On Tue, Sep 25, 2018 at 7:02 AM Michael Chan <michael.c...@broadcom.com> wrote: > > On Mon, Sep 24, 2018 at 2:18 PM Song Liu <songliubrav...@fb.com> wrote: > > > > > > > > > On Sep 24, 2018, at 2:05 PM, Eric Dumazet <eduma...@google.com> wrote: > > > > > >> > > >> Interesting, maybe a bnxt specific issue. > > >> > > >> It seems their model is to process TX/RX notification in the same queue, > > >> they throw away RX events if budget == 0 > > >> > > >> It means commit e7b9569102995ebc26821789628eef45bd9840d8 is wrong and > > >> must be reverted. > > >> > > >> Otherwise, we have a possibility of blocking a queue under netpoll > > >> pressure. > > > > > > Hmm, actually a revert might not be enough, since code at lines 2030-2031 > > > would fire and we might not call napi_complete_done() anyway. > > > > > > Unfortunately this driver logic is quite complex. > > > > > > Could you test on other NIC eventually ? > > > > > > > It actually runs OK on ixgbe. > > > > @Michael, could you please help us with this? > > > I've taken a quick look using today's net tree plus Eric's > poll_one_napi() patch. The problem I'm seeing is that netpoll calls > bnxt_poll() with budget 0. And since work_done >= budget of 0, we > return without calling napi_complete_done() and without arming the > interrupt. netpoll doesn't always call us back until we call > napi_complete_done(), right? So I think if there are in-flight TX > completions, we'll miss those.
That's the whole point of netpoll : We drain the TX queues, without interrupts being involved at all, by calling ->napi() with a zero budget. napi_complete(), even if called from ->napi() while budget was zero, should do nothing but return early. budget==0 means that ->napi() should process all TX completions. So it looks like bnxt has a bug, that is showing up after the latest poll_one_napi() patch. This latest patch is needed otherwise the cpu attempting the netpoll-TX-drain might drain nothing at all, since it does not anymore call ndo_poll_controller() that was grabbing SCHED bits on all queues (napi_schedule() like calls)
