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

[Leon Romanovsky <leon-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

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On Tue, Sep 26, 2017 at 11:24:41PM +0800, Wei Hu (Xavier) wrote:
>
>
> On 2017/9/26 21:13, Wei Hu (Xavier) wrote:
> >
> >
> > On 2017/9/26 1:36, Doug Ledford wrote:
> > > 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...
> > Hi, Doug
> >     Driver issues the cmds in cmq, and firmware gets and processes them.
> >     The firmware process only one cmd at the same time, and it will take
> >     about serveral to 200 us in one cmd currently, so the driver need
> >     not to use stream mode to issue cmd.
> >
> >     Regards
> > Wei Hu
> Hi, Doug
>     We can replace usleep_range(1000, 2000); with the following statement:
>         udelay(1);
>     to avoid spinning too long time and using usleep_range function in
>     spin_lock_bh locked region.
>
>     Can you give some suggestions?

He already suggested number of options, while the simple one was to reduce
HNS_ROCE_CMQ_TX_TIMEOUT from 200 to be something sane (for example 5).

Thanks
>
>     Regards
> Wei Hu
> > > > > > +#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
> > > > >      Key fingerprint = AE6B 1BDA 122B 23B4 265B  1274 B826 A333 0E57
> > > > > 2FDD
> > > > >
> > > > > --
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