On Thu, Apr 06, 2023 at 03:32:06PM +0200, Peter Zijlstra wrote: > On Thu, Apr 06, 2023 at 09:49:22AM -0300, Marcelo Tosatti wrote: > > > > > 2) Depends on the application and the definition of "occasional". > > > > > > > > For certain types of applications (for example PLC software or > > > > RAN processing), upon occurrence of an event, it is necessary to > > > > complete a certain task in a maximum amount of time (deadline). > > > > > > If the application is properly NOHZ_FULL and never does a kernel entry, > > > it will never get that IPI. If it is a pile of shit and does kernel > > > entries while it pretends to be NOHZ_FULL it gets to keep the pieces and > > > no amount of crying will get me to care. > > > > I suppose its common practice to use certain system calls in latency > > sensitive applications, for example nanosleep. Some examples: > > > > 1) cyclictest (nanosleep) > > cyclictest is not a NOHZ_FULL application, if you tihnk it is, you're > deluded. On the field (what end-users do on production): cyclictest runs on NOHZ_FULL cores. PLC type programs run on NOHZ_FULL cores. So accordingly to physical reality i observe, i am not deluded. > > 2) PLC programs (nanosleep) > > What's a PLC? Programmable Logic Circuit? Programmable logic controller. > > A system call does not necessarily have to take locks, does it ? > > This all is unrelated to locks OK. > > Or even if application does system calls, but runs under a VM, > > then you are requiring it to never VM-exit. > > That seems to be a goal for performance anyway. Not sure what you mean. > > This reduces the flexibility of developing such applications. > > Yeah, that's the cards you're dealt, deal with it. This is not what happens on the field.
