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From: 858585 jemmy <jemmy858585@gmail.com>
Date: Tue, 22 May 2018 18:12:24 +0800
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Subject: Re: [Qemu-devel] [PATCH 2/2] migration: not wait
 RDMA_CM_EVENT_DISCONNECTED event after rdma_disconnect
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To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Aviad Yehezkel <aviadye@mellanox.com>, Juan Quintela <quintela@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Gal Shachaf <galsha@mellanox.com>, Adi Dotan <adido@mellanox.com>, Lidong Chen <lidongchen@tencent.com>

On Wed, May 16, 2018 at 9:13 PM, Dr. David Alan Gilbert
<dgilbert@redhat.com> wrote:
> * 858585 jemmy (jemmy858585@gmail.com) wrote:
>
> <snip>
>
>> >> >> > I wonder why dereg_mr takes so long - I could understand if reg_mr
>> >> >> > took a long time, but why for dereg, that sounds like the easy side.
>> >> >>
>> >> >> I use perf collect the information when ibv_dereg_mr is invoked.
>> >> >>
>> >> >> -   9.95%  client2  [kernel.kallsyms]  [k] put_compound_page
>> >> >>                                                           `
>> >> >>    - put_compound_page
>> >> >>       - 98.45% put_page
>> >> >>            __ib_umem_release
>> >> >>            ib_umem_release
>> >> >>            dereg_mr
>> >> >>            mlx5_ib_dereg_mr
>> >> >>            ib_dereg_mr
>> >> >>            uverbs_free_mr
>> >> >>            remove_commit_idr_uobject
>> >> >>            _rdma_remove_commit_uobject
>> >> >>            rdma_remove_commit_uobject
>> >> >>            ib_uverbs_dereg_mr
>> >> >>            ib_uverbs_write
>> >> >>            vfs_write
>> >> >>            sys_write
>> >> >>            system_call_fastpath
>> >> >>            __GI___libc_write
>> >> >>            0
>> >> >>       + 1.55% __ib_umem_release
>> >> >> +   8.31%  client2  [kernel.kallsyms]  [k] compound_unlock_irqrestore
>> >> >> +   7.01%  client2  [kernel.kallsyms]  [k] page_waitqueue
>> >> >> +   7.00%  client2  [kernel.kallsyms]  [k] set_page_dirty
>> >> >> +   6.61%  client2  [kernel.kallsyms]  [k] unlock_page
>> >> >> +   6.33%  client2  [kernel.kallsyms]  [k] put_page_testzero
>> >> >> +   5.68%  client2  [kernel.kallsyms]  [k] set_page_dirty_lock
>> >> >> +   4.30%  client2  [kernel.kallsyms]  [k] __wake_up_bit
>> >> >> +   4.04%  client2  [kernel.kallsyms]  [k] free_pages_prepare
>> >> >> +   3.65%  client2  [kernel.kallsyms]  [k] release_pages
>> >> >> +   3.62%  client2  [kernel.kallsyms]  [k] arch_local_irq_save
>> >> >> +   3.35%  client2  [kernel.kallsyms]  [k] page_mapping
>> >> >> +   3.13%  client2  [kernel.kallsyms]  [k] get_pageblock_flags_group
>> >> >> +   3.09%  client2  [kernel.kallsyms]  [k] put_page
>> >> >>
>> >> >> the reason is __ib_umem_release will loop many times for each page.
>> >> >>
>> >> >> static void __ib_umem_release(struct ib_device *dev, struct ib_umem
>> >> >> *umem, int dirty)
>> >> >> {
>> >> >>     struct scatterlist *sg;
>> >> >>     struct page *page;
>> >> >>     int i;
>> >> >>
>> >> >>     if (umem->nmap > 0)
>> >> >>          ib_dma_unmap_sg(dev, umem->sg_head.sgl,
>> >> >>                                         umem->npages,
>> >> >>                                         DMA_BIDIRECTIONAL);
>> >> >>
>> >> >>          for_each_sg(umem->sg_head.sgl, sg, umem->npages, i) {      <<
>> >> >> loop a lot of times for each page.here
>> >> >
>> >> > Why 'lot of times for each page'?  I don't know this code at all, but
>> >> > I'd expected once per page?
>> >>
>> >> sorry, once per page, but a lot of page for a big size virtual machine.
>> >
>> > Ah OK; so yes it seems best if you can find a way to do the release in
>> > the migration thread then;  still maybe this is something some
>> > of the kernel people could look at speeding up?
>>
>> The kernel code seem is not complex, and I have no idea how to speed up.
>
> Me neither; but I'll ask around.
>
>> >> >
>> >> > With your other kernel fix, does the problem of the missing
>> >> > RDMA_CM_EVENT_DISCONNECTED events go away?
>> >>
>> >> Yes, after kernel and qemu fixed, this issue never happens again.
>> >
>> > I'm confused; which qemu fix; my question was whether the kernel fix by
>> > itself fixed the problem of the missing event.
>>
>> this qemu fix:
>> migration: update index field when delete or qsort RDMALocalBlock
>
> OK good; so then we shouldn't need this 2/2 patch.
>
>> this issue also cause by some ram block is not released. but I do not
>> find the root cause.
>
> Hmm, we should try and track that down.
>
>> >
>> >> Do you think we should remove rdma_get_cm_event after rdma_disconnect?
>> >
>> > I don't think so; if 'rdma_disconnect' is supposed to generate the event
>> > I think we're supposed to wait for it to know that the disconnect is
>> > really complete.
>>
>> After move qemu_fclose to migration thread, it will not block the main
>> thread when wait
>> the disconnection event.
>
> I'm not sure about moving the fclose to the migration thread; it worries
> me with the interaction with cancel and other failures.

Bad news, I find the destination qemu also have this problem today.
when the precopy migration finished, the destination qemu main thread
process_incoming_migration_co will call qemu_fclose(f), and cause the
guest os hang for a while.

so I think it's better to solve this problem by rdma.c itself, not
change the migration.c code
when calling qemu_fclose.

we can create a separate thread in qemu_rdma_cleanup, and release the
resource in the
separate thread.

>
> Dave
>
>> >
>> > Dave
>> >
>> >>
>> >> >
>> >> > Dave
>> >> >
>> >> >>               page = sg_page(sg);
>> >> >>               if (umem->writable && dirty)
>> >> >>                   set_page_dirty_lock(page);
>> >> >>               put_page(page);
>> >> >>          }
>> >> >>
>> >> >>          sg_free_table(&umem->sg_head);
>> >> >>          return;
>> >> >> }
>> >> >>
>> >> >> >
>> >> >> > Dave
>> >> >> >
>> >> >> >
>> >> >> >> >
>> >> >> >> > Dave
>> >> >> >> >
>> >> >> >> >> >
>> >> >> >> >> >> Anyway, it should not invoke rdma_get_cm_event in main thread, and the event channel
>> >> >> >> >> >> is also destroyed in qemu_rdma_cleanup.
>> >> >> >> >> >>
>> >> >> >> >> >> Signed-off-by: Lidong Chen <lidongchen@tencent.com>
>> >> >> >> >> >> ---
>> >> >> >> >> >>  migration/rdma.c       | 12 ++----------
>> >> >> >> >> >>  migration/trace-events |  1 -
>> >> >> >> >> >>  2 files changed, 2 insertions(+), 11 deletions(-)
>> >> >> >> >> >>
>> >> >> >> >> >> diff --git a/migration/rdma.c b/migration/rdma.c
>> >> >> >> >> >> index 0dd4033..92e4d30 100644
>> >> >> >> >> >> --- a/migration/rdma.c
>> >> >> >> >> >> +++ b/migration/rdma.c
>> >> >> >> >> >> @@ -2275,8 +2275,7 @@ static int qemu_rdma_write(QEMUFile *f, RDMAContext *rdma,
>> >> >> >> >> >>
>> >> >> >> >> >>  static void qemu_rdma_cleanup(RDMAContext *rdma)
>> >> >> >> >> >>  {
>> >> >> >> >> >> -    struct rdma_cm_event *cm_event;
>> >> >> >> >> >> -    int ret, idx;
>> >> >> >> >> >> +    int idx;
>> >> >> >> >> >>
>> >> >> >> >> >>      if (rdma->cm_id && rdma->connected) {
>> >> >> >> >> >>          if ((rdma->error_state ||
>> >> >> >> >> >> @@ -2290,14 +2289,7 @@ static void qemu_rdma_cleanup(RDMAContext *rdma)
>> >> >> >> >> >>              qemu_rdma_post_send_control(rdma, NULL, &head);
>> >> >> >> >> >>          }
>> >> >> >> >> >>
>> >> >> >> >> >> -        ret = rdma_disconnect(rdma->cm_id);
>> >> >> >> >> >> -        if (!ret) {
>> >> >> >> >> >> -            trace_qemu_rdma_cleanup_waiting_for_disconnect();
>> >> >> >> >> >> -            ret = rdma_get_cm_event(rdma->channel, &cm_event);
>> >> >> >> >> >> -            if (!ret) {
>> >> >> >> >> >> -                rdma_ack_cm_event(cm_event);
>> >> >> >> >> >> -            }
>> >> >> >> >> >> -        }
>> >> >> >> >> >> +        rdma_disconnect(rdma->cm_id);
>> >> >> >> >> >
>> >> >> >> >> > I'm worried whether this change could break stuff:
>> >> >> >> >> > The docs say for rdma_disconnect that it flushes any posted work
>> >> >> >> >> > requests to the completion queue;  so unless we wait for the event
>> >> >> >> >> > do we know the stuff has been flushed?   In the normal non-cancel case
>> >> >> >> >> > I'm worried that means we could lose something.
>> >> >> >> >> > (But I don't know the rdma/infiniband specs well enough to know if it's
>> >> >> >> >> > really a problem).
>> >> >> >> >>
>> >> >> >> >> In qemu_fclose function, it invoke qemu_fflush(f) before invoke f->ops->close.
>> >> >> >> >> so I think it's safe for normal migration case.
>> >> >> >> >>
>> >> >> >> >> For the root cause why not receive RDMA_CM_EVENT_DISCONNECTED event
>> >> >> >> >> after rdma_disconnect,
>> >> >> >> >> I loop in Aviad Yehezkel<aviadye@mellanox.com>, Aviad will help us
>> >> >> >> >> find the root cause.
>> >> >> >> >>
>> >> >> >> >> >
>> >> >> >> >> > Dave
>> >> >> >> >> >
>> >> >> >> >> >>          trace_qemu_rdma_cleanup_disconnect();
>> >> >> >> >> >>          rdma->connected = false;
>> >> >> >> >> >>      }
>> >> >> >> >> >> diff --git a/migration/trace-events b/migration/trace-events
>> >> >> >> >> >> index d6be74b..64573ff 100644
>> >> >> >> >> >> --- a/migration/trace-events
>> >> >> >> >> >> +++ b/migration/trace-events
>> >> >> >> >> >> @@ -125,7 +125,6 @@ qemu_rdma_accept_pin_state(bool pin) "%d"
>> >> >> >> >> >>  qemu_rdma_accept_pin_verbsc(void *verbs) "Verbs context after listen: %p"
>> >> >> >> >> >>  qemu_rdma_block_for_wrid_miss(const char *wcompstr, int wcomp, const char *gcompstr, uint64_t req) "A Wanted wrid %s (%d) but got %s (%" PRIu64 ")"
>> >> >> >> >> >>  qemu_rdma_cleanup_disconnect(void) ""
>> >> >> >> >> >> -qemu_rdma_cleanup_waiting_for_disconnect(void) ""
>> >> >> >> >> >>  qemu_rdma_close(void) ""
>> >> >> >> >> >>  qemu_rdma_connect_pin_all_requested(void) ""
>> >> >> >> >> >>  qemu_rdma_connect_pin_all_outcome(bool pin) "%d"
>> >> >> >> >> >> --
>> >> >> >> >> >> 1.8.3.1
>> >> >> >> >> >>
>> >> >> >> >> > --
>> >> >> >> >> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>> >> >> >> > --
>> >> >> >> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>> >> >> > --
>> >> >> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>> >> > --
>> >> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>> > --
>> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK