Re: [PATCH 0/7] blk-mq: fix queue quiescing

Jens Axboe
2017-05-25T17:42:59Z

On 05/25/2017 11:24 AM, Bart Van Assche wrote:
> On Thu, 2017-05-25 at 17:09 +0800, Ming Lei wrote:
>> Another big issue is that 'srcu_struct' is very big, which shouldn't
>> be embedded into hctx, since we only have one real user of
>> BLK_MQ_F_BLOCKING.
>>
>> So I will fix that too.
>
> Hello Ming,
>
> Is something like the (untested) patch below perhaps what you had in mind?
>
> Subject: [PATCH] blk-mq: Reduce blk_mq_hw_ctx size
>
> Since the srcu structure is rather large (184 bytes on an x86-64
> system), only allocate it if needed.
>
> Reported-by: Ming Lei <ming.lei@redhat.com>
> ---
> block/blk-mq.c | 13 ++++++++++++-
> include/linux/blk-mq.h | 5 +++--
> 2 files changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 1e330de4e3c5..15b7d4077638 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2233,6 +2233,17 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
> }
> EXPORT_SYMBOL(blk_mq_init_queue);
>
> +static int blk_mq_hw_ctx_size(struct request_queue *q)
> +{
> + BUILD_BUG_ON(ALIGN(offsetof(struct blk_mq_hw_ctx, queue_rq_srcu) +
> + sizeof(((struct blk_mq_hw_ctx *)NULL)->queue_rq_srcu), 64) !=
> + sizeof(struct blk_mq_hw_ctx));
> +
> + return q->tag_set->flags & BLK_MQ_F_BLOCKING ?
> + sizeof(struct blk_mq_hw_ctx) :
> + offsetof(struct blk_mq_hw_ctx, queue_rq_srcu);
> +}
> +
> static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
> struct request_queue *q)
> {
> @@ -2247,7 +2258,7 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
> continue;
>
> node = blk_mq_hw_queue_to_node(q->mq_map, i);
> - hctxs[i] = kzalloc_node(sizeof(struct blk_mq_hw_ctx),
> + hctxs[i] = kzalloc_node(blk_mq_hw_ctx_size(q),
> GFP_KERNEL, node);
> if (!hctxs[i])
> break;
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index c0d59330b5e0..8467e1f83524 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -39,8 +39,6 @@ struct blk_mq_hw_ctx {
> struct blk_mq_tags *tags;
> struct blk_mq_tags *sched_tags;
>
> - struct srcu_struct queue_rq_srcu;
> -
> unsigned long queued;
> unsigned long run;
> #define BLK_MQ_MAX_DISPATCH_ORDER 7
> @@ -62,6 +60,9 @@ struct blk_mq_hw_ctx {
> struct dentry *debugfs_dir;
> struct dentry *sched_debugfs_dir;
> #endif
> +
> + /* Must be the last member - see also blk_mq_hw_ctx_size(). */
> + struct srcu_struct queue_rq_srcu;
> };

Why not make it

/* Must be the last member - see also blk_mq_hw_ctx_size(). */
struct srcu_struct queue_rq_srcu[0];

and fixup blk_mq_hw_ctx_size()

static int blk_mq_hw_ctx_size(struct request_queue *q)
{
int size = sizeof(struct blk_mq_hw_ctx);

if (q->tag_set->flags & BLK_MQ_F_BLOCKING)
size += sizeof(struct srcu_struct);

return size;
}

I think that'd be cleaner. Keep the end-of-struct checking, just to be
on the safe side.

Neither one is super pretty though, and still doesn't fix the fact that
the srcu_struct is _half_ the blk_mq_hw_ctx in total.

--
Jens Axboe