Re: [PATCH 2/2] radix-tree: Fix optimisation problem

From: Cedric Blancher
Date: Mon Sep 26 2016 - 17:48:51 EST

You might also try to use valid, plain ISO C99 instead of perverted
gcc extensions which only cause a lot of trouble in the long run.


On 26 September 2016 at 23:28, Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> wrote:
> From: linus971@xxxxxxxxx [mailto:linus971@xxxxxxxxx] On Behalf Of Linus Torvalds
>> On Sun, Sep 25, 2016 at 12:04 PM, Linus Torvalds
>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> > It gets rid of
>> > the ad-hoc arithmetic in radix_tree_descend(), and just makes all that
>> > be inside the is_sibling_entry() logic instead. Which got renamed and
>> > made to actually return the main sibling.
>> Sadly, it looks like gcc generates bad code for this approach. Looks
>> like it ends up testing the resulting sibling pointer twice (because
>> we explicitly disable -fno-delete-null-pointer-checks in the kernel,
>> and we have no way to say "look, I know this pointer I'm returning is
>> non-null").
>> So a smaller patch that keeps the old boolean "is_sibling_entry()" but
>> then actually *uses* that inside radix_tree_descend() and then tries
>> to make the nasty cast to "void **" more legible by making it use a
>> temporary variable seems to be a reasonable balance.
>> At least I feel like I can still read the code, but admittedly by now
>> that may be because I've stared at those few lines so much that I feel
>> like I know what's going on. So maybe the code isn't actually any more
>> legible after all.
>> .. and unlike my previous patch, it actually generates better code
>> than the original (while still passing the fixed test-suite, of
>> course). The reason seems to be exactly that temporary variable,
>> allowing us to just do
>> entry = rcu_dereference_raw(*sibentry);
>> rather than doing
>> entry = rcu_dereference_raw(parent->slots[offset]);
>> with the re-computed offset.
>> So I think I'll commit this unless somebody screams.
> Acked-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> I don't love it. But I think it's a reasonable fix for this point in the release cycle, and I have an idea for changing the representation of sibling slots that will make this moot.
> (Basically adopting Konstantin's idea for using the *last* entry instead of the *first*, and then using entries of the form (offset << 2 | RADIX_TREE_INTERNAL_NODE), so we can identify sibling entries without knowing the parent pointer, and we can go straight from sibling entry to slot offset as a shift rather than as a pointer subtraction).

Cedric Blancher <cedric.blancher@xxxxxxxxx>
Institute Pasteur