On 18.08.2012 02:32, J. Bruce Fields wrote:
> On Fri, Aug 17, 2012 at 04:08:07PM -0400, J. Bruce Fields wrote:
>> Wait a minute, that assumption's a problem because that calculation
>> depends in part on xpt_reserved, which is changed here....
>>
>> In particular, svc_xprt_release() calls svc_reserve(rqstp, 0), which
>> subtracts rqstp->rq_reserved and then calls svc_xprt_enqueue, now with a
>> lower xpt_reserved value. That could well explain this.
>
> So, maybe something like this?
Well. What can I say? With the change below applied (to 3.2 kernel
at least), I don't see any stalls or high CPU usage on the server
anymore. It survived several multi-gigabyte transfers, for several
hours, without any problem. So it is a good step forward ;)
But the whole thing seems to be quite a bit fragile. I tried to follow
the logic in there, and the thing is quite a bit, well, "twisted", and
somewhat difficult to follow. So I don't know if this is the right
fix or not. At least it works! :)
And I really wonder why no one else reported this problem before.
Is me the only one in this world who uses linux nfsd? :)
Thank you for all your patience and the proposed fix!
/mjt
> commit c8136c319ad85d0db870021fc3f9074d37f26d4a
> Author: J. Bruce Fields <bfields@xxxxxxxxxx>
> Date: Fri Aug 17 17:31:53 2012 -0400
>
> svcrpc: don't add to xpt_reserved till we receive
>
> The rpc server tries to ensure that there will be room to send a reply
> before it receives a request.
>
> It does this by tracking, in xpt_reserved, an upper bound on the total
> size of the replies that is has already committed to for the socket.
>
> Currently it is adding in the estimate for a new reply *before* it
> checks whether there is space available. If it finds that there is not
> space, it then subtracts the estimate back out.
>
> This may lead the subsequent svc_xprt_enqueue to decide that there is
> space after all.
>
> The results is a svc_recv() that will repeatedly return -EAGAIN, causing
> server threads to loop without doing any actual work.
>
> Reported-by: Michael Tokarev <mjt@xxxxxxxxxx>
> Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
>
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index ec99849a..59ff3a3 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -366,8 +366,6 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
> rqstp, rqstp->rq_xprt);
> rqstp->rq_xprt = xprt;
> svc_xprt_get(xprt);
> - rqstp->rq_reserved = serv->sv_max_mesg;
> - atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
> pool->sp_stats.threads_woken++;
> wake_up(&rqstp->rq_wait);
> } else {
> @@ -644,8 +642,6 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
> if (xprt) {
> rqstp->rq_xprt = xprt;
> svc_xprt_get(xprt);
> - rqstp->rq_reserved = serv->sv_max_mesg;
> - atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
>
> /* As there is a shortage of threads and this request
> * had to be queued, don't allow the thread to wait so
> @@ -743,6 +739,10 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
> len = xprt->xpt_ops->xpo_recvfrom(rqstp);
> dprintk("svc: got len=%d\n", len);
> }
> + if (len > 0) {
> + rqstp->rq_reserved = serv->sv_max_mesg;
> + atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
> + }
> svc_xprt_received(xprt);
>
> /* No data, incomplete (TCP) read, or accept() */
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
--
To unsubscribe from this list: send the line "unsubscribe linux-nfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
