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From: Waiman Long <longman@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	"Huang, Ying" <ying.huang@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	"Chen, Rong A" <rong.a.chen@intel.com>, "lkp@01.org" <lkp@01.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Andi Kleen <ak@linux.intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Tim C Chen <tim.c.chen@intel.com>
Subject: Re: [LKP] [page cache] eb797a8ee0: vm-scalability.throughput -16.5% regression
Date: Tue, 26 Feb 2019 15:29:42 -0500	[thread overview]
Message-ID: <1c33a91c-a436-a879-ca14-7eebcbf971c2@redhat.com> (raw)
In-Reply-To: <CAHk-=whn+F+vFgCR0a04atTQuBmXhq0oU1Y1f0YqMiUFHj28JQ@mail.gmail.com>

On 02/26/2019 12:30 PM, Linus Torvalds wrote:
> On Tue, Feb 26, 2019 at 12:17 AM Huang, Ying <ying.huang@intel.com> wrote:
>> As for fixing.  Should we care about the cache line alignment of struct
>> inode?  Or its size is considered more important because there may be a
>> huge number of struct inode in the system?
> Thanks for the great analysis.
>
> I suspect we _would_ like to make sure inodes are as small as
> possible, since they are everywhere. Also, they are usually embedded
> in other structures (ie "struct inode" is embedded into "struct
> ext4_inode_info"), and unless we force alignment (and thus possibly
> lots of padding), the actual alignment of 'struct inode' will vary
> depending on filesystem.
>
> So I would suggest we *not* do cacheline alignment, because it will
> result in random padding.
>
> But it sounds like maybe the solution is to make sure that the
> different fields of the inode can and should be packed differently?
>
> So one thing to look at is to see what fields in 'struct inode' might
> be best moved together, to minimize cache accesses.
>
> And in particular, if this is *only* an issue of "struct
> rw_semaphore", maybe we should look at the layout of *that*. In
> particular, I'm getting the feeling that we should put the "owner"
> field right next to the "count" field, because the normal
> non-contended path only touches those two fields.

That is true. Putting the two next to each other reduces the chance of
needing to touch 2 cachelines to acquire a rwsem.

> Right now those two fields are pretty far from each other in 'struct
> rw_semaphore', which then makes the "oops they got allocated in
> different cachelines" much more likely.
>
> So even if 'struct inode' layout itself isn't changed, maybe just
> optimizing the layout of 'struct rw_semaphore' a bit for the common
> case might fix it all up.
>
> Waiman, I didn't check if your rewrite already possibly fixes this?

My current patch doesn't move the owner field, but I will add one to do
it. That change alone probably won't solve the regression we see here.
The optimistic spinner is spinning on the on_cpu flag of the task
structure as well as the rwsem->owner value (looking for change). The
lock holder only need to touch the count/owner values once at unlock.
However, if other hot data variables are in the same cacheline as
rwsem->owner, we will have cacaheline bouncing problem. So we need to
pad some rarely touched variables right before the rwsem in order to
reduce the chance of false cacheline sharing.

-Longman



  reply	other threads:[~2019-02-26 20:29 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-14  9:22 kernel test robot
2018-11-14 14:17 ` Matthew Wilcox
2019-02-26  8:17   ` Huang, Ying
2019-02-26 17:30     ` Linus Torvalds
2019-02-26 20:29       ` Waiman Long [this message]
2019-02-28  1:18         ` Huang, Ying
2019-02-28  1:32           ` Linus Torvalds
2019-03-02  8:26             ` Huang, Ying
2019-02-28  2:37           ` Waiman Long
2019-02-28  3:26             ` Huang, Ying

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