On Tue, Aug 29, 2017 at 05:56:05PM +0800, Ming Lei wrote:
> On Thu, Aug 24, 2017 at 12:24:53PM -0700, Shaohua Li wrote:
> > From: Shaohua Li <shli@xxxxxx>
> >
> > Currently loop disables merge. While it makes sense for buffer IO mode,
> > directio mode can benefit from request merge. Without merge, loop could
> > send small size IO to underlayer disk and harm performance.
>
> Hi Shaohua,
>
> IMO no matter if merge is used, loop always sends page by page
> to VFS in both dio or buffer I/O.
Why do you think so?
> Also if merge is enabled on loop, that means merge is run
> on both loop and low level block driver, and not sure if we
> can benefit from that.
why does merge still happen in low level block driver?
>
> So Could you provide some performance data about this patch?
In my virtual machine, a workload improves from ~20M/s to ~50M/s. And I clearly
see the request size becomes bigger.
> >
> > Reviewed-by: Omar Sandoval <osandov@xxxxxx>
> > Signed-off-by: Shaohua Li <shli@xxxxxx>
> > ---
> > drivers/block/loop.c | 66 ++++++++++++++++++++++++++++++++++++++++------------
> > drivers/block/loop.h | 1 +
> > 2 files changed, 52 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index 428da07..75a8f6e 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -213,10 +213,13 @@ static void __loop_update_dio(struct loop_device *lo, bool dio)
> > */
> > blk_mq_freeze_queue(lo->lo_queue);
> > lo->use_dio = use_dio;
> > - if (use_dio)
> > + if (use_dio) {
> > + queue_flag_clear_unlocked(QUEUE_FLAG_NOMERGES, lo->lo_queue);
> > lo->lo_flags |= LO_FLAGS_DIRECT_IO;
> > - else
> > + } else {
> > + queue_flag_set_unlocked(QUEUE_FLAG_NOMERGES, lo->lo_queue);
> > lo->lo_flags &= ~LO_FLAGS_DIRECT_IO;
> > + }
> > blk_mq_unfreeze_queue(lo->lo_queue);
> > }
> >
> > @@ -464,6 +467,8 @@ static void lo_rw_aio_complete(struct kiocb *iocb, long ret, long ret2)
> > {
> > struct loop_cmd *cmd = container_of(iocb, struct loop_cmd, iocb);
> >
> > + kfree(cmd->bvec);
> > + cmd->bvec = NULL;
> > cmd->ret = ret;
> > blk_mq_complete_request(cmd->rq);
> > }
> > @@ -473,22 +478,50 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
> > {
> > struct iov_iter iter;
> > struct bio_vec *bvec;
> > - struct bio *bio = cmd->rq->bio;
> > + struct request *rq = cmd->rq;
> > + struct bio *bio = rq->bio;
> > struct file *file = lo->lo_backing_file;
> > + unsigned int offset;
> > + int segments = 0;
> > int ret;
> >
> > - /* nomerge for loop request queue */
> > - WARN_ON(cmd->rq->bio != cmd->rq->biotail);
> > + if (rq->bio != rq->biotail) {
> > + struct req_iterator iter;
> > + struct bio_vec tmp;
> > +
> > + __rq_for_each_bio(bio, rq)
> > + segments += bio_segments(bio);
> > + bvec = kmalloc(sizeof(struct bio_vec) * segments, GFP_KERNEL);
>
> The allocation should have been GFP_NOIO.
Sounds good. To make this completely correct isn't easy though, we are calling
into the underlayer fs operations which can do allocation.
Thanks,
Shaohua
