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

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Date:	Fri, 10 Feb 2012 16:48:49 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Tejun Heo <tj@...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()

2012/2/10 Shaohua Li <shli@...nel.org>:
> 2012/2/10 Tejun Heo <tj@...nel.org>:
>> Hello, Shaohua.
>>
>> 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.
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