linux-kernel - spinlock contention of files->file

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Message-ID: <1380589503.5326.13.camel@edumazet-glaptop.roam.corp.google.com>
Date:	Mon, 30 Sep 2013 18:05:03 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: spinlock contention of files->file_lock

Speaking of spinlock contention, the files->file_lock is a good example.

Multi threaded programs hit this spinlock three times per fd : 

alloc_fd() / fd_install() / close() 

I think fd_install() could be done without this spinlock.

we want to protect fd_install() from a concurrent resize of the fd
table.

We could either :

- Add a spinlock used for resize, and fd_install().
  A bit suboptimal but easy to code and review.

- Add a seqcount, and cmpxchg() to synchronize fd_install() with the
potential resizer. (Might need additional RCU locking)



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