linux-kernel - Re: [RFC][PATCH 0/5] Optimize percpu-rwsem

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Message-ID: <20150605210857.GA24905@redhat.com>
Date:	Fri, 5 Jun 2015 23:08:57 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	der.herr@...r.at
Subject: Re: [RFC][PATCH 0/5] Optimize percpu-rwsem

On 06/05, Al Viro wrote:
>
> FWIW, I hadn't really looked into stop_machine uses, but fs/locks.c one
> is really not all that great - there we have a large trashcan of a list
> (every file_lock on the system) and the only use of that list is /proc/locks
> output generation.  Sure, additions take this CPU's spinlock.  And removals
> take pretty much a random one - losing the timeslice and regaining it on
> a different CPU is quite likely with the uses there.
>
> Why do we need a global lock there, anyway?  Why not hold only one for
> the chain currently being traversed?  Sure, we'll need to get and drop
> them in ->next() that way; so what?

And note that fs/seq_file.c:seq_hlist_next_percpu() has no other users.

And given that locks_delete_global_locks() takes the random lock anyway,
perhaps the hashed lists/locking makes no sense, I dunno.

Oleg.

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