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

[Ming Lei]

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. 


Thanks,
Ming