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From: Thierry Reding <thierry.reding@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	kernel test robot <rong.a.chen@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jann Horn <jannh@google.com>, LKML <linux-kernel@vger.kernel.org>,
	x86-ml <x86@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	Borislav Petkov <bp@suse.de>,
	lkp@lists.01.org
Subject: Re: [futex] 8019ad13ef: will-it-scale.per_process_ops -97.8% regression
Date: Wed, 11 Mar 2020 09:20:00 +0100
Message-ID: <20200311082000.GA724@ulmo> (raw)
In-Reply-To: <87h7yy90ve.fsf@nanos.tec.linutronix.de>


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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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      parent reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-08 14:02 kernel test robot
     [not found] ` <CAHk-=wgJV7xrxsTkOG203huPShzZBEC6N_BG0KGwHBmEq4yqWg@mail.gmail.com>
2020-03-08 15:28   ` Linus Torvalds
2020-03-08 18:07     ` Thomas Gleixner
2020-03-09  8:26       ` kernel test robot
2020-03-09  8:51       ` Peter Zijlstra
2020-03-09  9:34         ` Thomas Gleixner
2020-03-09 21:42       ` [tip: locking/urgent] futex: Unbreak futex hashing tip-bot2 for Thomas Gleixner
2020-03-11  8:20       ` Thierry Reding [this message]

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