Re: [PATCH for-next 05/20] RDMA/hns: Add command queue support for hip08 RoCE driver

[Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Mon, 2017-09-25 at 20:18 +0300, Leon Romanovsky wrote:
> On Mon, Sep 25, 2017 at 01:06:53PM -0400, Doug Ledford wrote:
> > On Wed, 2017-08-30 at 17:23 +0800, Wei Hu (Xavier) wrote:
> > 
> > > +	/*
> > > +	 * If the command is sync, wait for the firmware to
> > > write
> > > back,
> > > +	 * if multi descriptors to be sent, use the first one to
> > > check
> > > +	 */
> > > +	if ((desc->flag) & HNS_ROCE_CMD_FLAG_NO_INTR) {
> > > +		do {
> > > +			if (hns_roce_cmq_csq_done(hr_dev))
> > > +				break;
> > > +			usleep_range(1000, 2000);
> > > +			timeout++;
> > > +		} while (timeout < priv->cmq.tx_timeout);
> > > +	}
> > 
> > then we spin here for a maximum amount of time between 200 and
> > 400ms,
> > so 1/4 to 1/2 a second.  All the time we are holding the bh lock on
> > this CPU.  That seems excessive to me.  If we are going to spin
> > that
> > long, can you find a way to allocate/reserve your resources, send
> > the
> > command, then drop the bh lock while you spin, and retake it before
> > you
> > complete once the spinning is done?
> 
> They don't allocate anything in this loop, but checking the pointers
> are
> the same, see hns_roce_cmq_csq_done.

I'm not sure I understand your intended implication of your comment.  I
wasn't concerned about them allocating anything, only that if the
hardware is hung, then this loop will hang out for 1/4 to 1/2 a second
and hold up all bottom half processing on this CPU in the meantime. 
That's the sort of things that provides poor overall system behavior.

Now, since they are really only checking to see if the hardware has
gotten around to their particular command, and their command is part of
a ring structure, it's possible to record the original head command,
and our new head command, and then release the spin_lock_bh around the
entire do{ }while construct, and in hns_roce_cmd_csq_done() you could
check that head is not in the range old_head:new_head.  That would
protect you in case something in the bottom half processing queued up
some more commands and from one sleep to the next the head jumped from
something other than the new_head to something past new_head, so that
head == priv->cmq.csq.next_to_use ends up being perpetually false. 
But, that's just from a quick read of the code, I could easily be
missing something here...

> > 
> > > +#define HNS_ROCE_CMQ_TX_TIMEOUT			200
> > 
> > or you could reduce the size of this define...
> > 
> > --
> > Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> >     GPG KeyID: B826A3330E572FDD
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> > 2FDD
> > 
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-- 
Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
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