On Wed, Sep 09, 2020 at 12:39:00PM -0700, Paul E. McKenney wrote: > > > > > > When do you need this by? > > > > > > Left to myself, I will aim for the merge window after the upcoming one, > > > and then backport to the prior -stable versions having RCU tasks trace. > > > > That would be too late. > > We would have to disable sleepable bpf progs or convert them to srcu. > > bcc/bpftrace have a limit of 1000 probes for regexes to make sure > > these tools don't add too many kprobes to the kernel at once. > > Right now fentry/fexit/freplace are using trampoline which does > > synchronize_rcu_tasks(). My measurements show that it's roughly > > equal to synchronize_rcu() on idle box and perfectly capable to > > be a replacement for kprobe based attaching. > > It's not uncommon to attach a hundred kprobes or fentry probes at > > a start time. So bpf trampoline has to be able to do 1000 in a second. > > And it was the case before sleepable got added to the trampoline. > > Now it's doing: > > synchronize_rcu_mult(call_rcu_tasks, call_rcu_tasks_trace); > > and it's causing this massive slowdown which makes bpf trampoline > > pretty much unusable and everything that builds on top suffers. > > I can add a counter of sleepable progs to trampoline and do > > either sync rcu_tasks or sync_mult(tasks, tasks_trace), > > but we've discussed exactly that idea few months back and concluded that > > rcu_tasks is likely to be heavier than rcu_tasks_trace, so I didn't > > bother with the counter. I can still add it, but slow rcu_tasks_trace > > means that sleepable progs are not usable due to slow startup time, > > so have to do something with sleepable anyway. > > So "when do you need this by?" the answer is asap. > > I'm considering such changes to be a bugfix, not a feture. > > Got it. > > With the patch below, I am able to reproduce this issue, as expected. I think your tests is more stressful than mine. test_progs -t trampoline_count doesn't run the sleepable progs. So there is no lock/unlock_trace at all. It's updating trampoline and doing sync_mult() that's all. > My plan is to try the following: > > 1. Parameterize the backoff sequence so that RCU Tasks Trace > uses faster rechecking than does RCU Tasks. Experiment as > needed to arrive at a good backoff value. > > 2. If the tasks-list scan turns out to be a tighter bottleneck > than the backoff waits, look into parallelizing this scan. > (This seems unlikely, but the fact remains that RCU Tasks > Trace must do a bit more work per task than RCU Tasks.) > > 3. If these two approaches, still don't get the update-side > latency where it needs to be, improvise. > > The exact path into mainline will of course depend on how far down this > list I must go, but first to get a solution. I think there is a case of 4. Nothing is inside rcu_trace critical section. I would expect single ipi would confirm that.
