Re: [PATCH v7 17/25] block/rnbd: client: main functionality

From: Jinpu Wang <jinpu.wang@cloud.ionos.com>
To: Leon Romanovsky <leon@kernel.org>
Cc: Jack Wang <jinpuwang@gmail.com>,
	linux-block@vger.kernel.org, linux-rdma@vger.kernel.org,
	Jens Axboe <axboe@kernel.dk>,
	Christoph Hellwig <hch@infradead.org>,
	Sagi Grimberg <sagi@grimberg.me>,
	Bart Van Assche <bvanassche@acm.org>,
	Doug Ledford <dledford@redhat.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Danil Kipnis <danil.kipnis@cloud.ionos.com>,
	Roman Penyaev <rpenyaev@suse.de>
Subject: Re: [PATCH v7 17/25] block/rnbd: client: main functionality
Date: Wed, 22 Jan 2020 14:12:19 +0100	[thread overview]
Message-ID: <CAMGffEmpRvruSn6iz6EfgfAjE9xrnsihwPaQU8Ft9e7qLD5avw@mail.gmail.com> (raw)
In-Reply-To: <20200122122548.GB7018@unreal>

On Wed, Jan 22, 2020 at 1:25 PM Leon Romanovsky <leon@kernel.org> wrote:
>
> On Wed, Jan 22, 2020 at 12:22:43PM +0100, Jinpu Wang wrote:
> > > > +/**
> > > > + * rnbd_get_cpu_qlist() - finds a list with HW queues to be rerun
> > > > + * @sess:    Session to find a queue for
> > > > + * @cpu:     Cpu to start the search from
> > > > + *
> > > > + * Description:
> > > > + *     Each CPU has a list of HW queues, which needs to be rerun.  If a list
> > > > + *     is not empty - it is marked with a bit.  This function finds first
> > > > + *     set bit in a bitmap and returns corresponding CPU list.
> > > > + */
> > > > +static struct rnbd_cpu_qlist *
> > > > +rnbd_get_cpu_qlist(struct rnbd_clt_session *sess, int cpu)
> > > > +{
> > > > +     int bit;
> > > > +
> > > > +     /* First half */
> > > > +     bit = find_next_bit(sess->cpu_queues_bm, nr_cpu_ids, cpu);
> > >
> > > Is it protected by any lock?
> > We hold requeue_lock when set/clear bit, and disable preemption via
> > get_cpu_ptr when find_next_bit.
> > even it fails to get latest bit, it just cause an rerun the queue.
>
> It is not clear here at all.
>
> > >
> > > > +     if (bit < nr_cpu_ids) {
> > > > +             return per_cpu_ptr(sess->cpu_queues, bit);
> > > > +     } else if (cpu != 0) {
> > > > +             /* Second half */
> > > > +             bit = find_next_bit(sess->cpu_queues_bm, cpu, 0);
> > > > +             if (bit < cpu)
> > > > +                     return per_cpu_ptr(sess->cpu_queues, bit);
> > > > +     }
> > > > +
> > > > +     return NULL;
> > > > +}
> > > > +
> > > > +static inline int nxt_cpu(int cpu)
> > > > +{
> > > > +     return (cpu + 1) % nr_cpu_ids;
> > > > +}
> > > > +
> > > > +/**
> > > > + * rnbd_rerun_if_needed() - rerun next queue marked as stopped
> > > > + * @sess:    Session to rerun a queue on
> > > > + *
> > > > + * Description:
> > > > + *     Each CPU has it's own list of HW queues, which should be rerun.
> > > > + *     Function finds such list with HW queues, takes a list lock, picks up
> > > > + *     the first HW queue out of the list and requeues it.
> > > > + *
> > > > + * Return:
> > > > + *     True if the queue was requeued, false otherwise.
> > > > + *
> > > > + * Context:
> > > > + *     Does not matter.
> > > > + */
> > > > +static inline bool rnbd_rerun_if_needed(struct rnbd_clt_session *sess)
> > >
> > > No inline function in C files.
> > First time saw such request, there are so many inline functions in C
>
> 15) The inline disease
> https://elixir.bootlin.com/linux/latest/source/Documentation/process/coding-style.rst#L882
ok, will review the inline usage, and drop some.
>
> > files across the tree
> > grep inline drivers/infiniband/core/*.c
> > drivers/infiniband/core/addr.c:static inline bool
> > ib_nl_is_good_ip_resp(const struct nlmsghdr *nlh)
> > drivers/infiniband/core/cma.c:static inline u8 cma_get_ip_ver(const
> > struct cma_hdr *hdr)
> > drivers/infiniband/core/cma.c:static inline void cma_set_ip_ver(struct
> > cma_hdr *hdr, u8 ip_ver)
> > drivers/infiniband/core/cma.c:static inline void release_mc(struct kref *kref)
> > drivers/infiniband/core/cma.c:static inline struct sockaddr
> > *cma_src_addr(struct rdma_id_private *id_priv)
> > drivers/infiniband/core/cma.c:static inline struct sockaddr
> > *cma_dst_addr(struct rdma_id_private *id_priv)
> >
> > >
> > > > +{
> > > > +     struct rnbd_queue *q = NULL;
> > > > +     struct rnbd_cpu_qlist *cpu_q;
> > > > +     unsigned long flags;
> > > > +     int *cpup;
> > > > +
> > > > +     /*
> > > > +      * To keep fairness and not to let other queues starve we always
> > > > +      * try to wake up someone else in round-robin manner.  That of course
> > > > +      * increases latency but queues always have a chance to be executed.
> > > > +      */
> > > > +     cpup = get_cpu_ptr(sess->cpu_rr);
> > > > +     for (cpu_q = rnbd_get_cpu_qlist(sess, nxt_cpu(*cpup)); cpu_q;
> > > > +          cpu_q = rnbd_get_cpu_qlist(sess, nxt_cpu(cpu_q->cpu))) {
> > > > +             if (!spin_trylock_irqsave(&cpu_q->requeue_lock, flags))
> > > > +                     continue;
> > > > +             if (likely(test_bit(cpu_q->cpu, sess->cpu_queues_bm))) {
> > >
> > > Success oriented approach please.
> > sorry, I don't quite get your point.
>
> The flows are better to be written:
> if (err)
>   return or conitnue
> <...>
> do_something
>
> in your case
> if (!test_bit(...))
>  continue;
> do_work_here.
In our case,
 if we failed to get requeue_lock, we continue to next cpu_q and do the work
I guess you miss read the code.

Thanks