On 2/3/20 5:50 PM, Ilya Dryomov wrote:
> On Fri, Jan 31, 2020 at 11:38 AM Hannes Reinecke <hare@xxxxxxx> wrote:
>>
>> The mapping size is changed only very infrequently, so we don't
>> need to take the header mutex for checking; using READ_ONCE()
>> is sufficient here. And it avoids having to take a mutex in the
>> hot path.
>>
>> Signed-off-by: Hannes Reinecke <hare@xxxxxxx>
>> ---
>> drivers/block/rbd.c | 12 ++++++------
>> 1 file changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>> index db80b964d8ea..792180548e89 100644
>> --- a/drivers/block/rbd.c
>> +++ b/drivers/block/rbd.c
>> @@ -4788,13 +4788,13 @@ static void rbd_queue_workfn(struct work_struct *work)
>>
>> blk_mq_start_request(rq);
>>
>> - down_read(&rbd_dev->header_rwsem);
>> - mapping_size = rbd_dev->mapping.size;
>> + mapping_size = READ_ONCE(rbd_dev->mapping.size);
>> if (op_type != OBJ_OP_READ) {
>> + down_read(&rbd_dev->header_rwsem);
>> snapc = rbd_dev->header.snapc;
>> ceph_get_snap_context(snapc);
>> + up_read(&rbd_dev->header_rwsem);
>> }
>> - up_read(&rbd_dev->header_rwsem);
>>
>> if (offset + length > mapping_size) {
>> rbd_warn(rbd_dev, "beyond EOD (%llu~%llu > %llu)", offset,
>> @@ -4981,9 +4981,9 @@ static int rbd_dev_refresh(struct rbd_device *rbd_dev)
>> u64 mapping_size;
>> int ret;
>>
>> - down_write(&rbd_dev->header_rwsem);
>> - mapping_size = rbd_dev->mapping.size;
>> + mapping_size = READ_ONCE(rbd_dev->mapping.size);
>>
>> + down_write(&rbd_dev->header_rwsem);
>> ret = rbd_dev_header_info(rbd_dev);
>> if (ret)
>> goto out;
>> @@ -4999,7 +4999,7 @@ static int rbd_dev_refresh(struct rbd_device *rbd_dev)
>> }
>>
>> rbd_assert(!rbd_is_snap(rbd_dev));
>> - rbd_dev->mapping.size = rbd_dev->header.image_size;
>> + WRITE_ONCE(rbd_dev->mapping.size, rbd_dev->header.image_size);
>>
>> out:
>> up_write(&rbd_dev->header_rwsem);
>
> Does this result in a measurable performance improvement?
>
> I'd rather not go down the READ/WRITE_ONCE path and continue using
> proper locking, especially given that it's only for reads. FWIW the
> plan is to replace header_rwsem with a spin lock, after refactoring
> header read-in code to use a private buffer instead of reading into
> rbd_dev directly.
>
Well ... Not sure if I like the spin_lock idea.
Thing is, the mapping size is evaluated exactly _once_ when assembling
the request. So any change to the mapping size just after we've read it
would go unnoticed.
Hence it should be possible to combine both approaches; use READ_ONCE()
to read the mapping size, but use a spin lock for updating it as you
suggested. That way we'll eliminate a lock in the hot path, but would be
getting safe updates.
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
hare@xxxxxxx +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), GF: Felix Imendörffer
