On Fri, Apr 24, 2020 at 03:17:48PM +0200, Hannes Reinecke wrote:
> On 4/24/20 12:23 PM, Ming Lei wrote:
> > Now request is thought as in-flight only when its state is updated as
> > MQ_RQ_IN_FLIGHT, which is done by dirver via blk_mq_start_request().
> >
>
> driver
>
> > Actually from blk-mq's view, one rq can be thought as in-flight
> > after its tag is >= 0.
> >
> Well, and that we should clarify to avoid any misunderstanding.
> To my understanding, 'in-flight' are request which are submitted to
> the LLD. IE we'll have a lifetime rule like
>
> internal_tag >= tag > in-flight
>
> If the existence of a 'tag' would be equivalent to 'in-flight' we could
> do away with all the convoluted code managing the MQ_RQ_IN_FLIGHT state,
> wouldn't we?
Yeah, I have been thinking about that.
>
> > Passing one rq filter callback so that we can iterating requests very
> > flexiable.
> >
>
> flexible
>
> > Meantime blk_mq_all_tag_busy_iter is defined as public, which will be
> > called from blk-mq internally.
> >
> Maybe:
>
> Implement blk_mq_all_tag_busy_iter() which accepts a 'busy_fn' argument
> to filter over which commands to iterate, and make the existing
> blk_mq_tag_busy_iter() a wrapper for the new function.
Fine.
>
> > Cc: John Garry <john.garry@xxxxxxxxxx>
> > Cc: Bart Van Assche <bvanassche@xxxxxxx>
> > Cc: Hannes Reinecke <hare@xxxxxxxx>
> > Cc: Christoph Hellwig <hch@xxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > ---
> > block/blk-mq-tag.c | 39 +++++++++++++++++++++++++++------------
> > block/blk-mq-tag.h | 4 ++++
> > 2 files changed, 31 insertions(+), 12 deletions(-)
> >
> > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> > index 586c9d6e904a..2e43b827c96d 100644
> > --- a/block/blk-mq-tag.c
> > +++ b/block/blk-mq-tag.c
> > @@ -255,6 +255,7 @@ static void bt_for_each(struct blk_mq_hw_ctx *hctx, struct sbitmap_queue *bt,
> > struct bt_tags_iter_data {
> > struct blk_mq_tags *tags;
> > busy_tag_iter_fn *fn;
> > + busy_rq_iter_fn *busy_rq_fn;
> > void *data;
> > bool reserved;
> > };
> > @@ -274,7 +275,7 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
> > * test and set the bit before assining ->rqs[].
> > */
> > rq = tags->rqs[bitnr];
> > - if (rq && blk_mq_request_started(rq))
> > + if (rq && iter_data->busy_rq_fn(rq, iter_data->data, reserved))
> > return iter_data->fn(rq, iter_data->data, reserved);
> > return true;
> > @@ -294,11 +295,13 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
> > * bitmap_tags member of struct blk_mq_tags.
> > */
> > static void bt_tags_for_each(struct blk_mq_tags *tags, struct sbitmap_queue *bt,
> > - busy_tag_iter_fn *fn, void *data, bool reserved)
> > + busy_tag_iter_fn *fn, busy_rq_iter_fn *busy_rq_fn,
> > + void *data, bool reserved)
> > {
> > struct bt_tags_iter_data iter_data = {
> > .tags = tags,
> > .fn = fn,
> > + .busy_rq_fn = busy_rq_fn,
> > .data = data,
> > .reserved = reserved,
> > };
> > @@ -310,19 +313,30 @@ static void bt_tags_for_each(struct blk_mq_tags *tags, struct sbitmap_queue *bt,
> > /**
> > * blk_mq_all_tag_busy_iter - iterate over all started requests in a tag map
> > * @tags: Tag map to iterate over.
> > - * @fn: Pointer to the function that will be called for each started
> > - * request. @fn will be called as follows: @fn(rq, @priv,
> > - * reserved) where rq is a pointer to a request. 'reserved'
> > - * indicates whether or not @rq is a reserved request. Return
> > - * true to continue iterating tags, false to stop.
> > + * @fn: Pointer to the function that will be called for each request
> > + * when .busy_rq_fn(rq) returns true. @fn will be called as
> > + * follows: @fn(rq, @priv, reserved) where rq is a pointer to a
> > + * request. 'reserved' indicates whether or not @rq is a reserved
> > + * request. Return true to continue iterating tags, false to stop.
> > + * @busy_rq_fn: Pointer to the function that will be called for each request,
> > + * @busy_rq_fn's type is same with @fn. Only when @busy_rq_fn(rq,
> > + * @priv, reserved) returns true, @fn will be called on this rq.
> > * @priv: Will be passed as second argument to @fn.
> > */
> > -static void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags,
> > - busy_tag_iter_fn *fn, void *priv)
> > +void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags,
> > + busy_tag_iter_fn *fn, busy_rq_iter_fn *busy_rq_fn,
> > + void *priv)
> > {
> > if (tags->nr_reserved_tags)
> > - bt_tags_for_each(tags, &tags->breserved_tags, fn, priv, true);
> > - bt_tags_for_each(tags, &tags->bitmap_tags, fn, priv, false);
> > + bt_tags_for_each(tags, &tags->breserved_tags, fn, busy_rq_fn,
> > + priv, true);
> > + bt_tags_for_each(tags, &tags->bitmap_tags, fn, busy_rq_fn, priv, false);
> > +}
> > +
> > +static bool blk_mq_default_busy_rq(struct request *rq, void *data,
> > + bool reserved)
> > +{
> > + return blk_mq_request_started(rq);
> > }
> > /**
> > @@ -342,7 +356,8 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
> > for (i = 0; i < tagset->nr_hw_queues; i++) {
> > if (tagset->tags && tagset->tags[i])
> > - blk_mq_all_tag_busy_iter(tagset->tags[i], fn, priv);
> > + blk_mq_all_tag_busy_iter(tagset->tags[i], fn,
> > + blk_mq_default_busy_rq, priv);
> > }
> > }
> > EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
> > diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
> > index 2b8321efb682..fdf095d513e5 100644
> > --- a/block/blk-mq-tag.h
> > +++ b/block/blk-mq-tag.h
> > @@ -21,6 +21,7 @@ struct blk_mq_tags {
> > struct list_head page_list;
> > };
> > +typedef bool (busy_rq_iter_fn)(struct request *, void *, bool);
> > extern struct blk_mq_tags *blk_mq_init_tags(unsigned int nr_tags, unsigned int reserved_tags, int node, int alloc_policy);
> > extern void blk_mq_free_tags(struct blk_mq_tags *tags);
> > @@ -34,6 +35,9 @@ extern int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
> > extern void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool);
> > void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
> > void *priv);
> > +void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags,
> > + busy_tag_iter_fn *fn, busy_rq_iter_fn *busy_rq_fn,
> > + void *priv);
> > static inline struct sbq_wait_state *bt_wait_ptr(struct sbitmap_queue *bt,
> > struct blk_mq_hw_ctx *hctx)
> >
> I do worry about the performance impact of this new filter function.
> From my understanding, the _busy_iter() functions are supposed to be
> efficient, such that they can be used as an alternative to having a global
No, blk_mq_tagset_busy_iter() won't be called in fast IO path, usually
it is run in EH code path.
Also I don't see how big the performance impact can be given what the
patch is doing is just to add blk_mq_default_busy_rq() to replace the
check of blk_mq_request_started().
> atomic counter.
> (cf the replacement of the global host_busy counter).
>
> But if we're adding ever more functionality to the iterator itself there's a
> good chance we'll kill the performance rendering this assumption invalid.
>
> Have you measured the performance impact of this?
As I mentioned, we don't call such busy_iter() in fast path. Or do you
see such usage in fast path?
thanks,
Ming
