Re: [V9fs-developer] [PATCH 4/5] 9p: introduce async read requests

[Stefano Stabellini]

On Fri, 9 Dec 2016, Dominique Martinet wrote:
> Stefano Stabellini wrote on Thu, Dec 08, 2016:
> > If the read is an async operation, send a 9p request and return
> > EIOCBQUEUED. Do not wait for completion.
> > 
> > Complete the read operation from a callback instead.
> > 
> > Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > ---
> >  net/9p/client.c | 88 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 86 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/9p/client.c b/net/9p/client.c
> > index eb589ef..f9f09db 100644
> > --- a/net/9p/client.c
> > +++ b/net/9p/client.c
> > @@ -28,6 +28,7 @@
> >  #include <linux/module.h>
> >  #include <linux/errno.h>
> >  #include <linux/fs.h>
> > +#include <linux/pagemap.h>
> >  #include <linux/poll.h>
> >  #include <linux/idr.h>
> >  #include <linux/mutex.h>
> > @@ -1554,13 +1555,68 @@ int p9_client_unlinkat(struct p9_fid *dfid, const char *name, int flags)
> >  }
> >  EXPORT_SYMBOL(p9_client_unlinkat);
> >  
> > +static void
> > +p9_client_read_complete(struct p9_client *clnt, struct p9_req_t *req, int status)
> > +{
> > +	int err, count, n, i, total = 0;
> > +	char *dataptr, *to;
> > +
> > +	if (req->status == REQ_STATUS_ERROR) {
> > +		p9_debug(P9_DEBUG_ERROR, "req_status error %d\n", req->t_err);
> > +		err = req->t_err;
> > +		goto out;
> > +	}
> > +	err = p9_check_errors(clnt, req);
> > +	if (err)
> > +		goto out;
> > +
> > +	err = p9pdu_readf(req->rc, clnt->proto_version,
> > +			"D", &count, &dataptr);
> > +	if (err) {
> > +		trace_9p_protocol_dump(clnt, req->rc);
> > +		goto out;
> > +	}
> > +	if (!count) {
> > +		p9_debug(P9_DEBUG_ERROR, "count=%d\n", count);
> > +		err = 0;
> > +		goto out;
> > +	}
> > +
> > +	p9_debug(P9_DEBUG_9P, "<<< RREAD count %d\n", count);
> > +	if (count > req->rsize)
> > +		count = req->rsize;
> > +
> > +	for (i = 0; i < ((req->rsize + PAGE_SIZE - 1) / PAGE_SIZE); i++) {
> > +		to = kmap(req->pagevec[i]);
> > +		to += req->offset;
> > +		n = PAGE_SIZE - req->offset;
> > +		if (n > count)
> > +			n = count;
> > +		memcpy(to, dataptr, n);
> > +		kunmap(req->pagevec[i]);
> > +		req->offset = 0;
> > +		count -= n;
> > +		total += n;
> > +	}
> > +
> > +	err = total;
> > +	req->kiocb->ki_pos += total;
> > +
> > +out:
> > +	req->kiocb->ki_complete(req->kiocb, err, 0);
> > +
> > +	release_pages(req->pagevec, (req->rsize + PAGE_SIZE - 1) / PAGE_SIZE, false);
> > +	kvfree(req->pagevec);
> > +	p9_free_req(clnt, req);
> > +}
> > +
> >  int
> >  p9_client_read(struct p9_fid *fid, struct kiocb *iocb, u64 offset,
> >  				struct iov_iter *to, int *err)
> >  {
> >  	struct p9_client *clnt = fid->clnt;
> >  	struct p9_req_t *req;
> > -	int total = 0;
> > +	int total = 0, i;
> >  	*err = 0;
> >  
> >  	p9_debug(P9_DEBUG_9P, ">>> TREAD fid %d offset %llu %d\n",
> > @@ -1587,10 +1643,38 @@ int p9_client_unlinkat(struct p9_fid *dfid, const char *name, int flags)
> >  			req = p9_client_zc_rpc(clnt, P9_TREAD, to, NULL, rsize,
> >  					       0, 11, "dqd", fid->fid,
> >  					       offset, rsize);
> > -		} else {
> > +		/* sync request */
> > +		} else if(iocb == NULL || is_sync_kiocb(iocb)) {
> >  			non_zc = 1;
> >  			req = p9_client_rpc(clnt, P9_TREAD, "dqd", fid->fid, offset,
> >  					    rsize);
> > +		/* async request */
> > +		} else {
> 
> I'm not too familiar with iocb/how async IOs should work, but a logic
> question just to make sure that has been thought out:
> We prefer zc here to async, even if zc can be slow?
> 
> Ideally at some point zc and async aren't exclusive so we'll have async
> zc and async normal, but for now I'd say async comes before zc - yes
> there will be an extra copy in memory, but it will be done
> asynchronously.
> Was it intentional to prefer zc here?

I wasn't sure what to do about zc. The backends I am testing with don't
support zc, so I didn't feel confident in changing its behavior. I think
whether zc is faster than async+copy depends on the specific benchmark.
iodepth and blocksize parameters in fio, for example. With iodepth=1, zc
would be faster, the higher the iodepth, the faster async+copy would
become in comparison. At some point async+copy will be faster than zc,
but I am not sure where is the threshold, it would probably be storage
backend dependent too. Maybe around iodepth=3. This is a reasonable
guess but I haven't run any numbers to confirm it.

That said, I am happy to follow any strategy you suggest in regards to zc.


> > +			req = p9_client_get_req(clnt, P9_TREAD, "dqd", fid->fid, offset, rsize);
> > +			if (IS_ERR(req)) {
> > +				*err = PTR_ERR(req);
> > +				break;
> > +			}
> > +			req->rsize = iov_iter_get_pages_alloc(to, &req->pagevec, 
> > +					(size_t)rsize, &req->offset);
> > +			req->kiocb = iocb;
> > +			for (i = 0; i < req->rsize; i += PAGE_SIZE)
> > +				page_cache_get_speculative(req->pagevec[i/PAGE_SIZE]);
> > +			req->callback = p9_client_read_complete;
> > +
> > +			*err = clnt->trans_mod->request(clnt, req);
> > +			if (*err < 0) {
> > +				clnt->status = Disconnected;
> > +				release_pages(req->pagevec,
> > +						(req->rsize + PAGE_SIZE - 1) / PAGE_SIZE,
> > +						true);
> > +				kvfree(req->pagevec);
> > +				p9_free_req(clnt, req);
> > +				break;
> > +			}
> > +
> > +			*err = -EIOCBQUEUED;
> > +			break;
> 
> (Just a note to myself (or anyone who wants to do this) that a couple of
> places only look at err if size written is 0... They should check if err
> has been set)
> 
> >  		}
> >  		if (IS_ERR(req)) {
> >  			*err = PTR_ERR(req);
>