Re: [PATCH v9 06/25] RDMA/rtrs: client: main functionality

[Jinpu Wang <jinpu.wang@cloud.ionos.com>]

On Wed, Mar 4, 2020 at 5:49 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Wed, Mar 04, 2020 at 05:43:24PM +0100, Jinpu Wang wrote:
> > On Tue, Mar 3, 2020 at 5:04 PM Bart Van Assche <bvanassche@acm.org> wrote:
> > >
> > > On 3/2/20 5:20 AM, Danil Kipnis wrote:
> > > > On Sun, Mar 1, 2020 at 2:33 AM Bart Van Assche <bvanassche@acm.org> wrote:
> > > >> On 2020-02-21 02:47, Jack Wang wrote:
> > > >>> +static struct rtrs_permit *
> > > >>> +__rtrs_get_permit(struct rtrs_clt *clt, enum rtrs_clt_con_type con_type)
> > > >>> +{
> > > >>> +     size_t max_depth = clt->queue_depth;
> > > >>> +     struct rtrs_permit *permit;
> > > >>> +     int cpu, bit;
> > > >>> +
> > > >>> +     /* Combined with cq_vector, we pin the IO to the the cpu it comes */
> > > >>
> > > >> This comment is confusing. Please clarify this comment. All I see below
> > > >> is that preemption is disabled. I don't see pinning of I/O to the CPU of
> > > >> the caller.
> > > > The comment is addressing a use-case of the driver: The user can
> > > > assign (under /proc/irq/) the irqs of the HCA cq_vectors "one-to-one"
> > > > to each cpu. This will "force" the driver to process io response on
> > > > the same cpu the io has been submitted on.
> > > > In the code below only preemption is disabled. This can lead to the
> > > > situation that callers from different cpus will grab the same bit,
> > > > since find_first_zero_bit is not atomic. But then the
> > > > test_and_set_bit_lock will fail for all the callers but one, so that
> > > > they will loop again. This way an explicit spinlock is not required.
> > > > Will extend the comment.
> > >
> > > If the purpose of get_cpu() and put_cpu() calls is to serialize code
> > > against other threads, please use locking instead of disabling
> > > preemption. This will help tools that verify locking like lockdep and
> > > the kernel thread sanitizer (https://github.com/google/ktsan/wiki).
> > We can look into it, but I'm afraid converting to spinlock might have
> > a performance impact.
>
> I very much dislike seeing people inventing locking, rarely is it done
> right. Making assumptions about IRQ scheduling in a driver seems
> really sketchy.
>
> Why do you need preemption disabled when using an atomic varient of
> test_and_set_bit anyhow? It is atomic, just loop?

We have to admit, the code snip is from null_blk, get_tag function,
not invented by us.
the get_cpu/put_cpu was added to get/save the current cpu_id, which
can be removed around the do-while loop.,
we only need to raw_smp_processor_id to get current cpu, we use it
later to pick which connection to use.

Bart asked in the past,  we missed that, thanks Jason for bringing up it.

>
> > > >> I don't think that posting a signalled send from time to time is
> > > >> sufficient to prevent send queue overflow. Please address Jason's
> > > >> comment from January 7th: "Not quite. If the SQ depth is 16 and you post
> > > >> 16 things and then signal the last one, you *cannot* post new work until
> > > >> you see the completion. More SQ space *ONLY* becomes available upon
> > > >> receipt of a completion. This is why you can't have an unsignaled SQ."
> > > >
> > > >> See also https://lore.kernel.org/linux-rdma/20200107182528.GB26174@ziepe.ca/
> > > > In our case we set the send queue of each QP belonging to one
> > > > "session" to the one supported by the hardware (max_qp_wr) which is
> > > > around 5K on our hardware. The queue depth of our "session" is 512.
> > > > Those 512 are "shared" by all the QPs (number of CPUs on client side)
> > > > belonging to that session. So we have at most 512 and 512/num_cpus on
> > > > average inflights on each QP. We never experienced send queue full
> > > > event in any of our performance tests or production usage. The
> > > > alternative would be to count submitted requests and completed
> > > > requests, check the difference before submission and wait if the
> > > > difference multiplied by the queue depth of "session" exceeds the max
> > > > supported by the hardware. The check will require quite some code and
> > > > will most probably affect performance. I do not think it is worth it
> > > > to introduce a code path which is triggered only on a condition which
> > > > is known to never become true.
> > > > Jason, do you think it's necessary to implement such tracking?
> > >
> > > Please either make sure that send queues do not overflow by providing
> > > enough space for 512 in-flight requests fit or implement tracking for
> > > the number of in-flight requests.
> > We do have enough space for send queue.
>
> You have to do something to provably guarantee the send q cannot
> overflow. send q overflow is defined as calling post_send before a
> poll_cq has confirmed space is available for send.
>
> Jason
Shouldn't the cq api handle that already,  with IB_POLL_SOFTIRQ,
poll cq is done on very softirq run, so send queue space should be reclaimed
fast enough, with IB_POLL_WORKQUEUE, when cq->com_handler get called,
the ib_cq_poll_work will do the poll_cq, together with extra
send_queue size reserved,
the send queue can not overflow!

Thanks Jason for your input!