On Thu, 04 Sep 2014 19:30:18 -0600
Jens Axboe <ax...@kernel.dk> wrote:
> On 09/04/2014 07:26 PM, Chuck Ebbert wrote:
> > On Wed, 03 Sep 2014 14:35:29 -0600
> > Jens Axboe <ax...@kernel.dk> wrote:
> >
> >> On 09/03/2014 02:33 PM, Alexander Gordeev wrote:
> > <snip>
> >>> diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
> >>> index 6206ed1..795ec3f 100644
> >>> --- a/block/blk-mq-tag.h
> >>> +++ b/block/blk-mq-tag.h
> >>> @@ -66,23 +66,22 @@ enum {
> >>> BLK_MQ_TAG_MAX = BLK_MQ_TAG_FAIL - 1,
> >>> };
> >>>
> >>> -extern bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *);
> >>> +extern void __blk_mq_tag_busy(struct blk_mq_hw_ctx *);
> >>> extern void __blk_mq_tag_idle(struct blk_mq_hw_ctx *);
> >>>
> >>> static inline bool blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
> >>> {
> >>> - if (!(hctx->flags & BLK_MQ_F_TAG_SHARED))
> >>> - return false;
> >>> -
> >>> - return __blk_mq_tag_busy(hctx);
> >>> + if (hctx->flags & BLK_MQ_F_TAG_SHARED) {
> >>> + __blk_mq_tag_busy(hctx);
> >>> + return true;
> >>> + }
> >>> + return false;
> >>> }
> >>
> >> The normal/fast path here is the flag NOT being set, which is why
> >> it was coded that way to put the fast path inline.
> >>
> >>>
> >>> static inline void blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
> >>> {
> >>> - if (!(hctx->flags & BLK_MQ_F_TAG_SHARED))
> >>> - return;
> >>> -
> >>> - __blk_mq_tag_idle(hctx);
> >>> + if (hctx->flags & BLK_MQ_F_TAG_SHARED)
> >>> + __blk_mq_tag_idle(hctx);
> >>> }
> >>
> >> Ditto
> >
> > Shouldn't it just add unlikely() then? That way it's obvious what
> > the common case is, instead of relying on convoluted code.
>
> It's a common construct. Besides, if you find a flag-not-set check
> convoluted, then I hope you are not programming anything I use.
> That's a bit of a straw man, imho.
>
Sure, it's a common construct. But there's nothing there to prevent the
optimizer from rearranging things any way it pleases. Nor is there
anything keeping a human from doing the same. ;)
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