> Date: Mon, 10 Mar 2014 11:29:13 +0000
> Subject: Re: linux kernel flush_cache_all behaviour on a Big.LITTLE system
> From: karim.allah.ah...@gmail.com
> To: catalin.mari...@arm.com
> CC: linaro-dev@lists.linaro.org
>
> On Mon, Mar 10, 2014 at 10:52 AM, Catalin Marinas
> <catalin.mari...@arm.com> wrote:
> > On Mon, Mar 10, 2014 at 10:44:05AM +0000, karim.allah.ah...@gmail.com wrote:
> >> I have two questions:
> >>
> >> 1- I was wondering what should be the expected semantics of
> >> "flush_cache_all" on a Big.LITTLE architecture.
> >>
> >> I can see that the implementation of this function under linux kernel
> >> is doing the following:
> >>
> >> a- Read the value of LoC ( level of coherency )
> >> b- Flush each level of cache to that LoC value using DCCISW
> >> co-processor register.
> >>
> >> My expectation would be that if this is executed on one of the
> >> processors of the Big cluster it should flush all L1 and L2 caches on
> >> this cluster and then signal the CCI interconnect of the cache
> >> cleaning operation and then the CCI interconnect would propagate this
> >> signal downstream to the LITTLE cluster. This will mean that at the
> >> end all cache will be flushed.
> >
> > I am not sure exactly how the CCI behaves here but cache flushing by
> > set/way (like the flush_cache_all function) is not safe on SMP
> > (independent of big.LITTLE) and it should only be used in certain
> > contexts like suspend/resume where we have more control about cache
> > lines migration between CPUs/clusters.
> >
> >> Is that the proper semantics of this operation ?
> >>
> >> or it's only going to affect this CPU and no other CPUs in the cluster
> >> ( and consequently no other CPUs on the other cluster ). And if that's
> >> the case, does this mean that I've to do the cache flushing per_cpu ?
> >
> > The safe thing is to assume that it only affects a single CPU (and as an
> > optimisation we use a flush_cache_louis which does the L1 cache only).
> > When the whole cluster is going down and we know that only one CPU is
> > running, we can use flush_cache_all for that cluster but it does not
> > affect the caches in the other cluster.
>
> I see.
> I assumed that flush_cache_all is going to be seen by the other
> cluster as well! For example if my system was already declaring L2 as
> the LoC, my understanding is that if I flushed my caches till I
> reached L2 the CCI ( or something ) should be signalled to propagate
> this flush downstream to the other cluster in order to maintain the
> semantics of the Level of Coherency in ARM TRM. Is this a correct
> understanding of LoC ? Maybe I should explicitly notify the other
> observers as well after flushing to refresh their view of the memory (
> like you said by flushing using MVA ) otherwise they might see stale
> data ?
>
> CCI already have signals to propagate the cache maintenance operations
> downstream, I just didn't know when they are invoked and when they're
> not!
No.
Your assumption is correct for DCCIMVAC.
But flush_cache_all uses DCCISW. It only affects a single CPU. Other processors
and clusters won't observe it.
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