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

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Message-ID: <4F36526B.7070809@kernel.dk>
Date:	Sat, 11 Feb 2012 12:35:07 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	Shaohua Li <shli@...nel.org>, 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()

On 2012-02-11 03:17, Tejun Heo wrote:
> 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.

Was just thinking about that last night, the missing slice expire on
task exit makes a LOT more sense than changed locking.

I'm pushing off what I have to Linus today, since I'll be gone skiing
next week. I will check email regularly and be able to apply patches and
so forth, just a heads up on availability.

-- 
Jens Axboe

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