Re: Deadlock possibly caused by too_many_isolated.

[Jens Axboe <axboe@kernel.dk>]

On 2010-10-22 10:07, Wu Fengguang wrote:
>>> We surely need 1 set aside for each level of that stack that will
>>> potentially consume one. 1 should be enough for the generic pool, and
>>> then clones will use a separate pool. So md and friends should really
>>> have a pool per device, so that stacking will always work properly.
>>
>> Agreed for the deadlock problem.
>>
>>> There should be no throughput concerns, it should purely be a safe guard
>>> measure to prevent us deadlocking when doing IO for reclaim.
>>
>> It's easy to verify whether the minimal size will have negative
>> impacts on IO throughput. In Torsten's case, increase BIO_POOL_SIZE
>> by one and check how it performs.
> 
> Sorry it seems simply increasing BIO_POOL_SIZE is not enough to fix
> possible deadlocks. We need adding new mempool(s). Because when there
> BIO_POOL_SIZE=2 and there are two concurrent reclaimers each take 1
> reservation, they will deadlock each other when trying to take the
> next bio at the raid1 level.

Yes, plus it's not a practical solution since you don't know how deep
the stack is. As I wrote in the initial email, each consumer needs it's
own private mempool (and just 1 entry should suffice).

-- 
Jens Axboe

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