Re: [LSF/MM TOPIC][LSF/MM ATTEND] NAPI polling for block drivers

[Johannes Thumshirn <jthumshirn@suse.de>]

On Wed, Jan 18, 2017 at 04:27:24PM +0200, Sagi Grimberg wrote:
> 
> >So what you say is you saw a consomed == 1 [1] most of the time?
> >
> >[1] from http://git.infradead.org/nvme.git/commitdiff/eed5a9d925c59e43980047059fde29e3aa0b7836
> 
> Exactly. By processing 1 completion per interrupt it makes perfect sense
> why this performs poorly, it's not worth paying the soft-irq schedule
> for only a single completion.
> 
> What I'm curious is how consistent is this with different devices (wish
> I had some...)

Hannes just spotted this:
static int nvme_queue_rq(struct blk_mq_hw_ctx *hctx,
                         const struct blk_mq_queue_data *bd)
{
[...]
        __nvme_submit_cmd(nvmeq, &cmnd);
        nvme_process_cq(nvmeq);
        spin_unlock_irq(&nvmeq->q_lock);
        return BLK_MQ_RQ_QUEUE_OK;
out_cleanup_iod:
        nvme_free_iod(dev, req);
out_free_cmd:
        nvme_cleanup_cmd(req);
        return ret;
}

So we're draining the CQ on submit. This of cause makes polling for
completions in the IRQ handler rather pointless as we already did in the
submission path. 

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Johannes Thumshirn                                          Storage
jthumshirn@suse.de                                +49 911 74053 689
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