Re: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network troubles "bisected"

Sander Eikelenboom
2014-03-26T16:53:59Z

Wednesday, March 26, 2014, 5:25:21 PM, you wrote:

>> -----Original Message-----
>> From: Sander Eikelenboom [mailto:linux@eikelenboom.it]
>> Sent: 26 March 2014 16:07
>> To: Paul Durrant
>> Cc: Wei Liu; annie li; Zoltan Kiss; xen-devel@lists.xen.org; Ian Campbell; linux-
>> kernel; netdev@vger.kernel.org
>> Subject: Re: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network
>> troubles "bisected"
>>
>>
>> Wednesday, March 26, 2014, 4:50:30 PM, you wrote:
>>
>> >> -----Original Message-----
>> >> From: Sander Eikelenboom [mailto:linux@eikelenboom.it]
>> >> Sent: 26 March 2014 15:23
>> >> To: Paul Durrant
>> >> Cc: Wei Liu; annie li; Zoltan Kiss; xen-devel@lists.xen.org; Ian Campbell;
>> linux-
>> >> kernel; netdev@vger.kernel.org
>> >> Subject: Re: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network
>> >> troubles "bisected"
>> >>
>> >>
>> >> Wednesday, March 26, 2014, 3:44:42 PM, you wrote:
>> >>
>> >> >> -----Original Message-----
>> >> >> From: Sander Eikelenboom [mailto:linux@eikelenboom.it]
>> >> >> Sent: 26 March 2014 11:11
>> >> >> To: Paul Durrant
>> >> >> Cc: Wei Liu; annie li; Zoltan Kiss; xen-devel@lists.xen.org; Ian Campbell;
>> >> linux-
>> >> >> kernel; netdev@vger.kernel.org
>> >> >> Subject: Re: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network
>> >> >> troubles "bisected"
>> >> >>
>> >> >> Paul,
>> >> >>
>> >> >> You have been awfully silent for this whole thread while this is a
>> >> regression
>> >> >> caused by a patch of you
>> >> >> (ca2f09f2b2c6c25047cfc545d057c4edfcfe561c as clearly stated much
>> earlier
>> >> in
>> >> >> this thread).
>> >> >>
>> >>
>> >> > Sorry, I've been distracted...
>> >>
>> >> >> The commit messages states:
>> >> >> "net_rx_action() is the place where we could do with an accurate
>> >> >> predicition but,
>> >> >> since that has proven tricky to calculate, a cheap worse-case (but not
>> >> too
>> >> >> bad)
>> >> >> estimate is all we really need since the only thing we *must* prevent
>> is
>> >> >> xenvif_gop_skb()
>> >> >> consuming more slots than are available."
>> >> >>
>> >> >> Your "worst-case" calculation stated in the commit message is clearly
>> not
>> >> the
>> >> >> worst case,
>> >> >> since it doesn't take calls to "get_next_rx_buffer" into account.
>> >> >>
>> >>
>> >> > It should be taking into account the behaviour of
>> start_new_rx_buffer(),
>> >> which should be true if a slot is full or a frag will overflow the current slot
>> and
>> >> doesn't require splitting.
>> >> > The code in net_rx_action() makes the assumption that each frag will
>> >> require as many slots as its size requires, i.e. it assumes no packing of
>> >> multiple frags into a single slot, so it should be a worst case.
>> >> > Did I miss something in that logic?
>> >>
>> >> Yes.
>> >> In "xenvif_gop_skb()" this loop:
>> >>
>> >> for (i = 0; i < nr_frags; i++) {
>> >> xenvif_gop_frag_copy(vif, skb, npo,
>> >> skb_frag_page(&skb_shinfo(skb)->frags[i]),
>> >> skb_frag_size(&skb_shinfo(skb)->frags[i]),
>> >> skb_shinfo(skb)->frags[i].page_offset,
>> >> &head);
>> >> }
>> >>
>> >> Is capable of using up (at least) 1 slot more than is anticipated for in
>> >> "net_rx_action()" by this code:
>> >>
>> >> for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
>> >> unsigned int size;
>> >> size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
>> >> max_slots_needed += DIV_ROUND_UP(size, PAGE_SIZE);
>> >> }
>> >>
>> >> And this happens when it calls "get_next_rx_buffer()" from
>> >> "xenvif_gop_frag_copy()" where it's breaking down the frag.
>> >>
>>
>> > The function that determines whether to consume another slot is
>> start_new_rx_buffer() and for each frag I don't see why this would return
>> true more than DIV_ROUND_UP(size, PAGE_SIZE) times.
>> > It may be called more times than that since the code in
>> xenvif_gop_frag_copy() must also allow for the offset of the frag but should
>> not return true in all cases.
>> > So, I still cannot see why a frag would ever consume more than
>> DIV_ROUND_UP(size, PAGE_SIZE) slots.
>>
>> Well here a case were a frag is broken down in 2 pieces:
>>
>> [ 1156.870372] vif vif-7-0 vif7.0: ?!? xenvif_gop_frag_copy Me here 1 npo-
>> >meta_prod:39 vif->rx.sring->req_prod:2105867 vif->rx.req_cons:2105867
>> npo->copy_gref:760 npo->copy_off:4096 MAX_BUFFER_OFFSET:4096
>> bytes:560 size:560 offset:0 head:1273462060 i:2 vif->rx.sring-
>> >req_event:2104275 estimated_slots_needed:4 reserved_slots_left:0
>> [ 1156.871971] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue !
>> min_slots_needed:1 min_slots_needed_2:0 min_slots_needed_3:0 vif-
>> >rx.sring->req_prod:2105867 vif->rx.req_cons:2105867 vif->rx.sring-
>> >req_event:2105868 skb->len:66 skb->data_len:0
>> [ 1156.964316] vif vif-7-0 vif7.0: ?!? get_next_rx_buffer before req npo-
>> >meta_prod:39 vif->rx.sring->req_prod:2105867 vif->rx.req_cons:2105867
>> vif->rx.sring->req_event:2105868, reserved_slots_left:0
>> [ 1157.001635] vif vif-7-0 vif7.0: ?!? get_next_rx_buffer after req npo-
>> >meta_prod:39 vif->rx.sring->req_prod:2105867 vif->rx.req_cons:2105868
>> req->gref:4325379 req->id:11 vif->rx.sring->req_event:2105868
>> reserved_slots_left:-1
>> [ 1157.039095] vif vif-7-0 vif7.0: ?!? xenvif_gop_frag_copy Me here 2 npo-
>> >meta_prod:40 vif->rx.sring->req_prod:2105867 vif->rx.req_cons:2105868
>> npo->copy_gref:4325379 npo->copy_off:0 MAX_BUFFER_OFFSET:4096
>> bytes:560 size:560 offset:0 head:1273462060 i:2 vif->rx.sring-
>> >req_event:2105868 estimated_slots_needed:4 reserved_slots_left:-1
>> [ 1157.095216] vif vif-7-0 vif7.0: ?!? xenvif_gop_frag_copy Me here end npo-
>> >meta_prod:40 vif->rx.sring->req_prod:2105867 vif->rx.req_cons:2105868
>> npo->copy_gref:4325379 npo->copy_off:560 MAX_BUFFER_OFFSET:4096
>> bytes:560 size:0 offset:560 head:1273462060 i:3 vif->rx.sring-
>> >req_event:2105868 gso_gaps:0 estimated_slots_needed:4
>> reserved_slots_left:-1
>> [ 1157.151338] vif vif-7-0 vif7.0: ?!? xenvif_gop_skb Me here 4 after npo-
>> >meta_prod:40 old_meta_prod:36 vif->rx.sring->req_prod:2105867 vif-
>> >rx.req_cons:2105868 meta->gso_type:1 meta->gso_size:1448 nr_frags:1
>> req->gref:657 req->id:7 estimated_slots_needed:4 i(frag):0 j(data):1
>> reserved_slots_left:-1
>> [ 1157.188908] vif vif-7-0 vif7.0: ?!? xenvif_gop_skb Me here 5 npo-
>> >meta_prod:40 old_meta_prod:36 vif->rx.sring->req_prod:2105867 vif-
>> >rx.req_cons:2105868 meta->gso_type:1 meta->gso_size:1448 nr_frags:1
>> req->gref:657 req->id:7 estimated_slots_needed:4 j(data):1
>> reserved_slots_left:-1 used in funcstart: 0 + 1 .. used_dataloop:1 ..
>> used_fragloop:3
>> [ 1157.244975] vif vif-7-0 vif7.0: ?!? xenvif_rx_action me here 2 .. vif-
>> >rx.sring->req_prod:2105867 vif->rx.req_cons:2105868 sco-
>> >meta_slots_used:4 max_upped_gso:1 skb_is_gso(skb):1
>> max_slots_needed:4 j:6 is_gso:1 nr_frags:1 firstpart:1 secondpart:2
>> reserved_slots_left:-1
>>
>> - When processing an SKB we end up in "xenvif_gop_frag_copy" while prod
>> == cons ... but we still have bytes and size left ..
>> - start_new_rx_buffer() has returned true ..
>> - so we end up in get_next_rx_buffer
>> - this does a RING_GET_REQUEST and ups cons ..
>> - and we end up with a bad grant reference.
>>
>> Sometimes we are saved by the bell .. since additional slots have become
>> free (you see cons become > prod in "get_next_rx_buffer" but shortly after
>> that prod is increased ..
>> just in time to not cause a overrun).
>>

> Ah, but hang on... There's a BUG_ON meta_slots_used > max_slots_needed, so if we are overflowing the worst-case calculation then why is that BUG_ON not firing?

You mean:
sco = (struct skb_cb_overlay *)skb->cb;
sco->meta_slots_used = xenvif_gop_skb(skb, &npo);
BUG_ON(sco->meta_slots_used > max_slots_needed);

in "get_next_rx_buffer" ?

I don't know .. at least now it doesn't crash dom0 and therefore not my complete machine and since tcp is recovering from a failed packet :-)

But probably because "npo->copy_prod++" seems to be used for the frags .. and it isn't added to npo->meta_prod ?

--
Sander

> Paul

>> If you need additional / other info, please cook up a debug patch with what
>> you need.
>>
>> --
>> Sander
>>
>>
>>
>>
>>
>>
>> > Paul
>>
>> >> Ultimately this results in bad grant reference warnings (and packets
>> marked
>> >> as "errors" in the interface statistics).
>> >>
>> >> In my case it always seems to be a skb with 1 frag which is broken down in
>> 5
>> >> or 6 pieces ..
>> >>
>> >> So "get_next_rx_buffer()" is called once .. and i'm overrunning the ring
>> with
>> >> 1 slot, but i'm not sure if that's not coincedence
>> >> since in the code there seem to be no explicit limitation on how often this
>> >> code path is taken. So perhaps it's implicitly limited
>> >> since packets and frags can't be arbitrarily large in comparison with the
>> >> page_size but that's not something i'm capable of figuring out :-)
>> >>
>> >>
>> >>
>> >> > Paul
>> >>
>> >> >> Problem is that a worst case calculation would probably be reverting to
>> >> the
>> >> >> old calculation,
>> >> >> and the problems this patch was trying to solve would reappear, but
>> >> >> introducing new regressions
>> >> >> isn't very useful either. And since it seems such a tricky and fragile
>> thing to
>> >> >> determine, it would
>> >> >> probably be best to be split into a distinct function with a comment to
>> >> explain
>> >> >> the rationale used.
>> >> >>
>> >> >> Since this doesn't seem to progress very fast .. CC'ed some more folks
>> ..
>> >> you
>> >> >> never know ..
>> >> >>
>> >> >> --
>> >> >> Sander
>> >> >>
>> >> >>
>> >> >> Tuesday, March 25, 2014, 4:29:42 PM, you wrote:
>> >> >>
>> >> >>
>> >> >> > Tuesday, March 25, 2014, 4:15:39 PM, you wrote:
>> >> >>
>> >> >> >> On Sat, Mar 22, 2014 at 07:28:34PM +0100, Sander Eikelenboom
>> wrote:
>> >> >> >> [...]
>> >> >> >>> > Yes there is only one frag .. but it seems to be much larger than
>> >> >> PAGE_SIZE .. and xenvif_gop_frag_copy brakes that frag down into
>> >> smaller
>> >> >> bits .. hence the calculation in xenvif_rx_action determining the slots
>> >> needed
>> >> >> by doing:
>> >> >> >>>
>> >> >> >>> > for (i = 0; i < nr_frags; i++) {
>> >> >> >>> > unsigned int size;
>> >> >> >>> > size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
>> >> >> >>> > max_slots_needed += DIV_ROUND_UP(size,
>> >> PAGE_SIZE);
>> >> >> >>> > }
>> >> >> >>>
>> >> >> >>> > But the code in xenvif_gop_frag_copy .. seems to be needing
>> one
>> >> >> more slot (from the emperical test) .. and calling "get_next_rx_buffer"
>> >> >> seems involved in that ..
>> >> >> >>>
>> >> >> >>> Hmm looked again .. and it seems this is it .. when your frags are
>> large
>> >> >> enough you have the chance of running into this.
>> >> >> >>>
>> >> >>
>> >> >> >> get_next_rx_buffer is guarded by start_new_rx_buffer. Do you
>> see
>> >> any
>> >> >> >> problem with that implementation?
>> >> >> > In general no, but "get_next_rx_buffer" up's cons .. and the
>> calculations
>> >> >> done in "xenvif_rx_action" for max_slots_needed to prevent the
>> overrun
>> >> >> > don't count in this possibility. So it's not the guarding of
>> >> >> "start_new_rx_buffer" that is at fault. It's the ones early in
>> >> >> "xenvif_rx_action".
>> >> >> > The ones that were changed by Paul's patch from MAX_SKB_FRAGS
>> to a
>> >> >> calculated value that should be a "slim fit".
>> >> >>
>> >> >> > The problem is in determining upfront in "xenvif_rx_action" when
>> and
>> >> how
>> >> >> often the "get_next_rx_buffer" path will be taken.
>> >> >> > Unless there are other non direct restrictions (from a size point of
>> view)
>> >> it
>> >> >> can be called multiple times per frag per skb.
>> >> >>
>> >> >> >>> Problem is .. i don't see an easy fix, the "one more slot" of the
>> >> empirical
>> >> >> test doesn't seem to be the worst case either (i think):
>> >> >> >>>
>> >> >> >>> - In my case the packets that hit this only have 1 frag, but i could
>> have
>> >> >> had more frags.
>> >> >> >>> I also think you can't rule out the possibility of doing the
>> >> >> "get_next_rx_buffer" for multiple subsequent frags from one packet,
>> >> >> >>> so in the worst (and perhaps even from a single frag since it's
>> looped
>> >> >> over a split of it in what seems PAGE_SIZE pieces.)
>> >> >> >>> - So an exact calculation of how much slots we are going to need
>> for
>> >> >> hitting this "get_next_rx_buffer" upfront in "xenvif_rx_action" seems
>> >> >> unfeasible.
>> >> >> >>> - A worst case gamble seems impossible either .. if you take
>> multiple
>> >> >> frags * multiple times the "get_next_rx_buffer" ... you would probably
>> be
>> >> >> back at just
>> >> >> >>> setting the needed_slots to MAX_SKB_FRAGS.
>> >> >> >>>
>> >> >> >>> - Other thing would be checking for the available slots before
>> doing
>> >> the
>> >> >> "get_next_rx_buffer" .. how ever .. we don't account for how many
>> slots
>> >> we
>> >> >> still need to
>> >> >> >>> just process the remaining frags.
>> >> >> >>>
>> >> >>
>> >> >> >> We've done a worst case estimation for whole SKB (linear area + all
>> >> >> >> frags) already, at the very first beginning. That's what
>> >> >> >> max_slots_needed is for.
>> >> >>
>> >> >> >>> - Just remove the whole "get_next_rx_buffer".. just tested it but
>> it
>> >> >> seems the "get_next_rx_buffer" is necessary .. when i make
>> >> >> start_new_rx_buffer always return false
>> >> >> >>> i hit the bug below :S
>> >> >> >>>
>> >> >> >>> So the questions are ... is the "get_next_rx_buffer" part really
>> >> necessary
>> >> >> ?
>> >> >> >>> - if not, what is the benefit of the "get_next_rx_buffer" and
>> does
>> >> that
>> >> >> outweigh the additional cost of accounting
>> >> >> >>> of needed_slots for the frags that have yet to be processed ?
>> >> >> >>> - if yes, erhmmm what would be the best acceptable solution ..
>> >> >> returning to using MAX_SKB_FRAGS ?
>> >> >> >>>
>> >> >>
>> >> >> >> I think you need to answer several questions first:
>> >> >> >> 1. is max_slots_needed actually large enough to cover whole SKB?
>> >> >> > No it's not if, you end up calling "get_next_rx_buffer" one or
>> >> multiple
>> >> >> times when processing the SKB
>> >> >> > you have the chance of overrunning (or be saved because prod
>> gets
>> >> >> upped before you overrun).
>> >> >>
>> >> >> >> 2. is the test of ring slot availability acurate?
>> >> >> > Seems to be.
>> >> >>
>> >> >> >> 3. is the number of ring slots consumed larger than
>> >> max_slots_needed? (I
>> >> >> >> guess the answer is yes)
>> >> >> > Yes that was the whole point.
>> >> >>
>> >> >> >> 4. which step in the break-down procedure causes backend to
>> overrun
>> >> >> the
>> >> >> >> ring?
>> >> >> > The "get_next_rx_buffer" call .. that is not accounted for (which
>> can
>> >> be
>> >> >> called
>> >> >> > multiple times per frag (unless some other none obvious size
>> >> >> restriction limits this
>> >> >> > to one time per frag or one time per SKB).
>> >> >> > In my errorneous case it is called one time, but it would be nice if
>> >> there
>> >> >> would be some theoretical proof
>> >> >> > instead of just some emperical test.
>> >> >>
>> >> >>
>> >> >> >> It doesn't matter how many frags your SKB has and how big a frag
>> is. If
>> >> >> >> every step is correct then you're fine. The code you're looking at
>> >> >> >> (start_new_rx_buffer / get_next_rx_buffer and friend) needs to
>> be
>> >> >> >> carefully examined.
>> >> >>
>> >> >> > Well it seems it only calls "get_next_rx_buffer" in some special cases
>> ..
>> >> >> (and that also what i'm seeing because it doesn't happen
>> >> >> > continously.
>> >> >>
>> >> >> > Question is shouldn't this max_needed_slots calc be reverted to
>> what it
>> >> >> was before 3.14 and take the time in 3.15 to figure out a
>> >> >> > the theoretical max (if it can be calculated upfront) .. or another way
>> to
>> >> >> guarantee the code is able to process the whole SKB ?
>> >> >>
>> >> >> >> Wei.
>> >> >>
>> >> >>
>> >> >>
>> >> >>
>> >>
>> >>
>>
>>