Re: [PATCH] blk-wbt: get back the missed wakeup from __wbt_done

[jianchao.wang]

This is a multi-part message in MIME format.

On 08/24/2018 07:14 AM, Jens Axboe wrote:
> On 8/23/18 5:03 PM, Jens Axboe wrote:
>>> Hi Jens, This patch looks much cleaner for sure as Frank pointed out
>>> too. Basically this looks similar to wake_up_nr only making sure that
>>> those woken up requests won't get reordered. This does solves the
>>> thundering herd issue. However, I tested the patch against my
>>> application and lock contention numbers rose to around 10 times from
>>> what I had from your last 3 patches.  Again this did add to drop in
>>> of total files read by 0.12% and rate at which they were read by
>>> 0.02% but this is not a very significant drop. Is lock contention
>>> worth the tradeoff?  I also added missing
>>> __set_current_state(TASK_RUNNING) to the patch for testing.
>>
>> Can you try this variant? I don't think we need a
>> __set_current_state() after io_schedule(), should be fine as-is.
>>
>> I'm not surprised this will raise contention a bit, since we're now
>> waking N tasks potentially, if N can queue. With the initial change,
>> we'd always just wake one.  That is arguably incorrect. You say it's
>> 10 times higher contention, how does that compare to before your
>> patch?
>>
>> Is it possible to run something that looks like your workload?
> 
> Additionally, is the contention you are seeing the wait queue, or the
> atomic counter? When you say lock contention, I'm inclined to think it's
> the rqw->wait.lock.
> 

I guess the increased lock contend is due to:
when the wake up is ongoing with wait head lock is held, there is still waiter
on wait queue, and __wbt_wait will go to wait and try to require the wait head lock.
This is necessary to keep the order on the rqw->wait queue.

The attachment does following thing to try to avoid the scenario above.
"
Introduce wait queue rqw->delayed. Try to lock rqw->wait.lock firstly, if fails, add
the waiter on rqw->delayed. __wbt_done will pick the waiters on rqw->delayed up and
queue them on the tail of rqw->wait before it do wake up operation.
"

Thanks
Jianchao