>
>
> On 1/17/2018 6:50 PM, Ananyev, Konstantin wrote:
> >
> >>> Hi Jianfeng,
> >>>
> >>>> -----Original Message-----
> >>>> From: Tan, Jianfeng
> >>>> Sent: Tuesday, January 16, 2018 8:11 AM
> >>>> To: Ananyev, Konstantin <konstantin.anan...@intel.com>; dev@dpdk.org;
> >>>> Burakov, Anatoly <anatoly.bura...@intel.com>
> >>>> Cc: Richardson, Bruce <bruce.richard...@intel.com>; tho...@monjalon.net
> >>>> Subject: Re: [PATCH v2 3/4] eal: add synchronous multi-process
> >>>> communication
> >>>>
> >>>> Thank you, Konstantin and Anatoly firstly. Other comments are well
> >>>> received and I'll send out a new version.
> >>>>
> >>>>
> >>>> On 1/16/2018 8:00 AM, Ananyev, Konstantin wrote:
> >>>>>> We need the synchronous way for multi-process communication,
> >>>>>> i.e., blockingly waiting for reply message when we send a request
> >>>>>> to the peer process.
> >>>>>>
> >>>>>> We add two APIs rte_eal_mp_request() and rte_eal_mp_reply() for
> >>>>>> such use case. By invoking rte_eal_mp_request(), a request message
> >>>>>> is sent out, and then it waits there for a reply message. The
> >>>>>> timeout is hard-coded 5 Sec. And the replied message will be copied
> >>>>>> in the parameters of this API so that the caller can decide how
> >>>>>> to translate those information (including params and fds). Note
> >>>>>> if a primary process owns multiple secondary processes, this API
> >>>>>> will fail.
> >>>>>>
> >>>>>> The API rte_eal_mp_reply() is always called by an mp action handler.
> >>>>>> Here we add another parameter for rte_eal_mp_t so that the action
> >>>>>> handler knows which peer address to reply.
> >>>>>>
> >>>>>> We use mutex in rte_eal_mp_request() to guarantee that only one
> >>>>>> request is on the fly for one pair of processes.
> >>>>> You don't need to do things in such strange and restrictive way.
> >>>>> Instead you can do something like that:
> >>>>> 1) Introduce new struct, list for it and mutex
> >>>>> struct sync_request {
> >>>>> int reply_received;
> >>>>> char dst[PATH_MAX];
> >>>>> char reply[...];
> >>>>> LIST_ENTRY(sync_request) next;
> >>>>> };
> >>>>>
> >>>>> static struct
> >>>>> LIST_HEAD(list, sync_request);
> >>>>> pthread_mutex_t lock;
> >>>>> pthead_cond_t cond;
> >>>>> } sync_requests;
> >>>>>
> >>>>> 2) then at request() call:
> >>>>> Grab sync_requests.lock
> >>>>> Check do we already have a pending request for that destination,
> >>>>> If yes - the release the lock and returns with error.
> >>>>> - allocate and init new sync_request struct, set reply_received=0
> >>>>> - do send_msg()
> >>>>> -then in a cycle:
> >>>>> pthread_cond_timed_wait(&sync_requests.cond, &sync_request.lock,
> >>>>> ×pec);
> >>>>> - at return from it check if sync_request.reply_received == 1, if
> >>>>> not
> >>>>> check if timeout expired and either return a failure or go to the start
> >>>>> of the cycle.
> >>>>>
> >>>>> 3) at mp_handler() if REPLY received - grab sync_request.lock,
> >>>>> search through sync_requests.list for dst[] ,
> >>>>> if found, then set it's reply_received=1, copy the received
> >>>>> message into reply
> >>>>> and call pthread_cond_braodcast((&sync_requests.cond);
> >>>> The only benefit I can see is that now the sender can request to
> >>>> multiple receivers at the same time. And it makes things more
> >>>> complicated. Do we really need this?
> >>> The benefit is that one thread is blocked waiting for response,
> >>> your mp_handler can still receive and handle other messages.
> >> This can already be done in the original implementation. mp_handler
> >> listens for msg, request from the other peer(s), and replies the
> >> requests, which is not affected.
> >>
> >>> Plus as you said - other threads can keep sending messages.
> >> For this one, in the original implementation, other threads can still
> >> send msg, but not request. I suppose the request is not in a fast path,
> >> why we care to make it fast?
> >>
> > +int
> > +rte_eal_mp_request(const char *action_name,
> > + void *params,
> > + int len_p,
> > + int fds[],
> > + int fds_in,
> > + int fds_out)
> > +{
> > + int i, j;
> > + int sockfd;
> > + int nprocs;
> > + int ret = 0;
> > + struct mp_msghdr *req;
> > + struct timeval tv;
> > + char buf[MAX_MSG_LENGTH];
> > + struct mp_msghdr *hdr;
> > +
> > + RTE_LOG(DEBUG, EAL, "request: %s\n", action_name);
> > +
> > + if (fds_in > SCM_MAX_FD || fds_out > SCM_MAX_FD) {
> > + RTE_LOG(ERR, EAL, "Cannot send more than %d FDs\n", SCM_MAX_FD);
> > + rte_errno = -E2BIG;
> > + return 0;
> > + }
> > +
> > + req = format_msg(action_name, params, len_p, fds_in, MP_REQ);
> > + if (req == NULL)
> > + return 0;
> > +
> > + if ((sockfd = open_unix_fd(0)) < 0) {
> > + free(req);
> > + return 0;
> > + }
> > +
> > + tv.tv_sec = 5; /* 5 Secs Timeout */
> > + tv.tv_usec = 0;
> > + if (setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO,
> > + (const void *)&tv, sizeof(struct timeval)) < 0)
> > + RTE_LOG(INFO, EAL, "Failed to set recv timeout\n");
> >
> > I f you set it just for one call, why do you not restore it?
>
> Yes, original code is buggy, I should have put it into the critical section.
>
> Do you mean we just create once and use for ever? if yes, we could put
> the open and setting into mp_init().
>
> > Also I don't think it is a good idea to change it here -
> > if you'll make timeout a parameter value - then it could be overwritten
> > by different threads.
>
> For simplicity, I'm not inclined to put the timeout as an parameter
> exposing to caller. So if you agree, I'll put it into the mp_init() with
> open.
My preference would be to have timeout value on a per call basis.
For one request user would like to wait no more than 5sec,
for another one user would probably be ok to wait forever.
>
> >
> > +
> > + /* Only allow one req at a time */
> > + pthread_mutex_lock(&mp_mutex_request);
> > +
> > + if (rte_eal_process_type() == RTE_PROC_PRIMARY) {
> > + nprocs = 0;
> > + for (i = 0; i < MAX_SECONDARY_PROCS; ++i)
> > + if (!mp_sec_sockets[i]) {
> > + j = i;
> > + nprocs++;
> > + }
> > +
> > + if (nprocs > 1) {
> > + RTE_LOG(ERR, EAL,
> > + "multi secondary processes not supported\n");
> > + goto free_and_ret;
> > + }
> > +
> > + ret = send_msg(sockfd, mp_sec_sockets[j], req, fds);
> >
> > As I remember - sndmsg() is also blocking call, so under some conditions
> > you can stall
> > there forever.
>
> From linux's unix_diagram_sendmsg(), we see:
> timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
Ok, but it would have effect only if (msg->msg_flags & MSG_DONTWAIT) != 0.
And for that, as I remember you need your socket in non-blocking mode, no?
>
> I assume it will not block for datagram unix socket in Linux. But I'm
> not sure what it behaves in freebsd.
>
> Anyway, better to add an explicit setsockopt() to make it not blocking.
You can't do that - at the same moment another thread might call your sendmsg()
and it might expect it to be blocking call.
>
> > As mp_mutex_requestis still held - next rte_eal_mp_request(0 will also
> > block forever here.
> >
> > + } else
> > + ret = send_msg(sockfd, eal_mp_unix_path(), req, fds);
> > +
> > + if (ret == 0) {
> > + RTE_LOG(ERR, EAL, "failed to send request: %s\n", action_name);
> > + ret = -1;
> > + goto free_and_ret;
> > + }
> > +
> > + ret = read_msg(sockfd, buf, MAX_MSG_LENGTH, fds, fds_out, NULL);
> >
> > if the message you receive is not a reply you are expecting -
> > it will be simply dropped - mp_handler() would never process it.
>
> We cannot detect if it's the right reply absolutely correctly, but just
> check the action_name, which means, it still possibly gets a wrong reply
> if an action_name contains multiple requests.
>
> Is just comparing the action_name acceptable?
As I can see the main issue here is that you can call recvmsg() from 2 different
points and they are not syncronised:
1. your mp_handler() doesn't aware about reply you are waiting and not
have any handler associated with it.
So if mp_handler() will receive a reply it will just drop it.
2. your reply() is not aware about any other messages and associated actions -
so again it can't handle them properly (and probably shouldn't).
The simplest (and most common) way - always call recvmsg from one place -
mp_handler() and have a special action for reply msg.
As I wrote before that action will be just find the appropriate buffer provided
by reply() - copy message into it and signal thread waiting in reply() that
it can proceed.
Konstantin
>
> >
> > + if (ret > 0) {
> > + hdr = (struct mp_msghdr *)buf;
> > + if (hdr->len_params == len_p)
> > + memcpy(params, hdr->params, len_p);
> > + else {
> > + RTE_LOG(ERR, EAL, "invalid reply\n");
> > + ret = 0;
> > + }
> > + }
> > +
> > +free_and_ret:
> > + free(req);
> > + close(sockfd);
> > + pthread_mutex_unlock(&mp_mutex_request);
> > + return ret;
> > +}
> >
> > All of the above makes me think that current implementation is erroneous
> > and needs to be reworked.
>
> Thank you for your review. I'll work on a new version.
>
> Thanks,
> Jianfeng
>
> > Konstantin
> >
> >
