On Wed, Jan 22, 2020 at 10:12:40PM +0900, Masami Hiramatsu wrote: > On Wed, 22 Jan 2020 07:11:15 -0500 > Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > > On Wed, 22 Jan 2020 16:23:17 +0900 > > Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > > > > > On Tue, 21 Jan 2020 19:29:05 -0500 > > > Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > > > > > > On Thu, 16 Jan 2020 23:44:52 +0900 > > > > Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > > > > > > > > > Since the 5 jiffies delay for the optimizer is too > > > > > short to wait for other probes, make it longer, > > > > > like 1 second. > > > > > > > > Hi Masami, > > > > > > > > Can you explain more *why* 5 jiffies is too short. > > > > > > Yes, I had introduced this 5 jiffies delay for multiple probe registration > > > and unregistration like systemtap, which will use array-based interface to > > > register/unregister. In that case, 5 jiffies will be enough for the delay > > > to wait for other kprobe registration/unregsitration. > > > > > > However, since perf and ftrace register/unregister probes one-by-one with > > > RCU synchronization interval, the optimizer will be started before > > > finishing to register/unregister all probes. > > > And the optimizer locks kprobe_mutex a while -- RCU-tasks synchronization. > > > Since the kprobe_mutex is also involved in disabling kprobes, this also > > > stops probe-event disabling. > > > > > > Maybe 5 jiffies is enough for adding/removing a few probe events, but > > > not enough for dozens of probe events. > > > > > > > Perhaps we should have a mechanism that can detect new probes being > > added, and just continue to delay the optimization, instead of having > > some arbitrary delay. > > Yes, that is what [03/13] does :) > Anyway, it seems that the RCU-synchronization takes more than 5 jiffies. > And in that case, [03/13] still doesn't work. That's why I added this patch > after that. If the RCU synchronization is synchronize_rcu_tasks(), then yes, it will often take way more than 5 jiffies. If it is synchronize_rcu(), 5 jiffies would not be unusual, especially on larger systems. But in the case of synchronize_rcu(), one option is to instead use synchronize_rcu_expedited(). It is not clear that this last is really justified in this case, but figured it might be worth mentioning. Thanx, Paul
