On 11/14/2017 05:31 PM, Peter Zijlstra wrote: > On Tue, Nov 14, 2017 at 08:16:09AM -0800, Andy Lutomirski wrote: >> What guarantees that there's an IPI? Do we never do a syscall, get >> migrated during syscall processing (due to cond_resched(), for >> example), and land on another CPU that just happened to already be >> scheduling? > > Possible, the other CPU could've pulled the task because it went idle. > No IPIs involved in that scenario. > > And if it was running a different thread of the same process prior to > that, we'll also not do switch_mm(). > > So yes, it is possible to construct a migration scenario without core > serializing instructions (of the CPUID/MOV-CR kind, not the LOCK prefix > kind). > > Note that that still requires a multi-threaded process. > > There is another scenario; where the NOHZ load-balancer moves the task; > such that the NOHZ load balancing CPU is a 3rd CPU. In that case there > is an interrupt (to affect the load-balancing) but it will not land on > the CPU that's going to run the task. > > This could happen for a single threaded task; since I suppose the NOHZ > idle CPU that's going to be the victim could have ran our task last and > still lazily have the mm. > > Very tricky to make work, not to mention that I suspect actually going > idle will kill a whole bunch of state real quick. > IIRC, if the dest cpu is idle and the system is with idle=poll, no IPI is fired as well, but that is not a very common case. -- Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
