Re: [PATCH 1/1] io_uring: use proper references for fallback_req locking

[Bijan Mottahedeh <bijan.mottahedeh@oracle.com>]

On 5/4/2020 9:12 AM, Jens Axboe wrote:
> On 5/3/20 6:52 AM, Pavel Begunkov wrote:
>> On 30/04/2020 17:52, Jens Axboe wrote:
>>> On 4/29/20 6:47 PM, Bijan Mottahedeh wrote:
>>>> Use ctx->fallback_req address for test_and_set_bit_lock() and
>>>> clear_bit_unlock().
>>> Thanks, applied.
>>>
>> How about getting rid of it? As once was fairly noticed, we're screwed in many
>> other ways in case of OOM. Otherwise we at least need to make async context
>> allocation more resilient.
> Not sure how best to handle it, it really sucks to have things fall apart
> under high memory pressure, a condition that isn't that rare in production
> systems. But as you say, it's only a half measure currently. We could have
> the fallback request have req->io already allocated, though. That would
> provide what we need for guaranteed forward progress, even in the presence
> of OOM conditions.
>
A somewhat related question, would it make sense to have (configurable) 
pre-allocated requests, to be used first if low latency is a priority 
for a ring, or would the allocation overhead be negligible compared to 
the actual I/O?  This would be the flip side of fallback in a sense.

Thanks.

--bijan

--bijan