On Thu, Jun 20, 2019 at 12:18:41PM -0400, Willem de Bruijn wrote: > On Thu, Jun 20, 2019 at 12:14 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. > > > > > Gladly. > > > > > > 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. > > > > > I'm not sure. On the surface, we certainly could do it, but inserting a > > call to > > do_softirq, either directly, or indirectly through some other mechanism > > seems > > like a non-obvious fix, and may lead to confusion down the road. I'm > > hesitant > > to pursue such a soultion without some evidence it would make a better > > solution. > > > > > > 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. > > > > > That would be the solution described above (i.e. honoring SK_SNDTIMEO. > > Basically you call sock_send_waittimeo, which returns a timeout value, or 0 > > if > > MSG_DONTWAIT is set), then you block for that period of time waiting for > > transmit completion. > > From an ABI point of view, starting to support SK_SNDTIMEO where it > currently is not implemented certainly seems fine. > I would agree, I was just thinking since AF_PACKET is something of a unique protocol (i.e. no real protocol), there may have been reason to ignore that option, but I agree, it probably makes sense, barring no counter reasoning.
> > I'm happy to implement that solution, but I'd like to get > > some clarity as to if there is a reason we don't currently honor that socket > > option now before I change the behavior that way. > > > > Dave, do you have any insight into AF_PACKET history as to why we would > > ignore > > the send timeout socket option here? > > On the point of calling schedule(): even if that is rare, there are a lot of > other cond_resched() calls that may have the same starvation issue > with SCHED_FIFO and ksoftirqd. > Agreed, but I've not found any yet Neil
