On Thu, Aug 11, 2022 at 08:58:54PM -0700, Linus Torvalds wrote:
> On Thu, Aug 11, 2022 at 3:43 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Oh, sadly, clang does much worse here.
> >
> > Gcc ends up being able to not have a stack frame at all for
> > __d_lookup_rcu() once that DCACHE_OP_COMPARE case has been moved out.
> > The gcc code really looks very nice.
> >
> > Clang, not so much, and it still has spills and reloads.
>
> I ended up looking at the clang code generation more than I probably
> should have, because I found it so odd.
>
> Our code is literally written to not need that many values, and it
> should be easy to keep everything in registers.
>
> It turns out that clang is trying much too hard to be clever in
> dentry_string_cmp(). The code is literally written so that we keep the
> count of remaining characters in 'tcount', and then at the end we can
> generate a 'mask' from that to ignore the parts of the pathname that
> are beyond the size.
[snip]
There's a cheap way to reduce the register pressure:
seq = raw_seqcount_begin(&dentry->d_seq);
if (dentry->d_parent != parent)
continue;
if (d_unhashed(dentry))
continue;
if (dentry->d_name.hash_len != hashlen)
continue;
if (dentry_cmp(dentry, str, hashlen_len(hashlen)) != 0)
continue;
*seqp = seq;
could move the last store to before dentry_cmp(). Sure, we might get
some extra stores out of that. Into a hot cacheline, and if we really
hit many of those extra stores, we already have a problem - a lot of
collisions both in ->d_parent and ->d_name.hash_len. If that happens,
the cost of those extra stores is going to be trivial noise.
>From correctness POV that should be fine; callers of __d_lookup_rcu()
getting NULL either entirely ignore *seqp (d_alloc_parallel()) or
proceed to wipe it out (lookup_fast(), by calling try_to_unlazy()).
Comments?
