Re: [futex] 8019ad13ef: will-it-scale.per_process_ops -97.8% regression

Thierry Reding
2020-03-11T08:20:09Z

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On Sun, Mar 08, 2020 at 07:07:17PM +0100, Thomas Gleixner wrote:
> Linus Torvalds <torvalds@linux-foundation.org> writes:
>
> > [ Just a re-send without html crud that makes all the lists unhappy.
> > I'm still on the road, the flight I was supposed to be on yesterday
> > got cancelled.. ]
> >
> > I do note that the futex hashing seems to be broken by that commit. Or
> > at least it's questionable. It keeps hashing on "both.word", and
> > doesn't use the u64 field at all for hashing.
> >
> > Maybe I'm mis-reading it - I didn't apply the patch, I just looked at
> > the patch and my source base separately.
> >
> > But the 98% regression sure says something went wrong ;)
>
> Right you are. The pointer needs to be the starting point as it moved
> ahead of word, which means it starts at word and hashes word and
> offset and an extra u32 beyond the end of the key.
>
> Thanks,
>
> tglx
> ----
> diff --git a/kernel/futex.c b/kernel/futex.c
> index e14f7cd45dbd..9f3251349f65 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -385,8 +385,8 @@ static inline int hb_waiters_pending(struct futex_hash_bucket *hb)
> */
> static struct futex_hash_bucket *hash_futex(union futex_key *key)
> {
> - u32 hash = jhash2((u32*)&key->both.word,
> - (sizeof(key->both.word)+sizeof(key->both.ptr))/4,
> + u32 hash = jhash2((u32*)&key->both.ptr,
> + (sizeof(key->both.ptr) + sizeof(key->both.word)) / 4,
> key->both.offset);
> return &futex_queues[hash & (futex_hashsize - 1)];
> }

I was running into an issue with sshfs mounts and git bisect pointed to
the "futex: Fix inode life-time issue" commit as being the culprit. The
issue I was seeing is that an sshfs mount would succeed, but trying to
then access the mount ("ls -l /mnt" would be enough) would cause a soft
lock-up.

Applying this fix on top of next-20200310 fixes the issue for me, so:

Tested-by: Thierry Reding <treding@nvidia.com>

I still need that fix on next-20200311, but it'll probably show up in
linux-next sometime soon.

Thierry

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