linux-kernel - Re: [LKP] [lkp] [dcache_{readdir, dir_lseek}() users] 4e82901cd6: reaim.jobs_per

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Date:	Thu, 2 Jun 2016 17:29:56 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Huang, Ying" <ying.huang@...el.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, lkp@...org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [LKP] [lkp] [dcache_{readdir, dir_lseek}() users] 4e82901cd6:
 reaim.jobs_per_min -49.1% regression

On Thu, Jun 02, 2016 at 02:28:36PM +0800, Huang, Ying wrote:
> "Huang, Ying" <ying.huang@...el.com> writes:

> Here is the comparison result with perf profile information.  You can
> find it via searching 'perf-profile'.
[snip]

Looks like this load is hitting the arseloads of contention cases of
spin_lock() on various ->d_lock inside dcache_readdir().  With exclusive
lock on directory most of them end up uncontended...

I'll try to see what can be done there; quite a bit of those are actually
due to moving the cursor on every step.  I wonder if we could do the list
traversals with something less drastic - the real contents changes are
prevented by the shared lock on directory being held, after all...

I'll play with that and post when I get somewhere with it...