[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"
Ping ? Saturday, March 22, 2014, 7:28:34 PM, you wrote: > Friday, March 21, 2014, 6:27:27 PM, you wrote: >> Friday, March 21, 2014, 5:49:58 PM, you wrote: >>> [...] >>>> > Those are different estimations. max_slots_needed estimates the worst >>>> > case while min_slots_needed estimates the best case. When >>>> > min_slots_needed is met, netback puts the SKB into its internal queue. >>>> > xenvif_rx_action then will dequeue that packet, check max_slots_needed, >>>> > if met, break SKB down. >>>> >>>> I realised that, though if you don't count the cost of calculation .. the >>>> best would be >>>> to have min_slots and max_slots the same .. and being correct. But cost of >>>> the calculation count .. >>>> on the other hand min_slots seems to be far too low in this case. Is the >>>> nr_frags the best thing to check for ? >>>> Wouldn't something like skbsize / PAGE_SIZE be a better estimate ? >>>> >>> It doesn't matter. It won't cause breakage at that point. >> That was more from a performance point of view ... if the estimate can be >> that much off >> it would do the more costly check .. and still bail out. So the better the >> first estimation >> (at about the same cost of course) the better. >>>> >>>> > What I would expect is, even if they yield different values, checking >>>> > the ring availability is enough before breaking SKB down. >>>> >>>> > In your above case, max_slots_needed was met and SKB broken down. And as >>>> > you said in your empirical test, bumping max_slots_needed by 1 prevented >>>> > issues, then we might have problem calculating max_slots_needed. If you >>>> > can work out what went wrong that can be very useful. >>>> >>>> OK ran the whole night with "max_slots_needed+1" and i saw no triggering >>>> of both: >>>> - the non-fatal oddity (only a temporary cons > prod ..) >>>> - the fatal case (leading to the grantref error) >>>> >>>> So it seems we are off by one .. somewhere >>>> >>>> >>>> At first the non-fatal oddity .. this one i really can't grasp: >>>> >>>> [ 567.880282] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:4 vif->rx.sring->req_prod:562972 vif->rx.req_cons:562969 >>>> vif->rx.sring->req_event:562973 >>>> [ 569.893801] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:4 vif->rx.sring->req_prod:571188 vif->rx.req_cons:571187 >>>> vif->rx.sring->req_event:571189 >>>> [ 571.892419] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:4 vif->rx.sring->req_prod:577141 vif->rx.req_cons:577138 >>>> vif->rx.sring->req_event:577142 >>>> [ 575.451383] vif vif-7-0 vif7.0: ?!? get_next_rx_buffer before req >>>> npo->meta_prod:6 vif->rx.sring->req_prod:590603 vif->rx.req_cons:590603 >>>> vif->rx.sring->req_event:589615, reserved_slots_left:0 >>>> [ 575.487721] vif vif-7-0 vif7.0: ?!? get_next_rx_buffer after req >>>> npo->meta_prod:6 vif->rx.sring->req_prod:590660 vif->rx.req_cons:590604 >>>> req->gref:569 req->id:11 vif->rx.sring->req_event:589615 >>>> reserved_slots_left:-1 >>>> [ 575.524427] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:4 vif->rx.sring->req_prod:590660 vif->rx.req_cons:590660 >>>> vif->rx.sring->req_event:590661 >>>> [ 576.508063] vif vif-7-0 vif7.0: ?!? get_next_rx_buffer before req >>>> npo->meta_prod:16 vif->rx.sring->req_prod:594343 vif->rx.req_cons:594343 >>>> vif->rx.sring->req_event:591619, reserved_slots_left:0 >>>> [ 576.544708] vif vif-7-0 vif7.0: ?!? get_next_rx_buffer after req >>>> npo->meta_prod:16 vif->rx.sring->req_prod:594389 vif->rx.req_cons:594344 >>>> req->gref:668 req->id:167 vif->rx.sring->req_event:591619 >>>> reserved_slots_left:-1 >>>> [ 576.581595] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:4 vif->rx.sring->req_prod:594389 vif->rx.req_cons:594389 >>>> vif->rx.sring->req_event:594390 >>>> [ 577.325826] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:2 vif->rx.sring->req_prod:597248 vif->rx.req_cons:597247 >>>> vif->rx.sring->req_event:597249 >>>> [ 577.576973] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:4 vif->rx.sring->req_prod:598146 vif->rx.req_cons:598143 >>>> vif->rx.sring->req_event:598147 >>>> >>>> What i don't get is: >>>> - i do warn just before the RING_GET_REQUEST on >>>> vif->rx.sring->req_prod:590603 vif->rx.req_cons:590603 .. so cons == prod >>>> - i warn because req = RING_GET_REQUEST(&vif->rx, vif->rx.req_cons++); is >>>> going to add one to cons .. leading to a situation that should not exist .. >>>> - however in the warn just after this RING_GET_REQUEST .. prod is somehow >>>> increased a lot .. probably the "next_rx_buffer" aspect of things .. so >>>> cons is much smaller than prod again .. >>> Frontend is also accessing the ring. So it's normal that some index >>> changed. >> Ah ok .. so here the frontend is "saving us" by putting more stuff on the >> ring so we don't run dry .. >>>> - How everi don't see anything in the RING_GET_REQUEST macro fiddling with >>>> vif->rx.sring->req_prod .. >>>> - so who has been fiddling with vif->rx.sring->req_prod in the mean time >>>> .. my very limited c skills fail to see how this works :-) >>>> >>>> >>>> Then the fatal case ... >>>> >>>> [ 1108.566486] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:1 vif->rx.sring->req_prod:1843278 >>>> vif->rx.req_cons:1843278 vif->rx.sring->req_event:1843279 >>>> [ 1109.775757] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:4 vif->rx.sring->req_prod:1847779 >>>> vif->rx.req_cons:1847776 vif->rx.sring->req_event:1847780 >>>> [ 1110.041661] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:4 vif->rx.sring->req_prod:1848795 >>>> vif->rx.req_cons:1848794 vif->rx.sring->req_event:1848796 >>>> [ 1110.301778] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:4 vif->rx.sring->req_prod:1849755 >>>> vif->rx.req_cons:1849754 vif->rx.sring->req_event:1849756 >>>> [ 1111.565412] vif vif-7-0 vif7.0: ?!? get_next_rx_buffer before req >>>> npo->meta_prod:35 vif->rx.sring->req_prod:1854291 vif->rx.req_cons:1854291 >>>> vif->rx.sring->req_event:1853482, reserved_slots_left:0 >>>> [ 1111.565973] vif vif-7-0 vif7.0: ?!? xenvif_start_xmit stopping queue ! >>>> min_slots_needed:1 vif->rx.sring->req_prod:1854291 >>>> vif->rx.req_cons:1854291 vif->rx.sring->req_event:1854292 >>>> [ 1111.638425] vif vif-7-0 vif7.0: ?!? get_next_rx_buffer after req >>>> npo->meta_prod:35 vif->rx.sring->req_prod:1854291 vif->rx.req_cons:1854292 >>>> req->gref:4325377 req->id:83 vif->rx.sring->req_event:1854292 >>>> reserved_slots_left:-1 >>>> [ 1111.675042] vif vif-7-0 vif7.0: ?!? xenvif_gop_frag_copy Me here 2 >>>> npo->meta_prod:36 vif->rx.sring->req_prod:1854291 vif->rx.req_cons:1854292 >>>> npo->copy_gref:4325377 npo->copy_off:0 MAX_BUFFER_OFFSET:4096 bytes:1168 >>>> size:1168 i:5 vif->rx.sring->req_event:1854292 estimated_slots_needed:7 >>>> reserved_slots_left:-1 >>>> [ 1111.711833] vif vif-7-0 vif7.0: ?!? xenvif_gop_frag_copy Me here end >>>> npo->meta_prod:36 vif->rx.sring->req_prod:1854291 vif->rx.req_cons:1854292 >>>> npo->copy_gref:4325377 npo->copy_off:1168 MAX_BUFFER_OFFSET:4096 >>>> bytes:1168 size:0 i:6 vif->rx.sring->req_event:1854292 gso_gaps:0 >>>> estimated_slots_needed:7 reserved_slots_left:-1 >>>> [ 1111.766836] vif vif-7-0 vif7.0: ?!? xenvif_gop_skb Me here 4 after >>>> npo->meta_prod:36 old_meta_prod:29 vif->rx.sring->req_prod:1854291 >>>> vif->rx.req_cons:1854292 meta->gso_type:1 meta->gso_size:1448 nr_frags:1 >>>> req->gref:674 req->id:76 estimated_slots_needed:7 i(frag):0 j(data):1 >>>> reserved_slots_left:-1 >>>> [ 1111.803702] vif vif-7-0 vif7.0: ?!? xenvif_gop_skb Me here 5 >>>> npo->meta_prod:36 old_meta_prod:29 vif->rx.sring->req_prod:1854291 >>>> vif->rx.req_cons:1854292 meta->gso_type:1 meta->gso_size:1448 nr_frags:1 >>>> req->gref:674 req->id:76 estimated_slots_needed:7 j(data):1 >>>> reserved_slots_left:-1 used in funcstart: 0 + 1 .. used_dataloop:1 .. >>>> used_fragloop:6 >>>> [ 1111.858602] vif vif-7-0 vif7.0: ?!? xenvif_rx_action me here 2 .. >>>> vif->rx.sring->req_prod:1854291 vif->rx.req_cons:1854292 >>>> sco->meta_slots_used:7 max_upped_gso:1 skb_is_gso(skb):1 >>>> max_slots_needed:7 j:3 is_gso:1 nr_frags:1 firstpart:1 secondpart:5 >>>> reserved_slots_left:-1 >>>> >>>> Ok so if i try to pull this apart (from bottom to top): >>>> >>>> - it starts out with "xenvif_rx_action" and calculated we would need: >>>> 1 slot for the DIV_ROUND_UP(offset_in_page(skb->data) + >>>> skb_headlen(skb), PAGE_SIZE); >>>> 5 slots for 1 frag >>>> 1 slot for GSO >>>> So 7 slots in total .. >>>> >>>> - it checks if there are enough slots available if >>>> (!xenvif_rx_ring_slots_available(vif, max_slots_needed)) and aparently >>>> there are .. so it continues >>>> >>>> - We end up in "xenvif_gop_skb" processing .. and everything seems to go >>>> well .. >>>> - In the start of this function we do one unconditional >>>> RING_GET_REQUEST .. and potentially one conditional (if gso) >>>> - Question: in "xenvif_rx_action" we only reserved ONE slot for >>>> GSO stuff .. so this could cost TWO slots .. is that right ? >>> The slot reserved for GSO is for meta data, so 1 is enough. >>>> - But in this case we only use one ... see "used in funcstart: 0 >>>> + 1" >>>> >>>> - Now we loop 1 time through the "while (data < >>>> skb_tail_pointer(skb))" loop ... since j(data)=1 >>>> - and call "xenvif_gop_frag_copy" from inside this loop here .. >>>> which potentially calls .. get_next_rx_buffer >>>> - If i understand it correctly the number of slots this loop >>>> should take at a maximum should be what we calculated before with the >>>> DIV_ROUND_UP(offset_in_page(skb->data) + skb_headlen(skb), >>>> PAGE_SIZE) ? (1 slot in this case) >>>> - Nothing is logged yet because cons is still smaller than prod >>>> at this time, but from the aftermath we see this was "used_dataloop:1" >>>> - So that seems correct .. >>>> >>>> - Now we are at the loop through "nr_frags" and loop the 1 frag we >>>> have .. >>>> - This all goes well until the sixth iteration (i from >>>> 0..5) .. and call "xenvif_gop_frag_copy" from inside this loop ... >>> Isn't there only one frag? >> 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. > 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. > - 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 ? > -- > Sander > [ 235.655964] BUG: unable to handle kernel paging request at ffffc9001223b018 > [ 235.660380] IP: [<ffffffff8177d30e>] xenvif_gop_frag_copy+0x13e/0x580 > [ 235.664887] PGD 59a2a067 PUD 59a2b067 PMD 55c38067 PTE 0 > [ 235.669236] Oops: 0002 [#1] SMP > [ 235.673611] Modules linked in: > [ 235.677936] CPU: 0 PID: 10518 Comm: vif9.0 Not tainted > 3.13.6-20140322-nbdebug30+ #1 > [ 235.682377] Hardware name: MSI MS-7640/890FXA-GD70 (MS-7640) , BIOS > V1.8B1 09/13/2010 > [ 235.686711] task: ffff88004dc40000 ti: ffff88004b74a000 task.ti: > ffff88004b74a000 > [ 235.691185] RIP: e030:[<ffffffff8177d30e>] [<ffffffff8177d30e>] > xenvif_gop_frag_copy+0x13e/0x580 > [ 235.695869] RSP: e02b:ffff88004b74bb28 EFLAGS: 00010203 > [ 235.700613] RAX: ffff880080000000 RBX: ffff88004b74bd40 RCX: > 000000000126d940 > [ 235.705522] RDX: ffffc9001223aff8 RSI: ffff880000000000 RDI: > 0000000000049b65 > [ 235.710488] RBP: ffff88004b74bc08 R08: 000000000000012c R09: > 00000000000000ae > [ 235.715515] R10: ffff88004b74bd2c R11: 0000000000000001 R12: > 0000000000000000 > [ 235.720570] R13: 000000000000099e R14: 0000000000000662 R15: > ffff88004a6d0940 > [ 235.725688] FS: 00007f4cbed78700(0000) GS:ffff88005f600000(0000) > knlGS:0000000000000000 > [ 235.730961] CS: e033 DS: 0000 ES: 0000 CR0: 000000008005003b > [ 235.736061] CR2: ffffc9001223b018 CR3: 00000000572b1000 CR4: > 0000000000000660 > [ 235.741247] Stack: > [ 235.746320] ffff880000000000 ffffffff810fa475 ffff88004dc40000 > ffff88004dc40000 > [ 235.751848] ffffffff81b4ca4d 00000000000002dc 0000000000000000 > ffff88004b74be18 > [ 235.757195] ffff88004b74bb88 ffffffff810f6251 ffffffff822ee9a0 > 00000000000002dc > [ 235.762802] Call Trace: > [ 235.768086] [<ffffffff810fa475>] ? lock_acquire+0xe5/0x150 > [ 235.773386] [<ffffffff81b4ca4d>] ? _raw_spin_unlock_irqrestore+0x6d/0x90 > [ 235.778783] [<ffffffff810f6251>] ? trace_hardirqs_on_caller+0x101/0x240 > [ 235.784252] [<ffffffff810f639d>] ? trace_hardirqs_on+0xd/0x10 > [ 235.789726] [<ffffffff8177f32e>] xenvif_rx_action+0x86e/0x1670 > [ 235.795167] [<ffffffff810f7fc8>] ? __lock_acquire+0x418/0x2220 > [ 235.800560] [<ffffffff810df5f6>] ? finish_task_switch+0x46/0xf0 > [ 235.805782] [<ffffffff817810e0>] xenvif_kthread+0x40/0x190 > [ 235.811064] [<ffffffff810f05e0>] ? __init_waitqueue_head+0x60/0x60 > [ 235.816442] [<ffffffff817810a0>] ? xenvif_stop_queue+0x60/0x60 > [ 235.821888] [<ffffffff810d4f24>] kthread+0xe4/0x100 > [ 235.827688] [<ffffffff81b4cc10>] ? _raw_spin_unlock_irq+0x30/0x50 > [ 235.833925] [<ffffffff810d4e40>] ? __init_kthread_worker+0x70/0x70 > [ 235.839890] [<ffffffff81b4de3c>] ret_from_fork+0x7c/0xb0 > [ 235.845920] [<ffffffff810d4e40>] ? __init_kthread_worker+0x70/0x70 > [ 235.852174] Code: 89 c2 83 c0 01 48 8d 0c 92 48 8b 53 10 89 03 48 b8 00 00 > 00 80 00 88 ff ff 48 8d 14 ca 48 b9 00 00 00 00 00 16 00 00 48 03 4d c0 <66> > 44 89 62 20 66 c7 42 08 f0 7f 66 c7 42 22 02 00 48 c1 f9 06 > [ 235.865664] RIP [<ffffffff8177d30e>] xenvif_gop_frag_copy+0x13e/0x580 > [ 235.872106] RSP <ffff88004b74bb28> > [ 235.878358] CR2: ffffc9001223b018 > [ 235.885099] ---[ end trace 5971b65cc816997b ]--- >>>> - "xenvif_gop_frag_copy" calls "get_next_rx_buffer" .. >>>> cons becomes larger than prod ..and the "miracle" from the non-fatal case >>>> of prod suddenly becoming larger does not happen >>>> >>>> - So the calculation of how many slots are required for the frags in >>>> "xenvif_rx_action" does not match wath the code in "xenvif_gop_frag_copy" >>>> seems to need, >>>> I don't see why .. it's rounding up .. should behave correct if >>>> the left over happens to be 0 size .. >>>> >>>> >>>> >>>> > Wei. >>>> _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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