[RFC] Efficiency of the phandle_cache on ppc64/SLOF

[Rob Herring]

On Tue, Dec 10, 2019 at 2:17 AM Frank Rowand <frowand.list at gmail.com> wrote:
>
> On 12/9/19 7:51 PM, Rob Herring wrote:
> > On Mon, Dec 9, 2019 at 7:35 AM Sebastian Andrzej Siewior
> > <bigeasy at linutronix.de> wrote:
> >>
> >> On 2019-12-05 20:01:41 [-0600], Frank Rowand wrote:
> >>> Is there a memory usage issue for the systems that led to this thread?
> >>
> >> No, no memory issue led to this thread. I was just testing my patch and
> >> I assumed that I did something wrong in the counting/lock drop/lock
> >> acquire/allocate path because the array was hardly used. So I started to
> >> look deeper…
> >> Once I figured out everything was fine, I was curious if everyone is
> >> aware of the different phandle creation by dtc vs POWER. And I posted
> >> the mail in the thread.
> >> Once you confirmed that everything is "known / not an issue" I was ready
> >> to take off [0].
> >>
> >> Later more replies came in such as one mail [1] from Rob describing the
> >> original reason with 814 phandles. _Here_ I was just surprised that 1024
> >> were used over 64 entries for a benefit of 60ms. I understand that this
> >> is low concern for you because that memory is released if modules are
> >> not enabled. I usually see that module support is left enabled.
> >>
> >> However, Rob suggested / asked about the fixed size array (this is how I
> >> understood it):
> >> |And yes, as mentioned earlier I don't like the complexity. I didn't
> >> |from the start and I'm  I'm still of the opinion we should have a
> >> |fixed or 1 time sized true cache (i.e. smaller than total # of
> >> |phandles). That would solve the RT memory allocation and locking issue
> >> |too.
> >>
> >> so I attempted to ask if we should have the fixed size array maybe
> >> with the hash_32() instead the mask. This would make my other patch
> >> obsolete because the fixed size array should not have a RT issue. The
> >> hash_32() part here would address the POWER issue where the cache is
> >> currently not used efficiently.
> >>
> >> If you want instead to keep things as-is then this is okay from my side.
> >> If you want to keep this cache off on POWER then I could contribute a
> >> patch doing so.
> >
> > It turns out there's actually a bug in the current implementation. If
> > we have multiple phandles with the same mask, then we leak node
> > references if we miss in the cache and re-assign the cache entry.
>
> Aaargh.  Patch sent.
>
> > Easily fixed I suppose, but holding a ref count for a cached entry
> > seems wrong. That means we never have a ref count of 0 on every node
> > with a phandle.
>
> It will go to zero when the cache is freed.
>
> My memory is that we free the cache as part of removing an overlay.  I'll
> verify whether my memory is correct.

Yes, as part of having entries for every phandle we release and
realloc when number of phandles changes. If the size is fixed, then we
can stop doing that. We only need to remove entries in
of_detach_node() as that should always happen before nodes are
removed, right?

Rob