Re: [PATCH v3 09/11] dm: support IO polling for bio-based dm device

[JeffleXu <jefflexu@linux.alibaba.com>]

On 2/9/21 11:11 AM, Ming Lei wrote:
> On Mon, Feb 08, 2021 at 04:52:41PM +0800, Jeffle Xu wrote:
>> DM will iterate and poll all polling hardware queues of all target mq
>> devices when polling IO for dm device. To mitigate the race introduced
>> by iterating all target hw queues, a per-hw-queue flag is maintained
> 
> What is the per-hw-queue flag?

Sorry I forgot to update the commit message as the implementation
changed. Actually this mechanism is implemented by patch 10 of this
patch set.

> 
>> to indicate whether this polling hw queue currently being polled on or
>> not. Every polling hw queue is exclusive to one polling instance, i.e.,
>> the polling instance will skip this polling hw queue if this hw queue
>> currently is being polled by another polling instance, and start
>> polling on the next hw queue.
> 
> Not see such skip in dm_poll_one_dev() in which
> queue_for_each_poll_hw_ctx() is called directly for polling all POLL
> hctxs of the request queue, so can you explain it a bit more about this
> skip mechanism?
> 

It is implemented as patch 10 of this patch set. When spin_trylock()
fails, the polling instance will return immediately, instead of busy
waiting.


> Even though such skipping is implemented, not sure if good performance
> can be reached because hctx poll may be done in ping-pong style
> among several CPUs. But blk-mq hctx is supposed to have its cpu affinities.
> 

Yes, the mechanism of iterating all hw queues can make the competition
worse.

If every underlying data device has **only** one polling hw queue, then
this ping-pong style polling still exist, even when we implement split
bio tracking mechanism, i.e., acquiring the specific hw queue the bio
enqueued into. Because multiple polling instance has to compete for the
only polling hw queue.

But if multiple polling hw queues per device are reserved for multiple
polling instances, (e.g., every underlying data device has 3 polling hw
queues when there are 3 polling instances), just as what we practice on
mq polling, then the current implementation of iterating all hw queues
will indeed works in a ping-pong style, while this issue shall not exist
when accurate split bio tracking mechanism could be implemented.

As for the performance, I cite the test results here, as summarized in
the cover-letter
(https://lore.kernel.org/io-uring/20210208085243.82367-1-jefflexu@linux.alibaba.com/)

	    | IOPS (IRQ mode) | IOPS (iopoll=1 mode) | diff
----------- | --------------- | -------------------- | ----
without opt | 		 318k |		 	256k | ~-20%
with opt    |		 314k |		 	354k | ~13%

The 'opt' refers to the optimization of patch 10, i.e., the skipping
mechanism. There are 3 polling instances (i.e., 3 CPUs) in this test case.


Indeed the current implementation of iterating all hw queues is some
sort of compromise, as I find it really difficult to implement the
accurate split bio mechanism, and to achieve high performance at the
same time. Thus I turn to optimizing the original implementation of
iterating all hw queues, such as optimization of patch 10 and 11.


-- 
Thanks,
Jeffle