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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Xen-unstable Linux 3.14-rc3 and 3.13 Network troubles "bisected"
Wednesday, March 26, 2014, 5:25:21 PM, you wrote: >> -----Original Message----- >> From: Sander Eikelenboom [mailto:linux@xxxxxxxxxxxxxx] >> Sent: 26 March 2014 16:07 >> To: Paul Durrant >> Cc: Wei Liu; annie li; Zoltan Kiss; xen-devel@xxxxxxxxxxxxx; Ian Campbell; >> linux- >> kernel; netdev@xxxxxxxxxxxxxxx >> 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@xxxxxxxxxxxxxx] >> >> Sent: 26 March 2014 15:23 >> >> To: Paul Durrant >> >> Cc: Wei Liu; annie li; Zoltan Kiss; xen-devel@xxxxxxxxxxxxx; Ian Campbell; >> linux- >> >> kernel; netdev@xxxxxxxxxxxxxxx >> >> 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@xxxxxxxxxxxxxx] >> >> >> Sent: 26 March 2014 11:11 >> >> >> To: Paul Durrant >> >> >> Cc: Wei Liu; annie li; Zoltan Kiss; xen-devel@xxxxxxxxxxxxx; Ian >> >> >> Campbell; >> >> linux- >> >> >> kernel; netdev@xxxxxxxxxxxxxxx >> >> >> 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. >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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