linux-kernel - Re: [PATCH] block: strip out locking optimization in put_io

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Message-ID: <20120211021724.GO19392@google.com>
Date:	Fri, 10 Feb 2012 18:17:24 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Shaohua Li <shli@...nel.org>
Cc:	Jens Axboe <axboe@...nel.dk>, Vivek Goyal <vgoyal@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Knut Petersen <Knut_Petersen@...nline.de>, mroos@...ux.ee,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] block: strip out locking optimization in
 put_io_context()

Hello,

On Fri, Feb 10, 2012 at 04:48:49PM +0800, Shaohua Li wrote:
> >> Can you please test the following one?  It's probably the simplest
> >> version w/o RCU and wq deferring.  RCUfying isn't too bad but I'm
> >> still a bit hesitant because RCU coverage needs to be extended to
> >> request_queue via conditional synchronize_rcu() in queue exit path
> >> (can't enforce delayed RCU free on request_queues and unconditional
> >> synchronize_rcu() may cause excessive delay during boot for certain
> >> configurations).  It now can be done in the block core layer proper so
> >> it shouldn't be as bad tho.  If this too flops, I'll get to that.
> > doesn't work.
> I added trace in the schedule_work code path of put_io_context, which
> runs very rare. So it's not lock contention for sure.
> Sounds the only difference between the good/bad cases is the good
> case runs with rcu_lock_read/rcu_read_unlock. I also checked slab
> info, the cfq related slab doesn't use too many memory, unlikely
> because rcu latency uses too many memory.

Yeah, that makes much more sense.  It just isn't hot enough path for
this sort of micro locking changes to matter.  I think the problem is
that, after the change, the cfqq aren't being expired immediately on
task exit.  ie. While moving the cic destruction to release path, I
accidentally removed exit notification to cfq.  I'll come up with a
fix.

Thank you!

-- 
tejun
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