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@xxxxxxx> wrote:
> >
> > On 3/2/20 5:20 AM, Danil Kipnis wrote:
> > > On Sun, Mar 1, 2020 at 2:33 AM Bart Van Assche <bvanassche@xxxxxxx> 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?
> > >> 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@xxxxxxxx/
> > > 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
