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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"
> -----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? 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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