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

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Message-ID: <CANejiEVQav8mtQHD0FETxo1-Zc7exMyb6ecjrAzdZWw68OkTug@mail.gmail.com>
Date:	Mon, 13 Feb 2012 09:34:51 +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()

2012/2/11 Tejun Heo <tj@...nel.org>:
> 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.
ah, I felt a little strange looking at exit_io_context, but didn't realize
the exit notification is removed (confused by put_io_context). This
makes a lot of sense. Good catch!
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