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SubjectRe: [PATCH 08/10] blk-mq-sched: add framework for MQ capable IO schedulers
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On Fri, 2017-01-13 at 12:15 +0100, Hannes Reinecke wrote:
> On 01/11/2017 10:40 PM, Jens Axboe wrote:
> > This adds a set of hooks that intercepts the blk-mq path of
> > allocating/inserting/issuing/completing requests, allowing
> > us to develop a scheduler within that framework.
> >
> > We reuse the existing elevator scheduler API on the registration
> > side, but augment that with the scheduler flagging support for
> > the blk-mq interfce, and with a separate set of ops hooks for MQ
> > devices.
> >
> > We split driver and scheduler tags, so we can run the scheduling
> > independent of device queue depth.
> >
> > Signed-off-by: Jens Axboe <axboe@fb.com>
>
> [ .. ]
> > @@ -823,6 +847,35 @@ static inline unsigned int queued_to_index(unsigned int queued)
> > return min(BLK_MQ_MAX_DISPATCH_ORDER - 1, ilog2(queued) + 1);
> > }
> >
> > +static bool blk_mq_get_driver_tag(struct request *rq,
> > + struct blk_mq_hw_ctx **hctx, bool wait)
> > +{
> > + struct blk_mq_alloc_data data = {
> > + .q = rq->q,
> > + .ctx = rq->mq_ctx,
> > + .hctx = blk_mq_map_queue(rq->q, rq->mq_ctx->cpu),
> > + .flags = wait ? 0 : BLK_MQ_REQ_NOWAIT,
> > + };
> > +
> > + if (blk_mq_hctx_stopped(data.hctx))
> > + return false;
> > +
> > + if (rq->tag != -1) {
> > +done:
> > + if (hctx)
> > + *hctx = data.hctx;
> > + return true;
> > + }
> > +
> > + rq->tag = blk_mq_get_tag(&data);
> > + if (rq->tag >= 0) {
> > + data.hctx->tags->rqs[rq->tag] = rq;
> > + goto done;
> > + }
> > +
> > + return false;
> > +}
> > +
>
> What happens with the existing request at 'rqs[rq->tag]' ?
> Surely there is one already, right?
> Things like '->init_request' assume a fully populated array, so moving
> one entry to another location is ... interesting.
>
> I would have thought we need to do a request cloning here,
> otherwise this would introduce a memory leak, right?
> (Not to mention a potential double completion, as the request is now at
> two positions in the array)

Hello Hannes,

Have you noticed that there are two .rqs[] arrays - tags->rqs and
sched_tags->rqs[]? .init_request() loops over sched_tags->rqs[]. The
above assignment applies to tags->rqs[].

Bart.
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