Re: [PATCHSET v3 0/5] Support for RWF_UNCACHED

[Matthew Wilcox <willy@infradead.org>]

On Wed, Dec 11, 2019 at 05:41:16PM -0800, Linus Torvalds wrote:
> I too can see xas_create using 30% CPU time, but that's when I do a
> perf record on just kswapd - and when I actually look at it on a
> system level, it looks nowhere near that bad.
> 
> So I think people should look at this. Part of it might be for Willy:
> does that xas_create() need to be that expensive? I hate how "perf"
> callchains work, but it does look like it is probably
> page_cache_delete -> xas_store -> xas_create that is the bulk of the
> cost there.
> 
> Replacing the real page with the shadow entry shouldn't be that big of
> a deal, I would really hope.
> 
> Willy, that used to be a __radix_tree_lookup -> __radix_tree_replace
> thing, is there perhaps some simple optmization that could be done on
> the XArray case here?

It _should_ be the same order of complexity.  Since there's already
a page in the page cache, xas_create() just walks its way down to the
right node calling xas_descend() and then xas_store() does the equivalent
of __radix_tree_replace().  I don't see a bug that would make it more
expensive than the old code ... a 10GB file is going to have four levels
of radix tree node, so it shouldn't even spend that long looking for
the right node.