[PATCH v5 1/2] blk-mq: add tagset quiesce interface

[Ming Lei]

On Mon, Jul 27, 2020 at 08:23:15PM -0600, Jens Axboe wrote:
> On 7/27/20 8:17 PM, Ming Lei wrote:
> > On Mon, Jul 27, 2020 at 07:51:16PM -0600, Jens Axboe wrote:
> >> On 7/27/20 7:40 PM, Ming Lei wrote:
> >>> On Mon, Jul 27, 2020 at 04:10:21PM -0700, Sagi Grimberg wrote:
> >>>> drivers that have shared tagsets may need to quiesce potentially a lot
> >>>> of request queues that all share a single tagset (e.g. nvme). Add an interface
> >>>> to quiesce all the queues on a given tagset. This interface is useful because
> >>>> it can speedup the quiesce by doing it in parallel.
> >>>>
> >>>> For tagsets that have BLK_MQ_F_BLOCKING set, we use call_srcu to all hctxs
> >>>> in parallel such that all of them wait for the same rcu elapsed period with
> >>>> a per-hctx heap allocated rcu_synchronize. for tagsets that don't have
> >>>> BLK_MQ_F_BLOCKING set, we simply call a single synchronize_rcu as this is
> >>>> sufficient.
> >>>>
> >>>> Signed-off-by: Sagi Grimberg <sagi at grimberg.me>
> >>>> ---
> >>>>  block/blk-mq.c         | 66 ++++++++++++++++++++++++++++++++++++++++++
> >>>>  include/linux/blk-mq.h |  4 +++
> >>>>  2 files changed, 70 insertions(+)
> >>>>
> >>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
> >>>> index abcf590f6238..c37e37354330 100644
> >>>> --- a/block/blk-mq.c
> >>>> +++ b/block/blk-mq.c
> >>>> @@ -209,6 +209,42 @@ void blk_mq_quiesce_queue_nowait(struct request_queue *q)
> >>>>  }
> >>>>  EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue_nowait);
> >>>>  
> >>>> +static void blk_mq_quiesce_blocking_queue_async(struct request_queue *q)
> >>>> +{
> >>>> +	struct blk_mq_hw_ctx *hctx;
> >>>> +	unsigned int i;
> >>>> +
> >>>> +	blk_mq_quiesce_queue_nowait(q);
> >>>> +
> >>>> +	queue_for_each_hw_ctx(q, hctx, i) {
> >>>> +		WARN_ON_ONCE(!(hctx->flags & BLK_MQ_F_BLOCKING));
> >>>> +		hctx->rcu_sync = kmalloc(sizeof(*hctx->rcu_sync), GFP_KERNEL);
> >>>> +		if (!hctx->rcu_sync)
> >>>> +			continue;
> >>>
> >>> This approach of quiesce/unquiesce tagset is good abstraction.
> >>>
> >>> Just one more thing, please allocate a rcu_sync array because hctx is
> >>> supposed to not store scratch stuff.
> >>
> >> I'd be all for not stuffing this in the hctx, but how would that work?
> >> The only thing I can think of that would work reliably is batching the
> >> queue+wait into units of N. We could potentially have many thousands of
> >> queues, and it could get iffy (and/or unreliable) in terms of allocation
> >> size. Looks like rcu_synchronize is 48-bytes on my local install, and it
> >> doesn't take a lot of devices at current CPU counts to make an alloc
> >> covering all of it huge. Let's say 64 threads, and 32 devices, then
> >> we're already at 64*32*48 bytes which is an order 5 allocation. Not
> >> friendly, and not going to be reliable when you need it. And if we start
> >> batching in reasonable counts, then we're _almost_ back to doing a queue
> >> or two at the time... 32 * 48 is 1536 bytes, so we could only do two at
> >> the time for single page allocations.
> > 
> > We can convert to order 0 allocation by one extra indirect array. 
> 
> I guess that could work, and would just be one extra alloc + free if we
> still retain the batch. That'd take it to 16 devices (at 32 CPUs) per
> round, potentially way less of course if we have more CPUs. So still
> somewhat limiting, rather than do all at once.

With the approach in blk_mq_alloc_rqs(), each allocated page can be added
to one list, so the indirect array can be saved. Then it is possible to allocate
for any size queues/devices since every allocation is just for single page
in case that it is needed, even no pre-calculation is required.


Thanks,
Ming