On Fri, Jun 21, 2019 at 3:18 PM Neil Horman <nhor...@tuxdriver.com> wrote: > > On Fri, Jun 21, 2019 at 02:31:17PM -0400, Willem de Bruijn wrote: > > On Fri, Jun 21, 2019 at 12:42 PM Neil Horman <nhor...@tuxdriver.com> wrote: > > > > > > On Thu, Jun 20, 2019 at 11:16:13AM -0400, Willem de Bruijn wrote: > > > > On Thu, Jun 20, 2019 at 10:24 AM Neil Horman <nhor...@tuxdriver.com> > > > > wrote: > > > > > > > > > > On Thu, Jun 20, 2019 at 09:41:30AM -0400, Willem de Bruijn wrote: > > > > > > On Wed, Jun 19, 2019 at 4:26 PM Neil Horman <nhor...@tuxdriver.com> > > > > > > wrote: > > > > > > > > > > > > > > When an application is run that: > > > > > > > a) Sets its scheduler to be SCHED_FIFO > > > > > > > and > > > > > > > b) Opens a memory mapped AF_PACKET socket, and sends frames with > > > > > > > the > > > > > > > MSG_DONTWAIT flag cleared, its possible for the application to > > > > > > > hang > > > > > > > forever in the kernel. This occurs because when waiting, the > > > > > > > code in > > > > > > > tpacket_snd calls schedule, which under normal circumstances > > > > > > > allows > > > > > > > other tasks to run, including ksoftirqd, which in some cases is > > > > > > > responsible for freeing the transmitted skb (which in AF_PACKET > > > > > > > calls a > > > > > > > destructor that flips the status bit of the transmitted frame > > > > > > > back to > > > > > > > available, allowing the transmitting task to complete). > > > > > > > > > > > > > > However, when the calling application is SCHED_FIFO, its priority > > > > > > > is > > > > > > > such that the schedule call immediately places the task back on > > > > > > > the cpu, > > > > > > > preventing ksoftirqd from freeing the skb, which in turn prevents > > > > > > > the > > > > > > > transmitting task from detecting that the transmission is > > > > > > > complete. > > > > > > > > > > > > > > We can fix this by converting the schedule call to a completion > > > > > > > mechanism. By using a completion queue, we force the calling > > > > > > > task, when > > > > > > > it detects there are no more frames to send, to schedule itself > > > > > > > off the > > > > > > > cpu until such time as the last transmitted skb is freed, allowing > > > > > > > forward progress to be made. > > > > > > > > > > > > > > Tested by myself and the reporter, with good results > > > > > > > > > > > > > > Appies to the net tree > > > > > > > > > > > > > > Signed-off-by: Neil Horman <nhor...@tuxdriver.com> > > > > > > > Reported-by: Matteo Croce <mcr...@redhat.com> > > > > > > > CC: "David S. Miller" <da...@davemloft.net> > > > > > > > --- > > > > > > > > > > > > This is a complex change for a narrow configuration. Isn't a > > > > > > SCHED_FIFO process preempting ksoftirqd a potential problem for > > > > > > other > > > > > > networking workloads as well? And the right configuration to always > > > > > > increase ksoftirqd priority when increasing another process's > > > > > > priority? Also, even when ksoftirqd kicks in, isn't some progress > > > > > > still made on the local_bh_enable reached from schedule()? > > > > > > > > > > > > > > > > A few questions here to answer: > > > > > > > > Thanks for the detailed explanation. > > > > > > > > > Regarding other protocols having this problem, thats not the case, > > > > > because non > > > > > packet sockets honor the SK_SNDTIMEO option here (i.e. they sleep for > > > > > a period > > > > > of time specified by the SNDTIMEO option if MSG_DONTWAIT isn't set. > > > > > We could > > > > > certainly do that, but the current implementation doesn't (opting > > > > > instead to > > > > > wait indefinately until the respective packet(s) have transmitted or > > > > > errored > > > > > out), and I wanted to maintain that behavior. If there is consensus > > > > > that packet > > > > > sockets should honor SNDTIMEO, then I can certainly do that. > > > > > > > > > > As for progress made by calling local_bh_enable, My read of the code > > > > > doesn't > > > > > have the scheduler calling local_bh_enable at all. Instead schedule > > > > > uses > > > > > preempt_disable/preempt_enable_no_resched() to gain exlcusive access > > > > > to the cpu, > > > > > which ignores pending softirqs on re-enablement. > > > > > > > > Ah, I'm mistaken there, then. > > > > > > > > > Perhaps that needs to change, > > > > > but I'm averse to making scheduler changes for this (the > > > > > aforementioned concern > > > > > about complex changes for a narrow use case) > > > > > > > > > > Regarding raising the priority of ksoftirqd, that could be a > > > > > solution, but the > > > > > priority would need to be raised to a high priority SCHED_FIFO > > > > > parameter, and > > > > > that gets back to making complex changes for a narrow problem domain > > > > > > > > > > As for the comlexity of the of the solution, I think this is, given > > > > > your > > > > > comments the least complex and intrusive change to solve the given > > > > > problem. > > > > > > > > Could it be simpler to ensure do_softirq() gets run here? That would > > > > allow progress for this case. > > > > > > > > > We > > > > > need to find a way to force the calling task off the cpu while the > > > > > asynchronous > > > > > operations in the transmit path complete, and we can do that this > > > > > way, or by > > > > > honoring SK_SNDTIMEO. I'm fine with doing the latter, but I didn't > > > > > want to > > > > > alter the current protocol behavior without consensus on that. > > > > > > > > In general SCHED_FIFO is dangerous with regard to stalling other > > > > progress, incl. ksoftirqd. But it does appear that this packet socket > > > > case is special inside networking in calling schedule() directly here. > > > > > > > > If converting that, should it convert to logic more akin to other > > > > sockets, like sock_wait_for_wmem? I haven't had a chance to read up on > > > > the pros and cons of completion here yet, sorry. Didn't want to delay > > > > responding until after I get a chance. > > > > > > > So, I started looking at implementing SOCK_SNDTIMEO for this patch, and > > > something occured to me....We still need a mechanism to block in > > > tpacket_snd. > > > That is to say, other protocol use SK_SNDTIMEO to wait for socket memory > > > to > > > become available, and that requirement doesn't exist for memory mapped > > > sockets > > > in AF_PACKET (which tpacket_snd implements the kernel side for). We have > > > memory > > > mapped frame buffers, which we marshall with an otherwise empty skb, and > > > just > > > send that (i.e. no waiting on socket memory, we just product an error if > > > we > > > don't have enough ram to allocate an sk_buff). Given that, we only ever > > > need to > > > wait for a frame to complete transmission, or get freed in an error path > > > further > > > down the stack. This probably explains why SK_SNDTIMEO doesn't exist for > > > AF_PACKET. > > > > SNDTIMEO behavior would still be useful: to wait for frame slot to > > become available, but only up to a timeout? > > > Ok, thats fair. To be clear, memory_mapped packets aren't waiting here for a > frame to become available for sending (thats done in userspace, where the > application checks a specific memory location for the TP_AVAILABLE status to > be > set, so a new frame can be written). tpacket_snd is wating for the > transmission > of a specific frame to complete the transmit action, which is a different > thing.
Right. Until this report I was actually not even aware of this behavior without MSG_DONTWAIT. Though the wait is not for a specific frame, right? Wait as long as the pending_refcnt, which is incremented on every loop. > Still, I suppose theres no reason we couldn't contrain that wait on a timeout > set by SK_SNDTIMEO > > > To be clear, adding that is not a prerequisite for fixing this > > specific issue, of course. It would just be nice if the one happens to > > be fixed by adding the other. > > > > My main question is wrt the implementation details of struct > > completion. Without dynamic memory allocation, > > sock_wait_for_wmem/sk_stream_write_space obviously does not make > > sense. But should we still use sk_wq and more importantly does this > > need the same wait semantics (TASK_INTERRUPTIBLE) and does struct > > completion give those? > > > I suppose we could overload its use here, but I would be worried that such an > overload would be confusing. Nominally, sk_wq is used, as you note, to block > sockets whose allocated send space is full, until such time as enough frames > have been sent to make space for the next write. In this scenario, we already > know we have space to send a frame (by virtue of the fact that we are > executing > in tpakcet_snd, which is only called after userspace has written a frame to > the > memory mapped buffer already allocated for frame transmission). In this case > we > are simply waiting for the last frame that we have sent to complete > transmission, at which point we can look to see if more frames are available > to > send, or return from the system call. I'm happy to take an alternate > consensus > into account, but for the sake of clarity I think I would rather use the > completion queue, as it makes clear the correlation between the waiter and the > event we are waiting on. That will be less likely to have unexpected side-effects. Agreed. Thanks for the explanation. Only last question, does it have the right wait behavior? Should this be wait_for_completion_interruptible?
