Re: [PATCH 05/24] ibtrs: client: main functionality

Roman Penyaev
2018-02-05T13:27:34Z

On Fri, Feb 2, 2018 at 5:54 PM, Bart Van Assche <Bart.VanAssche@wdc.com> wrote:
> On Fri, 2018-02-02 at 15:08 +0100, Roman Pen wrote:
>> +static inline struct ibtrs_tag *
>> +__ibtrs_get_tag(struct ibtrs_clt *clt, enum ibtrs_clt_con_type con_type)
>> +{
>> + size_t max_depth = clt->queue_depth;
>> + struct ibtrs_tag *tag;
>> + int cpu, bit;
>> +
>> + cpu = get_cpu();
>> + do {
>> + bit = find_first_zero_bit(clt->tags_map, max_depth);
>> + if (unlikely(bit >= max_depth)) {
>> + put_cpu();
>> + return NULL;
>> + }
>> +
>> + } while (unlikely(test_and_set_bit_lock(bit, clt->tags_map)));
>> + put_cpu();
>> +
>> + tag = GET_TAG(clt, bit);
>> + WARN_ON(tag->mem_id != bit);
>> + tag->cpu_id = cpu;
>> + tag->con_type = con_type;
>> +
>> + return tag;
>> +}
>> +
>> +static inline void __ibtrs_put_tag(struct ibtrs_clt *clt,
>> + struct ibtrs_tag *tag)
>> +{
>> + clear_bit_unlock(tag->mem_id, clt->tags_map);
>> +}
>> +
>> +struct ibtrs_tag *ibtrs_clt_get_tag(struct ibtrs_clt *clt,
>> + enum ibtrs_clt_con_type con_type,
>> + int can_wait)
>> +{
>> + struct ibtrs_tag *tag;
>> + DEFINE_WAIT(wait);
>> +
>> + tag = __ibtrs_get_tag(clt, con_type);
>> + if (likely(tag) || !can_wait)
>> + return tag;
>> +
>> + do {
>> + prepare_to_wait(&clt->tags_wait, &wait, TASK_UNINTERRUPTIBLE);
>> + tag = __ibtrs_get_tag(clt, con_type);
>> + if (likely(tag))
>> + break;
>> +
>> + io_schedule();
>> + } while (1);
>> +
>> + finish_wait(&clt->tags_wait, &wait);
>> +
>> + return tag;
>> +}
>> +EXPORT_SYMBOL(ibtrs_clt_get_tag);
>> +
>> +void ibtrs_clt_put_tag(struct ibtrs_clt *clt, struct ibtrs_tag *tag)
>> +{
>> + if (WARN_ON(!test_bit(tag->mem_id, clt->tags_map)))
>> + return;
>> +
>> + __ibtrs_put_tag(clt, tag);
>> +
>> + /*
>> + * Putting a tag is a barrier, so we will observe
>> + * new entry in the wait list, no worries.
>> + */
>> + if (waitqueue_active(&clt->tags_wait))
>> + wake_up(&clt->tags_wait);
>> +}
>> +EXPORT_SYMBOL(ibtrs_clt_put_tag);
>
> Do these functions have any advantage over the code in lib/sbitmap.c? If not,
> please call the sbitmap functions instead of adding an additional tag allocator.

Indeed, seems sbitmap can be reused.

But tags is a part of IBTRS, and is not related to block device at all. One
IBTRS connection (session) handles many block devices (or any IO producers).
With a tag you get a free slot of a buffer where you can read/write, so once
you've allocated a tag you won't sleep on IO path inside a library. Also tag
helps a lot on IO fail-over to another connection (multipath implementation,
which is also a part of the transport library, not a block device), where you
simply reuse the same buffer slot (with a tag in your hands) forwarding IO to
another RDMA connection.

--
Roman