suggesting not to
> use per-cpu areas because of this is a good idea, since we lose a lot of
> performance in the most common cases.

If you modify a percpu area then that is usually done because that cpu
needs to take some action. An IPI is fine.

Otherwise yes I would suggest not use a percpu area but a separate data
structure for synchronization.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/