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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, 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). 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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