[dpdk-dev] [PATCH V1 2/3] mbuf: change free_cb interface to adapt to GSO case

[yang_y_yi]

At 2020-08-03 04:29:07, "Olivier Matz" <olivier.matz at 6wind.com> wrote:
>Hi,
>
>On Sun, Aug 02, 2020 at 07:12:36AM +0800, yang_y_yi wrote:
>> 
>> 
>> At 2020-07-31 23:15:43, "Olivier Matz" <olivier.matz at 6wind.com> wrote:
>> >Hi,
>> >
>> >On Thu, Jul 30, 2020 at 08:08:59PM +0800, yang_y_yi at 163.com wrote:
>> >> From: Yi Yang <yangyi01 at inspur.com>
>> >> 
>> >> In GSO case, segmented mbufs are attached to original
>> >> mbuf which can't be freed when it is external. The issue
>> >> is free_cb doesn't know original mbuf and doesn't free
>> >> it when refcnt of shinfo is 0.
>> >> 
>> >> Original mbuf can be freed by rte_pktmbuf_free segmented
>> >> mbufs or by rte_pktmbuf_free original mbuf. Two kind of
>> >> cases should have different behaviors. free_cb won't
>> >> explicitly call rte_pktmbuf_free to free original mbuf
>> >> if it is freed by rte_pktmbuf_free original mbuf, but it
>> >> has to call rte_pktmbuf_free to free original mbuf if it
>> >> is freed by rte_pktmbuf_free segmented mbufs.
>> >> 
>> >> In order to fix this issue, free_cb interface has to been
>> >> changed, __rte_pktmbuf_free_extbuf must deliver called
>> >> mbuf pointer to free_cb, argument opaque can be defined
>> >> as a custom struct by user, it can includes original mbuf
>> >> pointer, user-defined free_cb can compare caller mbuf with
>> >> mbuf in opaque struct, free_cb should free original mbuf
>> >> if they are not same, this corresponds to rte_pktmbuf_free
>> >> segmented mbufs case, otherwise, free_cb won't free original
>> >> mbuf because the caller explicitly called rte_pktmbuf_free
>> >> to free it.
>> >> 
>> >> Here is pseduo code to show two kind of cases.
>> >> 
>> >> case 1. rte_pktmbuf_free segmented mbufs
>> >> 
>> >> nb_tx = rte_gso_segment(original_mbuf, /* original mbuf */
>> >>                       &gso_ctx,
>> >>                       /* segmented mbuf */
>> >>                       (struct rte_mbuf **)&gso_mbufs,
>> >>                       MAX_GSO_MBUFS);
>> >
>> >I'm sorry but it is not very clear to me what operations are done by
>> >rte_gso_segment().
>> >
>> >In the current code, I only see calls to rte_pktmbuf_attach(),
>> >which do not deal with external buffers. Am I missing something?
>> >
>> >Are you able to show the issue only with mbuf functions? It would
>> >be helpful to understand what does not work.
>> >
>> >
>> >Thanks,
>> >Olivier
>> >
>> Oliver, thank you for comment, let me show you why it doesn't work for my use case.  In OVS DPDK, VM uses vhostuserclient to send large packets whose size is about 64K because we enabled TSO & UFO, these large packets use rte_mbufs allocated by DPDK virtio_net function 
>> virtio_dev_pktmbuf_alloc() (in file lib/librte_vhost/virtio_net.c. Please refer to [PATCH V1 3/3], I changed free_cb as below, these packets use the same allocate function and the same free_cb no matter they are TCP packet or UDP packets, in case of VXLAN TSO, most NICs can't support inner UDP fragment offload, so OVS DPDK has to do it by software, for UDP case, the original rte_mbuf only can be freed by segmented rte_mbufs which are output packets of rte_gso_segment, i.e. the original rte_mbuf only can freed by free_cb, you can see, it explicitly called rte_pktmbuf_free(arg->mbuf), the condition statement "if (caller_m != arg->mbuf)" is true for this case, this has no problem, but for TCP case, the original mbuf is delivered to rte_eth_tx_burst() but not segmented rte_mbufs output by rte_gso_segment, PMD driver will call rte_pktmbuf_free(original_rte_mbuf) but not rte_pktmbuf_free(segmented_rte_mbufs), the same free_cb will be called, that means original_rte_mbuf will be freed twice, you know what will happen, this is just the issue I'm fixing. I bring in caller_m argument, it can help work around this because caller_m is arg->mbuf and the condition statement "if (caller_m != arg->mbuf)" is false, you can't fix it without the change this patch series did.
>
>I'm sill not sure to get your issue. Please, if you have a simple test
>case using only mbufs functions (without virtio, gso, ...), it would be
>very helpful because we will be sure that we are talking about the same
>thing. In case there is an issue, it can easily become a unit test.

Oliver, I think you don't get the point, free operation can't be controlled by the application itself, 
it is done by PMD driver and triggered by rte_eth_tx_burst, I have shown pseudo code,
rte_gso_segment just segments a large mbuf to multiple mbufs, it won't send them, the application
will call rte_eth_tx_burst to send them finally.

>
>That said, I looked at vhost mbuf allocation and gso segmentation, and
>I found some strange things:
>
>1/ In virtio_dev_extbuf_alloc(), and I there are 2 paths to create the
>   ext mbuf.
>
>   a/ The first one stores the shinfo struct in the mbuf, basically
>      like this:
>
>	pkt = rte_pktmbuf_alloc(mp);
>	shinfo = rte_pktmbuf_mtod(pkt, struct rte_mbuf_ext_shared_info *);
>	buf = rte_malloc(NULL, buf_len, RTE_CACHE_LINE_SIZE);
>	shinfo->free_cb = virtio_dev_extbuf_free;
>	shinfo->fcb_opaque = buf;
>	rte_mbuf_ext_refcnt_set(shinfo, 1);
>
>      I don't think it is a good idea, because there is no guarantee that
>      the mbuf won't be freed before the buffer. For instance, doing
>      this will probably fail:
>
>	pkt2 = rte_pktmbuf_alloc(mp);
>	rte_pktmbuf_attach(pkt2, pkt);
>	rte_pktmbuf_free(pkt);  /* pkt is freed, but it contains shinfo ! */

pkt is created by the application I can control, so I can control it where it will be freed, right?

>
>      To do this properly, the mbuf refcnt should be increased, and
>      the mbuf should be freed in the callback. But I don't think it's
>      worth doing it, given the second path (b/) looks good to me.
>
>   b/ The second path stores the shinfo struct at the end of the
>      allocated buffer, like this:
>
>	pkt = rte_pktmbuf_alloc(mp);
>	buf_len += sizeof(*shinfo) + sizeof(uintptr_t);
>	buf_len = RTE_ALIGN_CEIL(total_len, sizeof(uintptr_t));
>	buf = rte_malloc(NULL, buf_len, RTE_CACHE_LINE_SIZE);
>	shinfo = rte_pktmbuf_ext_shinfo_init_helper(buf, &buf_len,
>					      virtio_dev_extbuf_free, buf);
>
>      I think this is correct, because we have the guarantee that shinfo
>      exists as long as the buffer exists.

What buffer does the allocated buffer you're saying here? The issue we're discussing how we can
free original mbuf which owns shinfo buffer.

>
>2/ in rte_gso_segment(), there is a loop like this:
>
>	while (pkt_seg) {
>		rte_mbuf_refcnt_update(pkt_seg, -1);
>		pkt_seg = pkt_seg->next;
>	}
>
>   You change it to take in account the refcnt for ext mbufs.
>
>   I may have missed something but I wonder why it is not simply:
>
>	rte_pktmbuf_free(pkt_seg);
>
>   It will decrease the proper refcnt, and free the mbufs if they
>   are not used.

Again, rte_gso_segment just decreases refcnt by one, this will ensure the last segmented 
mbuf free will trigger freeing original mbuf (only free_cb can do this).

>
>Again, sorry if this is not the issue your are referring to, but
>in this case I really think that having a simple example code that
>shows the issue would help.

Oliver, my statement in the patch I sent out has pseudo code to show this.  I don't think a simple
unit test can show it. Let me summarize it here again. For original mbuf, there are two cases freeing
it, case one is PMD driver calls free against segmented mbufs, last segmented mbuf free will trigger
free_cb call which will free original large & extended mbuf. Case two is PMD driver will call free against
original mbuf, that also will call free_cb to free attached extended buffer. In case one free_cb must call
rte_pktmbuf_free otherwise nobody will free original large & extended mbuf, in case two free_cb can't 
call rte_pktmbuf_free because the caller calling it is just rte_pktmbuf_free we need. That is to say, you
must use the same free_cb to handle these two cases, this is my issue and the point you don't get.


>
>Regards,
>Olivier