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SubjectRe: [PATCH] block: strip out locking optimization in put_io_context()
From
2012/2/10 Shaohua Li <shli@kernel.org>:
> 2012/2/10 Tejun Heo <tj@kernel.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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